Roadmap for Venus Exploration V11J · Web viewThe Venus Exploration Analysis Group is NASA’s community‐based forum designed to provide scientific input and technology development

ROADMAP FOR VENUS EXPLORATION

DRAFT FOR VENUS EXPLORATION COMMUNITY

REVIEW

Venus Exploration Assessment GroupVersion V11L

Jan 21, 2019

Pre-Decisional Information -- For Planning and Discussion Purposes Only 1


Table of Contents 1 Introduction........................................................................................................................................................4

2 Venus Exploration in NASA’s Science Program....................................................................................................5

2.1 Discovery Missions.....................................................................................................................................5

2.2 New Frontiers Missions..............................................................................................................................5

2.3 Flagship Mission Concepts..........................................................................................................................6

2.4 Missions of Opportunity – Venus Bridge....................................................................................................6

2.5 International Opportunities........................................................................................................................6

2.6 Technology Readiness................................................................................................................................7

3 Roadmap Development Process..........................................................................................................................9

3.1 Initial Inputs................................................................................................................................................9

3.2 Interactions with the GOI Focus Group.....................................................................................................10

3.3 Interactions with the Technology Focus Group........................................................................................10

3.4 Venus Exploration Roadmap – Phases......................................................................................................10

4 Venus Exploration Platforms.............................................................................................................................12

4.1 Orbital Investigations of Surface and Interior...........................................................................................13

4.2 Orbital Investigations of Atmosphere and interaction with Space Environment......................................13

4.3 Atmospheric Probes.................................................................................................................................15

4.4 Surface Platforms......................................................................................................................................16

4.5 Aerial Platforms........................................................................................................................................17

4.6 Venus Surface Sample Return...................................................................................................................18

4.7 Surface or Near Surface Platform with Regional Mobility.........................................................................19

5 Mapping to Goals-Objectives-Investigations.....................................................................................................20

5.1 Format of the GOI-Platform assessments tables......................................................................................20

5.2 Goal One – Evolution, Habitability, and Exoplanets..................................................................................21

5.3 Goal Two - Atmospheric Dynamics, Composition and Climate History......................................27

5.4 Goal Three - Surface (and Interior)............................................................................................35

5.5 Mapping to Goals-Objectives-Investigations - Summary Comments.........................................41

6 Venus Exploration Roadmap..............................................................................................................42

6.1 Near Term – Pre Decadal -2019 to 2022....................................................................................43

6.2 Mid Term – Decadal - 2023 to 2032).........................................................................................45

6.3 Far Term – Post Decadal 2033-2042..........................................................................................47

7 Summary...........................................................................................................................................49

Appendix A: Science from Aerial Platforms.............................................................................................50

Appendix B: Platform Interrelationships...................................................................................................52



Venus Exploration Assessment Group (VEXAG).The Venus Exploration Analysis Group is NASA’s community based forum designed to provide‐ scientific input and technology development plans for planning and prioritizing the exploration of Venus over the next several decades. VEXAG is chartered by NASA’s Planetary Science Division and reports its findings to NASA. Open to all interested scientists, technologists and engineers VEXAG regularly evaluates Venus exploration goals, scientific objectives, investigations, and critical measurement requirements, including recommendations on the NRC Decadal Survey and the Solar System Exploration Strategic Roadmap. This document, prepared by the VEXAG Venus Exploration Roadmap Focus Group is one of three VEXAG documents prepared to guide planetary exploration in 2019 and beyond

Venus Exploration Roadmap Focus Group – Members

James A. Cutts (Lead) Jet Propulsion Laboratory –California Institute of Technology Michael Amato NASA Goddard Space Flight CenterTibor Kremic NASA Glenn Research CenterMartha Gilmore Wesleyan UniversityCandace Gray New Mexico State UniversityScott Hensley Jet Propulsion Laboratory –California Institute of TechnologyGary Hunter NASA Glenn Research CenterNoam Izenberg John Hopkins University – Applied Physics LabWalter Kiefer Lunar and Planetary InstituteKevin McGouldrick University of ColoradoJoe O’Rourke Arizona State UniversitySuzanne Smrekar Jet Propulsion Laboratory – California Institute of Technology

© 2019 California Institute of Technology. All rights reserved.

Government sponsorship acknowledged

1 IntroductionVenus is so similar to Earth in size, composition, and distance from the Sun that it is frequently referred to as “Earth’s twin.” Despite these similarities, however, Venus has gone down a very different evolutionary path. Venus today is dominated by a greenhouse climate “gone wild” that resulted from a complex interplay of the same atmospheric, surface, and interior processes at work on Earth. Yet there is strong evidence that the planet was not always so inhospitable, and Venus may even have possessed significant water in the distant past. Thus, the study of Venus provides unique and important opportunities to understand not only the general processes that govern the inner planets, but it also serves as a comparison case to suggest how those processes affected Earth, and in particular why and how Venus and Earth diverged. Exploration of Venus is one of the great remaining challenges and opportunities of Solar System science. It will provide valuable insight into planetary processes, the past and future of the terrestrial planets, and the likelihood of habitable planets in other planetary systems around other stars.

This Venus Exploration Roadmap lays out a framework for the future exploration of Venus, encompassing observations of the atmosphere, surface, and interior using a variety of mission modes ranging from orbiters and probes to aerial platforms, and long duration landers. It was developed for the space science community by the Venus Exploration Analysis Group (VEXAG) to provide guidance to the Planetary Science Division and the Planetary Science Decadal Survey of 2021 (Reference) which is charged with framing a strategy for all of planetary exploration for the next decade and beyond. With the prolonged hiatus in U.S. led Venus exploration, the science and the technology are now ready for not just a single mission. In planetary exploration, the next decade can and should be the Decade of Venus.

Scientific guidance for this Exploration Roadmap is provided by the companion document Venus Goals, Objectives, and Investigations (GOI) (VEXAG_GOI_Focus_Group, 2018), which establishes the foundation and priorities for future Venus exploration. To facilitate the identification of specific mission concepts, the Exploration Roadmap considers the scientific contribution of different exploration platforms: orbiters, probes, surface platforms (landers) and aerial platforms. Many areas of progress in Venus exploration depends on advances in technology and the second companion document (Hunter &VEXAG Technology Focus Group, 2018). provides additional detail on the technologies that are propelling forward these capabilities- those which are now at hand and those which still represent formidable challenges.

Implementation of missions by NASA must be accomplished within a programmatic framework of competitive and assigned missions and increasingly reflects the role of missions conducted by other space agencies either independently or, as increasingly is the case, in collaboration with NASA. The relative accessibility of Venus means that many spacefaring nations can now reach Venus and contribute to its exploration. The representative mission sets described in this Venus Exploration Roadmap were derived based on assessment of their scientific potential, technology readiness, and programmatic considerations including feed-forward of science and capability in a logical fashion. This Roadmap is not a rigid prescription of mission implementations but a framework that can be used to choose the optimal pathway once the programmatic framework is better understood.


2 Venus Exploration in NASA’s Science ProgramImportant Venus science can be accomplished within all of the mission programs sponsored by the NASA Solar System Exploration Directorate. The diversity of compelling Venus science questions means that even highly focused measurements can be extremely productive, so even relatively low-cost missions can make major contributions. Conversely, the complex inter-relationships and variability of atmospheric, surface and interior phenomena on Venus make the planet an ideal candidate for larger multi-disciplinary missions. Our Venus exploration strategy, comprising missions from all classes, coupled with strategic technology investments, will enable major progress in the understanding of the key scientific issues. This Venus Exploration Roadmap envisages NASA missions funded through the established programmatic lines complemented by missions led by other space agencies. International collaboration in the planning and implementation of these missions, taking advantage of the unique strengths and interests of each participating agency, provides an opportunity to make more rapid scientific progress than would otherwise be possible. In this section, we review each of the NASA Programs that can be a vehicle for Venus Exploration, the status of International Collaboration and the importance of advancing technology in future Venus exploration, a topic explored in much more detail in the companion report (Hunter &VEXAG Technology Focus Group, 2018)

2.1 Discovery MissionsThe Discovery Program consists of Principal Investigator (PI)-led smaller missions that provide opportunities for targeted investigations with relatively rapid flight missions. The Discovery program is ideally suited for missions to Venus. The planet’s relative proximity means that flight times are short, and power and communications bandwidth are plentiful for orbital missions. The dense atmosphere can be used for aerobraking or aerocapture to reduce propellant requirements, and Venus also provides an attractive and scientifically rich environment for probes or aerial platforms. Important surface compositional and topographic measurements, including change detection, can be made from orbit using radar, altimetry, and emissivity techniques already proven at Venus. Discovery missions (including orbiters, probes, and aerial platforms) will represent critical steps toward understanding Venus and its scientific relationship to Earth, and they will serve as pathfinders for more complex multi-disciplinary missions.

In 2017, two Venus missions – VERITAS, and DaVINCI, were selected for a phase A study. VERITAS was an orbiter spacecraft and included a radar mapper and infrared surface imaging instruments. DaVINCI was an entry probe mission that included compositional, atmospheric structure measurements and surface imaging. Neither proposal was selected for flight. The next Discovery AO will be released in Mar 2019.

2.2 New Frontiers MissionsThe New Frontiers Program consists of PI-led medium-class missions addressing specific strategic scientific investigations endorsed by the Decadal Survey. The Decadal Survey of 2011 recommended a single Venus New Frontiers mission, the Venus In Situ Explorer (VISE) with science goals.

The New Frontier NF-4 AO released in December 2016 included a Venus In Situ Explorer (VISE) as one of the mission themes. The mission theme is focused on examining the physics and chemistry of Venus’s atmosphere and crust by characterizing variables that cannot be measured from orbit, including the detailed composition of the lower atmosphere, and the elemental and mineralogical composition of surface materials. Three Venus concepts were submitted to the call. The Venus In Situ Atmospheric and Geochemical Explorer (VISAGE) and Venus In Situ Composition Explorer (VICI), which were both landed missions. The Venus Origins eXplorer (VOX) was an orbiter mission similar in concept to


VERITAS and including a skimmer probe as an integral part of the mission and not just as a technology experiment. Only the VOX mission was determined to be Category 1 but it was not one of the two proposals that advanced to Step 2 in the call.

The NF-4 experience indicates that there is a high level of interest in Venus as a target for New Frontiers and there are highly competitive mission options. However, NASA and the next Decadal Survey should consider updating the description of the Venus mission theme to be less prescriptive with respect to the mission implementation and to place more emphasis on the nature and scope of the science that can be accomplished at Venus within the New Frontiers cost cap.

2.3 Flagship Mission ConceptsFlagship missions address those high-priority investigations that are so challenging that they cannot be achieved within the resources allocated to the Discovery and New Frontiers Programs. The 2011 Decadal Survey Inner Planets group selected a single small Venus flagship mission concept, the Venus Climate Mission (VCM), requiring little new technology, for the period 2013–2022. The VCM would make synergistic observations from multiple platforms (orbiter, balloon, mini-probe, and dropsondes) to enable global three-dimensional characterization of the atmosphere. VCM was ranked fourth in priority, along with an Encleadus mission, behind Flagship missions to Mars, Europa and Ice Giants.

In 2014, NASA also began exploring a potential collaboration with Russia on a Flagship-class mission in which NASA would be the junior partner. The mission Venera D would include an orbiter and short duration lander furnished by Russia and the substantial lift capability of the Angara A5 heavy lift launch vehicle would allow inclusion of major NASA contributed elements. A long duration surface platform using high temperature electronics and an aerial platform using conventional electronics and sensors have been considered for inclusion in this mission. In the Spring of 2019, NASA is initiating a study that will revisit concepts for Venus Flagship missions in the decade 2023 to 2032.

In revising the 2014 Roadmap, we have focused on three planning periods: Pre Decadal from 2019 to 2022, Decadal 2023 to 2022 and Post Decadal 2033 to 2042. Drawing on the last 25 years of experience the Mars Exploration Program, we have taken a more conservative position on future missions than appeared in the 2014 roadmap.

2.4 Missions of Opportunity – Venus BridgeAs the pressure on space science budgets grows more severe, it is important that NASA consider alternative mission modes that may be able to make contributions to Venus science at lower cost—perhaps less than half the cost of a Discovery mission. In 2018, VEXAG completed the Venus Bridge study (Grimm & Gilmore, 2018), which examined the feasibility of implement linked orbital and in situ missions exploiting SmallSat and CubeSat technologies. Several linked mission concepts were identified with highly focused objectives and a number of challenges that need still to be overcome of the potential of these small missions is to be realized. As such small spacecraft become increasingly capable, they could potentially conduct important short-duration scientific observations at very low cost, demonstrate new technologies for application at Venus and in the process help to refine the goals of future larger missions. NASA may consider whether a dedicated Venus program or a more general program addressing the inner planets should be the appropriate approach.

2.5 International OpportunitiesThe international community has demonstrated a strong interest in Venus, and future exploration of Venus is enhanced through international cooperation. The European Space Agency’s Venus Express, which ended in December 2014, has benefited from involvement of other agencies including NASA.


Likewise, the Japanese Space Agency’s Akatsuki, also has NASA participation. Looking forward, the Indian Space Research Organization (ISRO) is planning a Venus mission that is targeted for launch in 2023. A call for international participation in the experiment payload was issuedissued in November 2018 with responses expected in January 2019. The mission includes an orbiter with a comprehensive payload focused on both atmospheric and surface objectives and a balloon platform.

As noted in Section 2.3, Russia’s Roscosmos and Space Research Institute entered into a collaboration with NASA’s Planetary Science Division on Venera D, on a Flagship class mission with both landed and orbital elements. A second report from the Joint Science and Technology Definition Team (JSDT) is pending but Venera D the mission is still a pre-Phase A mission concept study and no firm commitment has been made by the Russian side to go forward with the mission.

ESA is studying a follow on to Venus Express. After pursuing both orbital and aerial platform missions as early candidates for the ESA M Class (Medium sized) mission, the ESA has settled on an orbital mission focused on investigations of the surface and interior. EnVision was originally one of 27 candidates submitted to the M4 call and although not selected at that time received sufficiently strong technical reviews that it was resubmitted for M5 (Ghail, 2018) and is now one of three candidate missions in a phase A study. NASA is participating in that study and considering potential NASA contributions. A selection decision is expected in 2021 with a launch in 2032.

While there are some similar capabilities proposed for Envision and VERITAS, the concepts are highly complementary. VERITAS focuses on global coverage, producing a geodetically controlled topography map at 250 m horz. x 5 m vert and SAR imaging at 30 m resolution, with targeted imaging at 15 m for 40% of the planet, and interferometry at specific targets. Envision produces 30 m SAR imaging and strereo topography for 20% of the planet, with 6 and 1 m SAR imaging for 2% and 0.1% of the planet, respectively. ESA’s Envision also includes a subsurface sounding capability and 2 spectrometers for atmospheric science. Both concepts include the Venus Emissivity Mapper (VEM) that examines surface composition and mineralogy (Helbert et al., 2018). The VERITAS VEM near-global coverage would provide a valuable baseline for change detection of surface weathering and volcanism. VERITAS, which could beimplemented by at least eight years before EnVision, would provide global imaging data relevant to the selection of targets for high-resolution data and global topographic maps needed for interferometry and clutter reduction in subsurface sounding. The approach of global imaging followed by targeted, higher resolution imaging has proved extremely valuable for the study of Mars and other planetary bodies.

NASA’s suite of mission opportunities, the potential for smallsats, and coordination of efforts between space agencies will enable the Decade of Venus exploration envisaged in this Roadmap. Missions such as Cassini-Huygens exemplify how scientific return increases when multiple partners collaborate on a single mission, and this could serve as a model for future Venus exploration. Furthermore, given the large number of focused science objectives that comprise this Venus Exploration Roadmap, coordination at the program level could result in significant cost savings.

2.6 Technology ReadinessThe key challenges facing future Venus orbital missions include the extreme environments that they must accommodate. New technologies as described in the companion technology plan (Hunter & VEXAGTechnology Focus Group, 2018) must play a vital role in future Venus exploration. Some high-priority Venus missions can be conducted with technologies that are ready today. A number of others can be brought to TRL 6 between project selection and mission preliminary design review to enable launches to occur of the next Planetary Science Decadal Survey (2023 to 2032). However, many of Venus’s secrets


will require capabilities that cannot be realized until the subsequent decade and beyond. This roadmap makes a sober andrealistic assessment of what is feasible within potential resources.

2.6.1 Long Lived Surface ExplorationThe surface of Venus is one of the most difficult places in the Solar System to explore. In addition to the obvious extreme thermal conditions, these missions must also contend with a corrosive chemical environment, lack of sunlight, uncertain terrain, and atmospheric limitations on communication and other spacecraft operations. While short-duration, scientifically significant missions to the surface or lower atmosphere are possible with current technology, longer duration missions will require substantial investments and a strategic approach to technology development and testing. The Venus Technology Plan developed as a companion to this Exploration Roadmap describes the challenges and priorities for Venus exploration and recommends an investment approach that can support all mission classes. This effort is already underway with the HotTech program (Mercer, 2018) which includes an integration program to assure that all of the needed technologies are in place.

2.6.2 Aerial PlatformsAnother approach to long duration in situ exploration is to conduct it from aerial platforms operating high in the atmosphere where temperatures and pressures are close to those of the Earth at sea level and consequently instruments using conventional technologies can be employed. Although the environmental challenges in the upper atmosphere are much less severe than those at the surface, there are environmental challenges from the corrosive cloud particles. NASA has recently initiated a program to address some of these challenges (Mercer, 2018).

2.6.3 SmallSat and CubeSat TechnologiesWith its proximity to Earth, Venus is a prime candidate for the application of advances in miniaturization of spacecraft systems and components. The Venus Bridge study explored how these capabilities might be applied at Venus. The recent flights of the first interplanetary CubeSats (MarCO1 and 2) in support of the InSight mission communications during atmospheric entry have demonstrated that these technologies can be sufficiently robust to support missions to the inner planets. However, realizing opportunities to conduct science at Venus will require advances in SmallSat and CubeSat power, propulsion, and telecommunications. NASA funded four mission concept studies for Venus under the Planetary Science Deep Space SmallSat Studies (PSDS3) program. Cupid’s Arrow is a SmallSat that skims the atmosphere to measure noble gases in Venus atmosphere (Sotin, 2018). CUVE is a CubeSat carrying a UV mapping spectrometer (Cottini, 2018). SAEVE carries out seismic and atmospheric exploration of Venus ( (Kremic, 2018). VAMOS measures the Venus airglow fluctuation to monitor seismicity (Komjathy 2018).


3 Roadmap Development ProcessThree focus groups were formed by VEXAG in May 2018 and were assigned the task of revising VEXAG’s guiding documents. These VEXAG documents define scientific goals and the missions and technology needs needed to implement them. The process for updating the Venus Exploration Roadmap, is shown in Figure 3-1, and is described in more detail below.

3.1 Initial InputsThe starting point for the Roadmap Focus Group was the Venus Exploration Roadmap of 2014. As it awaited inputs from the GOI Focus group, which began work contemporaneously, the Roadmap Focus group was briefed on new mission and experimental concepts that had emerged since the 2014 Roadmap was completed. These concepts included but were not limited to developments in small satellites and CubeSats, aerial platforms and high temperature electronics technologies enabling long duration surface missions. In addition, to the development of new platforms, there were also developments in instruments and experimental techniques including miniature instruments that could be deployed on SmallSats and CubeSats.

Figure 3-1 Process for developing the VEXAG 2018 Roadmap


3.2 Interactions with the GOI Focus GroupWhen the GOI Focus Group developed its draft Goals, Objectives and Investigations in Mid October 2018, the Roadmap Focus group provide feedback. This process benefited from the cross cutting membership of the GOI and Roadmap focus groups. Following the completion of the GOI, the two focus groups worked together to develop a consensus on how Roadmap missions would address investigations in the GOI>

3.2.1 Actionable InvestigationA key issue for the Roadmap Focus Group was assuring that investigations were defined with sufficient specifically that a platform and experimental approach could be identified. A number of iterations took place with the GOI Focus Group to achieve this.

3.2.2 New Types of InvestigationThe Roadmap Focus Group also identified investigations that enabled by either new experimental techniques or platform technologies. These were brought to the attention of the GOI focus group and included in later versions of the GOI.

3.2.3 Matrix of Investigations and PlatformsFollowing the completion of the GOI, the two focus groups worked together to develop a matrix describing how well Roadmap missions could address investigation in the GOI. This was an updated version of a similar matrix that appeared in the 2014 Roadmap

3.3 Interactions with the Technology Focus GroupThese interactions spanned the topics of technology needs, technology capabilities and technology maturity assessment. The process benefited from the cross cutting membership of the Roadmap and Technology focus groups.

3.3.1 Technology Needs: The technology needs for each of the Roadmap missions were identified and communicated to the Technology focus group. Because the formal Roadmap technology needs emerged later in the overall process, the Technology focus group had to work on provisional data.

3.3.2 Technology CapabilitiesThe VER focus group had the benefit of an existing technology plan and rapid updates to that plan that occurred in the first six months after the document update process began.

3.3.3 Technology Maturity AssessmentA key factor in identifying the sequence of the Roadmap missions was the determination of the technology maturity of the missions in the Roadmap. This drew on assessments of both subsystem and system level technology maturity that was conducted by the Technology focus group but it involved assessments of cost and risk tolerance that were provided by the Roadmap focus group.

3.4 Venus Exploration Roadmap – PhasesThe Venus Exploration Roadmap was broken into three phases of unequal length that reflected the timing of this assessment relative to the Planetary Science Decadal Survey


3.4.1 Near Term – Pre Decadal- 2019 to 2022The first phase dealt with mission opportunities that will exist in the four-year period before the next planetary science decadal survey is completed. While there may be no new Venus mission launches in this period, ISRO’s Venus mission may be launched shortly after in 2023, there will be Discovery, M5 and potentially New Frontiers selections during this time. Accordingly, this period deserves special consideration.

3.4.2 Mid Term – Decadal - 2023 to 2032This is the period for which recommendations will be made for the next Planetary Science Decadal Survey and is thus a primary focus of this roadmap

3.4.3 Far Term – Post- Decadal 2033 to 2042This follows the decade of prime interest to the Decadal Survey. Missions in three categories are included:

Missions that are already planned for this time horizon such as? EnVision? Missions for which the technology is sufficiently challenging that they are not feasible in the in

the earlier time frame, Missions that represent a logical follow on to missions in the earlier decade.


4 Venus Exploration PlatformsThe purpose of the Venus Exploration Roadmap is to define the opportunities for advancing scientific knowledge of Venus by means of missions that can carry out the investigations called for in the companion GOI document (VEXAG_GOI_Focus_Group, 2018). These investigations are implemented with different types of instrument platforms, orbiters, probes, surface platforms (landers) and aerial platforms (Table 4-1). Sample return, perhaps the most complex exploration modality, involves acquisition of a sample of surface or atmosphere and return of that sample to Earth. In this section, we examine the capability of each type of platform for making relevant scientific measurements at Venus. The impact of the Venus environment on the ability to conduct these investigations is key and the role of advances in technology in mitigating the impact of the environment is of particular relevance. In section 5, we describe how measurements made from these platforms can address the VEXAG Goals, Objectives and Investigations where we identify cases where measurements from a single platform are both necessary and sufficient for accomplishing an objective and cases where measurements from multiple platform either synchronously or sequentially are required. In section 6, we describe the Venus Exploration Roadmap by incorporating information on the technology maturity of each of the platform types and the mission opportunities in which they can be deployed. In Section 6, we also construct potential mission sequences that involve multiple platforms where required and provide the feedforward needed to provide a program that most effectively and efficiently addresses the Goals, Objectives and Investigations.

Table 4-1 Platforms included in the Venus Exploration Roadmap. Some platforms include the capability to get to Venus. Others required delivery by an auxiliary vehicle such as an orbiter or cruise state. Some platforms may be deployed in a single mission, some may be combined to form a larger and more capable mission.


4.1 Orbital Investigations of Surface and InteriorDespite the dense atmosphere and thick cloud cover of Venus, which present unique challenges for orbital investigations of the surface and the interior, a tremendous amount can be learned about the surface and interior of Venus from orbit.

Radar imaging at an order of magnitude better than Magellan, providing an opportunity to observe entirely new processes.

Topography at 2 orders of magnitude better than currently available. Topography is critical at Venus, where much of the surface has little dielectric contrast

Iron mineralogy and oxidation state, as well as thermal variations, can be obtained by observation in infrared windows at a scale of ~50 km to determine rock types, weathering reactions, and search for recent and active volcanism

Uniform resolution gravity field with sufficient resolution to determine global elastic thickness Radio sounders to probe the shallow (~100m) subsurface stratigraphy

Our roadmap contains two complementary orbiter missions.

4.1.1 Global Mapper: The next step in orbital surface exploration should be a global mapping mission that would improve the resolution of radar images by an order of magnitude over Magellan and the spatial resolution of topographic maps by an even larger amount. The technology for such a mission is ready today as reflected in the Category 1 rating of the VOX missions in the recent New Frontiers (NF-4) competition. It could be a candidate for upcoming Discovery and New Frontiers calls.

4.1.2 Targeted Investigations: A second mission that would logically follow the global mapper would aim for still higher spatial resolution but for areas targeted based on global mapping results. This mission would also include a radio sounder for probing the subsurface to look for buried structure indicative of recent sedimentary and volcanic processes. This mission would utilize the precision global topography maps to remove surface clutter. The Envision Mission, which is now being considered for the M5 call, meets these criteria. The earliest it could by launched under current ESA plans is 2032.

Both of these missions involve orbiters that carry out most of their scientific mission from a near polar and circular or near circular orbit with a period near 90 minutes. Because of the slow rotation of Venus, it is possible to obtain images of the same surface locations in order to detect any temporal changes during several successive orbits.sidereal 122

Global reconnaissance supports landed missions by identifying high. NASA has promoted mission sequences that first conduct reconnaissance, then conduct in-situ meausurements, followed by mobile expoloration. The Mars Program has been highly successful in implementing this approach. Global reconnaissance serves to


4.2 Orbital Investigations of Atmosphere and interaction with Space Environment

The dense atmosphere and thick clouds of Venus are accessible to investigation with a variety of remote sensing and some in situ techniques. Prior missions to Venus including the comparatively recent ESA Venus Express mission and the ongoing JAXA Akatsuki mission have contributed the most current knowledge of the planet. The atmospheric investigations conducted from an orbital vantage point have been grouped into three broad categories and are discussed below

Orbital requirements for atmospheric investigations depend on the nature of the investigation.

Hyperspectral Imagery: Many of these investigations require observations from sufficient distance to observe either the full disc or a significant part of it. Exceptions are techniques where the limb must be observed in emission at high resolution and the inherent resolution of the instrument is limited such as with millimeter and submillimeter spectroscopy. Although most of these investigations relate to processes occurring entirely within the atmosphere, the potential of modulation of the airglow by seismic events has been recognized since completion of the 2014 Roadmap ( (Stevenson, 2015) and would add an important science dimension to those included here.

Particles and Fields: These in situ measurements of the space environment require observations at many different ranges and geometries. Simultaneous orbital measurements at two or more orbital locations can sometimes be useful.

Solar/Stellar/Radio Occultation: These techniques involve observations of the limb of the planet and while range is not as critical as when the limb is being observed in emission, orbital location is still important. In addition to the traditional form of radio occultation technique such as implemented on Venus Express and Akatsuki, where the radio signal from the spacecraft is observed by a single ground based antenna or conversely, we have explicitly included the potential of CubeSats and SmallSats performing mutual occultations. This can vastly increase the number of locations where the atmosphere is probed over what is possible with a single spacecraft and does not require a commitment of costly ground bases antennas.

4.2.1 Orbiter – Atmosphere and Ionosphere - SynopticThe first class of orbiter considered here is a platform that would address atmospheric and ionospheric objectives using a comprehensive payload of instruments and is typified by those carried out on Venus Express and planned for the Venera D orbiter and ISRO’s proposed Venus mission. Typically to provide the ability to carry out both nadir viewing and limb scanning observations a highly eccentric and high inclination orbit is desirable for this purpose. However, low inclination and low eccentricity orbits are well-suited for investigations focused on atmospheric dynamics and composition, as demonstrated on Earth by the synergy of Geostationary platforms such as GOES and Himawari, and Low Earth Orbit platforms such as the A-train.

4.2.2 Orbiter- Atmosphere and Ionosphere - Small SatelliteA new type of approach to orbital observations has emerged in the context of Venus Bridge. This concept takes advantage of low cost SmallSat or even CubeSat technology for delivering the science payload to Venus. Typically, this means that the science payload is much more constrained. As a consequence, the orbit may be tailored to specific objectives rather than representing a compromise between many. With the advent of CubeSats, it is also possible to contemplate missions with multiple


platform sampling different parts of the space environment contemporaneously or performing mutual radio occultations in order to dramatically increased spatial and temporal sampling.

4.3 Atmospheric Probes Atmospheric probes short duration observations in the atmosphere. The three types of probe are considered in this section are distinguished by the manner in which they enter the atmosphere. They also differ sharply in terms of complexity and cost and hence the opportunities for integrating them with other platforms into mission concepts .

4.3.1 SkimmersA skimmer is a vehicle that passes through the upper reaches of the Venus atmosphere acquires a gas sample and then analyses the sample after it emerges from the atmosphere. A skimmer concept, the Sample Collection for Investigation of Mars (SCIM) mission (Leshin, 2002), proposed to the Mars Scout program. SCIM would have captured intact dust grain samples in aerogel for return to Earth. In the Venus application, the focus is on inert gases and there is no return to Earth but analysis is performed in space. A skimmer concept studied in NASA’s Planetary Science Deep Space SmallSat Studies (PSDS3) known as Cupid’s Arrow (Sotin, Avice, & Baker, 2018). Similar concepts have appeared under other names as part of Discovery and New Frontiers proposals.

The skimmer concept is clearly limited to sampling the higher reaches of the atmosphere and studies are ongoing with respect to whether the atmosphere at the sampled altitude is representative of the bulk composition as well as whether the hypervelocity sampling process induces fractionation. The strengths of the technique include the limited heating experienced at this altitude that greatly simplifies thermal protection relative to a deep probe and the large amount of time available for mass spectrometric analysis that can vastly improve the counting statistics for isotope present in trace amounts.

4.3.2 Atmospheric Entry ProbesAtmospheric probes that descend through the Venus atmosphere and reach the surface were used in the Pioneer Venus program in the early 1970s. Decades of instrument development have made it possible to implement a deep probe mission with greatly improved instrumentation including descent imaging. A key difference from the skimmer probe is the need to remove all of the energy of the probe as it enters the atmosphere and not just a small fraction. The development of the High Energy Entry Environment Technology (HEEET) for tolerating the severe conditions of Venus entry not only makes an entry mission possible again (the TPS material used on Pioneer Venus probe is no longer manufactured) but allows greater flexibility in entry conditions including shallower entry angles.

4.3.3 SondesThe advent of Aerial Platforms (see Section 4.5) enables a class of atmospheric probe that can be delivered to Venus with an aerial platform and hence does not require a separate entry and descent system. As a result, these probes, which we refer to as sondes in this document, can be smaller and much lower cost than conventional entry probes. In addition, data can be relayed through the aerial platform enabling much higher data return than direct to Earth or by using an orbiter or flyby spacecraft. Sondes were an integral part of the Venus Climate Mission study and include a large sonde that was released soon after entry of the aerial platform and a smaller sonde that was carried by the aerial platform and subsequently released from it. Sondes can capitalize on technologies developed for CubeSats. Missions with multiple sondes, released at different times and that probe only the upper


atmosphere, have been considered. Missions with deep sondes that descend to the surface and use guidance for targeting surface features identified in high-resolution radar imagers are considered for the second decade mission (2033 to 2042)

4.4 Surface PlatformsVehicles that descend to the surface and then conduct investigations on the surface of Venus constitute a key element of our Roadmap. In 2018, NASA’s Planetary Science Division initiated a Venus Surface Platforms study (Amato & Kremic, 2019). This report includes information presented during this study most recently at a meeting held on Nov 28-29 2018 at Glenn Research Center. It includes consideration of platforms that can survive and carry out science measurements for periods of a few hours, similar to the Venera-VEGA landers and long-lived platforms capable of months of operation that are being enabled by NASA’s technology programs.

4.4.1 Short Duration LandersA short duration lander, as considered here, is a vehicle that relies on conventional electronics and sensors for carrying out its missions and these sensors are maintained in their operational temperature range by means of passive thermal control. Passive, in this context, means a combination of thermal insulation and the use of phase change materials. These two approaches mitigate the temperature rise resulting from heat leaking into the payload compartment and that generated by power dissipation by the payload. The typical lifetime of these landed missions is measured in hours and not days.

A series of Venera and Vega lander missions of progressively advancing capability was carried out by the Soviet Union in the 1970s and 1980s. These missions form the primary basis for what we know about the elemental composition of the Venus surface and some missions included color images once the spacecraft had landed. No lander mission has been conducted since. The last two Planetary Science Decadal Surveys have called for a Venus In Situ Explorer (VISE) and a VISE mission theme has been included in three New Frontiers proposal calls. However, a NF VISE mission has not been selected for flight yet. In addition, Russia has been studying a mission concept called Venera D that includes a landed mission with a clear heritage to the Venera-VEGA landers of the 1970s and 1980s.

Three recent mission concepts, with platforms in this category, are described below:

The Venus In Situ Atmospheric and Geochemical Explorer (VISAGE), proposed by JP for NF-4L, would descend to the surface and samples would be brought on board for analysis by infrared and X-ray spectroscopy.

The Venus In Situ Composition Investigation (VICI), selected for technology development of a Venus Element and Mineralogy Camera under NF-4, uses lasers on the lander to measure the mineralogy and elemental composition of rocks and soils

Venera D is still in a Pre-Phase A study but the Jan 2017 report of the JSDT calls for samples to be brought inside the lander and elemental analysis to be conducted remotely using a gamma ray spectrometer.

4.4.2 Long Lived In Situ Surface Explorer (LLISSE)Another candidate capability for the decade 2023 to 2032 is a Long-Lived In Situ Surface Explorer (LLISSE). The LLISSE could be deployed either as a self-contained payload attached to a short duration lander or in a vehicle with its own entry descent and landing system. The technical challenges are described in the companion technology plan (Hunter & VEXAG Technology Focus Group, 2018). The


selection of instruments that can operate at Venus surface temperatures that will be available in the next decade will provide measurements of temperature, pressure, wind speed and atmospheric chemistry and the ability to make these measurements over a period up to or including a complete Venus day would represent a major step forward. A technology demonstration of a seismic experiment including measurements of the seismic background in the Venus surface environment is also possible.

4.4.3 Advanced LandersAdvances In technology, described in more detailed in the companion technology plan, can have a dramatic impact on the capabilities of surface platforms:

Landing Guidance: Improving the precision of landing or the ability to avoid hazards on landing Robust landing. Mitigating the risks of landing in regions of complex topography Extended surface lifetime: Extending lifetime significantly beyond 3 hours. Autonomy: Increasing the sophistication of surface operations. Instrument Performance: Increasing the speed at which chemical analyses are performed.

Several concepts for conducting a surface seismology investigation were considered in the Venus Surface Platforms study (Amato & Kremic, 2019). A surface seismology experiment, which requires major development of instrument components that can operate for months or even years in the Venus surface environment, will complement and build upon the seismic observations acquired from orbit (Section 4.2) and from aerial platforms using technologies that are much closer at hand. Precursor technology demonstrations on prior surface missions will be key to understanding the surface backgrounds. Key issues to be considered are:

Feasibility and affordability of single station (like InSight/SEIS) and multi station (network) concepts

Seismic sources – Venus quakes, landslides, bolide impacts, atmospheric excitation

The Advanced Lander envisaged here would integrate the evolving capabilities of short lived and long-lived platforms as described in Sections 4.4.1 and 4.4.2. In particular:

Entry descent and landing capability enhanced with Terrain Relative Navigations if feasible Enhanced thermal lifetime through improved insulation, phase changed materials and reduced

power consumption. A long-lived seismometer experiment that would b deployed to the surface by the spacecraft

arm prior to carrying out its sampling function. A long-lived heat flow experiment implemented in the sampling drill hole.

4.5 Aerial Platforms Aerial platforms conduct their measurements from a vantage point within the Venus atmosphere. This makes it possible to make measurements that cannot be made from orbit as well as to provide in situ verification of analyses based on orbital data. In addition, aerial platforms can be used to deploy sondes (4.3.3) and to capture data from them for relay to an orbiter or return to Earth. The assessment given here draws on recent study conducted for the Planetary Science Division (Venus_Aerial_Platform_Study_Team, 2018). The types of measurements that can made from aerial platforms and sondes have been grouped into five categories (see also Appendix A)


Atmospheric Gas: Measuring the composition of noble gases and their isotopes as well as the active chemical species

Cloud and haze particles: Measuring the size and scattering properties of these particles as well as their chemical and potentially biological nature.

Atmospheric Structure: Measuring temperature, pressure and upward and downward welling radiation as a function of altitude as well as all three components of velocity including turbulence.

Planetary Interior: Apply geophysical techniques to the study of the planetary interior including the use of passive electromagnetic sounding, infrasound, remanent magnetics and gravimetry.

Surface Imaging: Obtain nighttime images of the surface from the base of the clouds and visual imaging from a few kilometers above the surface from sondes deployed from the platform.

A series of platforms of progressively advancing capability have been identified for this Roadmap. The technologies required for these concepts are described in the companion Venus Technology Plan.

4.5.1 Fixed Altitude BalloonA fixed altitude balloon would be a more capable version of the Venera VEGA balloons mission of 1985. Advances in technology would enable the lifetime to be extended to up to 100 days using solar power to replenish batteries. The payload could be much larger and include instruments to study the atmosphere and interior.

4.5.2 Variable Altitude (50 to 60 km) BalloonWith comparatively modest advances in technology, balloons can be implemented with the ability to control altitude in the range 50 to 60 km. This is still within the temperature range accessible with conventional electronics. The ability to change altitude will enable the atmospheric cloud layer to be studied more completely and will also enhance the value of some of the geophysical and atmospheric structure observations.

4.5.3 Variable Altitude (40 to 60 km) BalloonA further increase in the altitude change capability would allow the platform to access the atmosphere below the bases of the clouds. In addition to extending the atmospheric science that can be accomplished this would also allow higher-resolution, sub-cloud, night time imaging of the surface in the infrared representing a dramatic gain over what can be accomplished from orbit.

4.5.4 Other Concepts Vehicles with varying levels of three-dimensional control were considered in a trade study but do not compete favorably with the lighter than air vehicles in terms of overall scientific productivity for long duration flight. For highly targeted science, some of these platforms may have a role. Platforms which can operate close to the surface have also been considered but would require high temperature technologies for implementation.

4.6 Venus Surface Sample ReturnA long-standing objective across all of planetary exploration is to return samples of the solid surface of a planet and the Roadmap considered how progress might be made towards surface sample return from Venus. Venus Surface Sample Return (VSSR) was included as a Far Term objective in the 2014 Roadmap. However, that document did not carefully consider the feasibility of the VSSR mission. Here we have


taken a critical look at VSSR drawing on experience with the Mars Surface Sample Return (MSSR) mission.

Compared to sample return from the Moon, Mercury and Mars and even for the moons of the outer planets, VSSR is a formidable goal. There are three aspects of the Venus environment that makes VSSR so challenging - the high gravity field (comparable to Earth), the dense atmosphere (90 bars at the surface) and the high surface temperature (460C).

The high gravity field means that even for launches that originate in tenuous reaches of the atmosphere, multi-stage Venus Ascent Vehicles (VAVs) comparable in capability with those for launching payloads into Earth orbit are needed

The high pressure (90 bars) means that buoyancy systems with two or more stages are needed to lift the sample from the surface to the upper atmosphere where the atmospheric density is low enough that launch of the VAV is feasible

The high surface temperatures means that if any equipment accompanies the sample in its ascent (VAV, sampling, buoyancy systems) must be protected from the near surface temperatures as well as pressures

A number of studies of VSSR were carried out in the late 1990s ( (Sweetser, 1999)and the early years of this century. In all of these concepts, the sample capsule launched by the VAV performs a rendezvous in Venus orbit with an orbital spacecraft. Then, the orbital spacecraft departs from Venus orbit on an Earth return trajectory. This approach is very similar to the current Mars Surface Sample Return (MSSR) architecture. However, the process of getting the sample to orbit is much more complex. In one approach, the VAV descends to the surface of Venus; an alternative is for the buoyancy system carrying the sample to rendezvous with the VAV in the high atmosphere. Regardless of the VSSR architecture ultimately selected, it will be more complex than that of MSSR and the individual architectural elements such as the VAV and the buoyant stage will much more technically challenging.

The last Planetary Science Decadal Survey recognized that MSSR required a sequence of at least three launches of mission elements, stretching out over more than one decade. Since VSSR is much more complex, requires more mission elements and extremely difficult technologies, implementation of such a mission would extend far beyond the timeframe of this roadmap. As a result, we are not including VSSR in this roadmap. However, in section 6.3 Error: Reference source not found, we will consider possible VSSR precursor missions that can be conducted within the time frame considered.

4.7 Surface or Near Surface Platform with Regional Mobility A platform capable of conducting surveys of the surface was specified in the 2014 Roadmap for Venus Exploration (RVE2014). The companion 2014 Technology plan provides both a brief description of near surface and surface vehicles as well as the technologies required for their realization. For most of the technologies, maturity was deemed to be very low. It is useful here to review the principal challenges.

Mobility appears to be the least of the challenges. Buoyant vehicles have been envisage that can plausibly traverse hundreds of kilometers drifting in the low level Venus winds such as the Venus Mobile Explorer (VME) which was studied in the last Decadal survey. The difficulty is generating power in the Venus environment and in particular sufficient power for cooling components that cannot be implemented as high temperature components.


Although this mission concept does not have the complexity of VSSR, it is nevertheless a very challenging mission. Like VSSR it is also a candidate for a precursor mission within the time frame of the Roadmap and in Section 6.3 we consider how such a selection might be made.


5 Mapping to Goals-Objectives-Investigations The experimental platforms described in sections 4 of this Venus Exploration Roadmap were identified based on assessment of their scientific potential, technology readiness, and programmatic considerations including a logical feed-forward of science and technology capability. In this section, we describe how these platforms address the Venus exploration scientific strategy described in the companion Goals, Objectives and Investigations document (VEXAG_GOI_Focus_Group, 2018).

5.1 Format of the GOI-Platform assessments tablesFor each of the three goals, we have generated a table in the form of a matrix that described role of measurements made from a platform in addressing each of the 27 investigations contained within the GOI. The table for Goal One (Table 5-2) can be used to illustrate the specifics of the format. Along the vertical axis, short hand descriptions of the goals, objectives and investigations are given along with the code number and rating of each investigation. Roadmap platforms are grouped by class horizontally, following the scheme used in Section 4. Directly beneath the shorthand description of each platform type is the earliest time-frame when this platform would be ready for deployment.

The contribution of each platform to the successful completion of each investigation is indicated by the color of the cell at the intersection of an investigation and a platform (e.g. Table 5-2). Measurements made from the platform can be either:

Comprehensive – Providing the measurements necessary and sufficient for the investigations (designated in blue)

Significant – Providing measurements that are necessary but not sufficient in themselves to complete the investigations (designated in green)

Supporting – Enabling measurements by other platforms to carry our needed measurements successful (designated in yellow).

For some investigations in the GOI, measurements a single platform can provide a Comprehensive answer. Where there are alternative platforms solutions for implementing the investigation, as is the case for I.A.1, the cell is labeled with the letter “A” (Alternative).

Many of the GOI investigations require measurements from more than one platform to provide a Comprehensive answer and in these cases the platforms necessary to accomplish this are denoted as Significant and coded with the letter “C” for complementary to indicate the platforms needed to complete the investigations.

In some cases, complementary measurements must be made sequentially in order that the results from one can be included in the design and deployment details for the next platform. For other investigations, they must be synchronous with one another to be useful.. There has been no attempt to draw this distinction in the tables but it is explained in the text accompanying the tables.

The final column in the table provides an overall assessment if measurements are made from all of the platforms that are identified that are not duplicative. For most investigations a comprehensive solution is deemed feasible.


5.2 Goal One – Evolution, Habitability, and Exoplanets

Table 5-2 describes the ability of Roadmap platforms to address the ten investigation that comprise Goal One. In this section, we discuss the contributions that made by each of these investigations.

Table 5-2 Goal One – Assessment of ability of Roadmap Platforms to address GOI Investigations

5.2.1 Objective A – Why did Venus evolve as it did?

There are five investigations required to address this objective. Here we describe how Roadmap platforms contribute to each investigation

Investigation I.A.1 Isotopic ratios of noble gases

Determine the isotopic ratios and abundances of D/H, noble gases, and other elements in Venus’ atmosphere to constrain its planetary accretion, atmospheric evolution, and the possibility of ancient

habitability

This approach requires measurements with mass spectrometers and six alternative platforms are identified in Table 5-2 that can be applied in this type of investigation. Each of them has strengths and weaknesses and a comparative assessment is beyond the scope of this roadmap. Each has been designated as providing a substantial solution to the investigation.


What were Venus’ initial and early states (including its inventory of volatiles including water), and how, when & why did it evolve as it did?

Understand Venus’ early evolution (including possible habitability), and the evolutionary paths of Earth-sized terrestrial (exoplanets.

Skimmer measurements: A skimmer platform collects gases from the upper atmosphere beneath the homopause in a single pass or potentially multiple passes. Analysis takes place after the skimmer vehicle leaves the atmosphere allowing very precise measurements of trace constituents. The weaknesses are that potential fractionation of gases occur during hypervelocity collection and gas compositions near and just below the homopause may not be representative of bulk composition. The strength of the technique is that it can be implemented with a mission without the need for high energy entry technology and it enables very precise measurements because the analysis time is not limited by the survival time in the Venus environment and can continue for days and weeks.

Descent Probe: Measurements from a descent probe, which is descending at a rate of a few meters per second, are not subject to hypervelocity fractionation and the samples are representative of bulk composition. However, with the limited analysis time available with a descent probe, the determination of certain trace species may be limited in precision.

Surface Platform: During the descent phase of a surface platform, measurements would be made in analogous fashion to those from a descent probe. The same issues arise with respect to the precision of the analyses.

Aerial Platform: With a lifetime approximately 1000 times the descent probe, a primary advantage of the aerial platform for these measurements is the ability to integrate over many samples in the mass spectrometer and achieve very high sensitivity. Any of the three aerial platforms considered could make this measurement.

In summary, a substantial solution to this investigation may be feasible with either of these platforms, although the specific trades are beyond the scope of this report.

Investigation I.A.2 Evidence for water derived rocks

Determine whether Venus shows evidence for abundant silicic (granitic) igneous rocks and/or ancient sedimentary rocks (including carbonates), as markers for abundant liquid water in its past.

The ultimate purpose of this investigation cannot be attained with either a single platform or a mission. A sequence of missions including orbital, aerial and surface platforms is needed in order to collect the necessary data.

Orbiter Surface and Interior: The first step is to characterize the surface with infrared imaging data in order to identify regions that potentially include granitic and or ancient sedimentary rocks. The spatial resolution that is achievable from orbit is about 50 km. Sedimentary rocks may be identifiable from morphological data. Either the global or targeted missions could accomplish this.

Aerial Platform and Targeted Sonde: Data from an aerial and sonde mission can provide more promising definition of potential regions of granitic rocks than is possible from orbit. The Aerial platform operating beneath the clouds can provide infrared data with resolution better than 10 m for targeted areas. Sondes could acquire data of even higher resolution but would require a guidance capability.

Advanced Surface Platform: Definitive proof of silicic and sedimentary rocks will require in situ compositional and imaging data. The platform will require advanced Terrain Relative Navigation (TRN) in order to reach areas identified in orbital and aerial imaging data (Hunter & VEXAG Technology Focus Group, 2018). While the ability to reach a single predesignated target would be preferable, it may be adequate to provide the capability to select from a set of potential targets during descent.


Investigations by each of these platforms are deemed as making substantial contributions. Collectively, they provide a comprehensive solution to the investigation.

Investigation I.A.3 - Evolution of the Venus Atmosphere

Characterize the processes by which the atmosphere of Venus has evolved (gains, losses, & changes) over time, including effects of magnetic fields, and incident ions and electrons.

Instruments on the Venus Express orbiter (ASPERA and MAG) have already been used to characterization of current atmospheric loss processes on Venus. For instance, the MAG instrument made the first ever measurements of atmospheric loss of the planet’s day side, and ASPERA has shown that oxygen and hydrogen are lost from the Venusian night side. Better quantification of these loss rates is now needed us to improve our understanding of the initial inventory of key gases like water vapor. An orbiter mission, optimized for remote and in situ observations of the atmosphere and ionosphere, is the appropriate vehicle for undertaking this task.

Orbiter Atmosphere and Ionosphere: The needs here are for better understanding of Venus’ magnetic field (including reconnection) under a range of solar conditions, better analyses of ion and neutral fluxes (incoming from the sun and outgoing from the atmosphere), and the modes of energy transfer from incoming solar radiation and particles to form hot ions. Better quantification of these loss rates, coupled with better quantification of current isotopic ratios will help us to improve our understanding of the initial inventory of key gases like water vapor.

Measurements from a single spacecraft with a comprehensive instrument set or one or more smallsats as described in the Venus Bridge study (Grimm & Gilmore, 2018)) are deemed as providing comprehensive alternative solutions to this investigation.

Investigation I.A.4 - Structure of Mantle and Core

Determine the structure and thermal state of Venus’ mantle, and the size and physical state of the core to place constraints on its accretion and early differentiation.

Orbiter Surface and Interior: Determining the size of the core and whether it remains at least partially liquid today requires better constraints on the tidal Love number (k2) and the moment of inertia of Venus. Doppler tracking of an orbiter such as VERITAS and/or EnVision could reduce the error on k2 from the current ~20% to the ~3% level required to discriminate between models with a solid versus at least partially liquid core. Changes in the precession rate and length-of-day for Venus that depend on its moment of inertia may produce detectable signatures in the orbital perturbations of an orbiting spacecraft.

Long-Lived Surface Platform: True Polar Wander may occur more rapidly and easily on Venus compared to Earth because relatively slow rotation provides little inhibition to solid-body rotation with respect to the spin axis. Predicted rates of motion driven by mass redistribution from mantle dynamics are on the order of a meter per year, requiring precise location of a surface station from space. Seismic investigations could also detect chemical heterogeneity in the lower mantle and core (analogous to the D’’ and E’ layers near Earth’s core/mantle boundary) that may be ancient remnants of chemical interactions and magma ocean solidification that occurred soon after planetary accretion and differentiation.

Ground-Based Investigations: In parallel with spacecraft missions, ground-based observations may enable substantial progress towards achieving this investigation. Simultaneous observations of radar echoes with the Goldstone Solar System Radar and the Green Bank Telescope allow measurement of the instantaneous spin period of Venus, which was not achieved with either Magellan or Venus Express.


Tracking the instantaneous length-of-day over years and decades place unique constraints on the mantle and core size and structure.

There are some issues here on the ability of Venus seismology to determine information on the mantle and core. Do we have any Road Mapper willing to undertake this? I

Investigation I.A.5 - Search for remanent magnetism

Search for remanent magnetism in Venus’ surface rocks, to constrain past tectonic regimes, composition of the core, and to climate regimes related to atmospheric loss processes.

Despite Venus’ high surface temperature, remanent magnetism could be preserved in the minerals magnetite and hematite, because their Curie temperatures are far higher (585°C and 680°C respectively). Any detection of crustal remanent magnetism would prove that Venus, like Earth, accreted under high-energy conditions that melted and homogenized its core. Furthermore, crustal remanence would indicate that the core once cooled fast enough to convect and that the surface was not much hotter in the recent past

Orbital Platform: Orbital measurements taken to date have not revealed either a permanent or remanent magnetic field on Venus. Future orbital measurements are limited in spatial resolution by how close it is possible for an orbiter to approach Venus, which is limited by atmospheric drag and by interference by magnetic fields in the solar wind. However, orbital measurements could be an important supporting measurement to provide data on the external field environment.

Aerial Platform: A fixed altitude aerial platform operating at 55 km in the atmosphere can carry out a search for remanent magnetism along the ground tracks accessible in the nominal 100 day mission. In a 100 day, an aerial platform would measure remanent fields along a profile corresponding to approximately 18 circumnavigations of Venus with a spatial resolution of approximately 50 km. Regions of magnetized crust with thicknesses of at least ~3–10 km would produce magnetic anomalies >40 nT, which exceed a conservative noise floor for magnetometer measurements at ~50 km altitude. These measurements can provide a comprehensive solution to the investigations as framed.

Surface Platform: Although magnetic measurements obtained from a surface platform would be sensitive to anomalies of small spatial extent, they would be limited to a single location on the planet Accordingly, surface platform magnetic measurements are seen as playing a supporting role.

Collectively, the measurements from three platform types will provide a comprehensive solution. While the orbital and landed measurements are not strictly necessary for the success of the investigation, they would provide important supporting data.

5.2.2 Objective B – Implications of Venus Evolution for Exoplanets

Investigation I.B.1 Volcano Tectonic Features

Investigate Venus’ unique volcano-tectonic surface features (e.g., coronae, tesserae, rigid block & mobile belts) at improved spatial and topographic resolution, to determine which physical parameters allowed

them to develop on Venus, and when in Venus’ geologic history these conditions were present.

Orbiter – Surface and Interior: The measurements that serve this Investigation are radar-related – no other technique can provide global and regional information about Venus’ surface at the required length


How do Venus’s current state and its evolution (in comparison with those of Earth and other terrestrial planets), inform us about planetary evolution paths in general and possible current states, including those of exoplanets in other stellar environments?

and height scales. For global coverage, orbital assets are required, and suitable radar orbiters have been proposed several times over the last decade. The orbital radar must operate in SAR mode to provide the requisite spatial resolution.. It would also be important to transmit and receive dual polarizations, as is done so effectively by Aricebo and other planetary radars. This investigation is most effectively implemented with an initial mission with the global survey platform and a later mission capable ofregional and local investigations at higher resolution.

Aerial Platform: Radar could also be very effective from a balloon platform – being much closer to Venus’ surface and thus with higher spatial resolution. In addition to SAR mode, it might be possible to utilize “Side Looking Airborne Radar” (SLAR) modes from a balloon. Include Surface imaging Sonde deployment for a limited number of features at ultrahigh resolution. This is deemed a support measurement

Either the Global or targeted mission can provide a substantial solution to this investigation. However, this does not mean they are strictly alternatives and they provide complementary information on the investigation. The aerial platform would be strictly in a supporting role.

Investigation I.B.2 Crust and Upper Mantle

Determine the current physical and thermal states of Venus' crust and upper mantle (e.g., mobility, spatial variability) to constrain past processes (e.g., crustal overturn) and for comparison to other planets.

Data to address this Investigation would come from indirect and direct geophysical measurements

Orbiter – Surface and Interior: A high-resolution gravity field, such as might be obtained with a GRAIL-like orbiter mission, would provide crucial information about Venus’ crust and upper mantle. Models of physical and thermal states would be tested against the observed gravity signature (necessarily augmented by high-resolution altimetry data). The thermal states of Venus’ interior can be inferred (to some extent) from gravity-derived densities, but would also be inferred from maps/distributions of surface temperature. Surface temperatures can be inferred from measurement of electromagnetic emission (Planck emission) and estimates of emissivities. It may be possible to detect thermal anomalies which would have their roots in crustal and upper mantle processes. As an indirect measurement, this is characterized as supporting.

Orbiter – Surface and Interior: Radar sounding is another approach for exploring Venus’ interior structure. The radar sounding developed for the EnVision mission proposal (Ghail, 2018) is predicted to reach depths of less than 1 km; the actual sounding depth is unclear, given the constraints of the ionosphere and the crustal conductivity at the high Venus surface temperatures. Accordingly, radar sounding is likely to be more suited to investigating sedimentary and volcanic deposits than deep crustal structure. This measurement is determined to be a substantial solution to investigating stratigraphy.

Orbiter Atmosphere and ionosphere: The most effective way of directly exploring spatial variability in the thickness of the crust is by monitoring the optical signatures in the upper atmosphere and ionosphere resulting from large quakes ( (Garcia, Lognonne, & Bonnin, 2005)(e.g., Garcia et al., 2005). The velocity of the Rayleigh waves indicate crustal properties and deviations from radial symmetry reflect spatial variability in the crust. In principle, this can be conducted from either type of orbiter in this category although a dedicated SmallSat orbiter in an optimal orbit would be the best solution. This approach is deemed as a substantial solution to characterize the crust and upper mantle.

Aerial Platform: .Electromagnetic sounding, exploiting the Schumann resonance energized by lightning in the Venus atmosphere, can determine the conductivity of the subsurface and the temperature profile. A promising technique in development is the detection of seismic activity using infrasound, which is enabled by the high atmospheric density and hence strong coupling to surface motions, which would be complementary to the orbital observations.


Surface Platform Advanced: Seismic measurements, tracking body and surface waves from Venus quakes could, in principle, provide detailed maps of the Venus subsurface. However, a seismic network, with or without active sources, is a distant goal.

Orbital measurements of seismic events alone can make a substantial contribution to addressing this question. In addition, there are other orbital and in situ measurements which add many other dimensions to characterizing the crust and upper mantle. Even a subset of these measurements would constitute a comprehensive solution to this investigation.

Investigation I.B.3 Plate Tectonics vs Mobile Lid

Determine if Venus transitioned over time among multiple tectonic regimes reflecting mantle processes), e.g., mobile, stagnant or episodic lid models, and/or other modes of mantle cooling (e.g., heat pipe,

squishy lid), and what physical parameters govern(ed) such global transitions.

Implementing this investigation will required a multifaceted endeavor including studies of global radar images, targeted radar data of higher resolution and high resolution visual imaging of targeted sites. Coupled with this, we need a variety of geophysical measurements to choose between models of the

Implementation Approach: The Focus Group is still working on this.

Investigation I.B.4 – Heat Producing and Volatile Elements

Determine Venus’ abundances and distributions of heat-producing and volatile elements (in crust, mantle, core), and how abundances and distributions influence differentiation, crust formation and rheology on

Venus and other planets.

These elemental abundances can only be determined from landed assets. Venus’ atmosphere is too thick for orbital remote sensing of gamma ray emission (as in the MRO GRS instrument) or X-ray emissions. Precursor orbital measurements may be useful in guiding the choice of representative terrains to be studies e.g. plains vs tessera.

Surface Platform: Abundances of heat-producing elements and some volatile elements might be detected by their emitted or induced gamma rays (as did the Venera and VEGA spacecraft); K is likely abundant enough, as are many of the ‘volatile’ elements, to be detected in x-ray fluorescence or LIBS analyses.

Advanced Surface Platform: Determination of surface heat flow would help understand if the surface abundance of heat producing elements is representative of Venus bulk composition. Measurement of heat flow with sufficient precision would require an advanced lander with the ability to drill holes to a depth of several meters and then monitor temperatures in the hole over a period of at least one Venus day. This appears to be feasible according to the findings of the Venus Surface Platform study (Amato & Kremic, 2019).

Investigation I.B.5 – Exoplanet Observables vs Venus

Determine the atmospheric properties of Venus that could be observable remotely by current and planned exoplanetary telescopes to constrain whether Venus (and Earth) represent typical or atypical

states in planetary evolution.

Investigations of exoplanets will be limited to remote sensing observations of their atmospheres. Accordingly the only platforms that are relevant to this objective are those that perform similar objectives for Venus.

Orbiter Atmospheric and Ionosphere: Observations of exoplanet Venus-analogs will be aided by measurements of Venus atmospheric D/H ratios and noble gases (which point to evolutionary processes


that may also occur on exoplanets (Investigation I.1.A). Observations of atmospheric loss processes that may occur to a more extreme degree on exo-Venus planets, and better constraints on Venusian catalytic photochemistry that affects atmospheric composition will be of most interest.

5.2.3 Goal I Overall AssessmentThe final column in Table 5-2, indicates the contribution that a combination of measurements made by all the platforms can make to the investigation. This does not mean that measurements from all the platforms are needed, since some platforms provide alternative solutions. For Goal I, comprehensive solutions to all of the investigations except one are possible. For a number of investigations, comprehensive solutions would still be possible with the use of a subset of the platforms.

5.3 Goal Two - Atmospheric Dynamics, Composition and Climate

History Table 5-3 describes the two objectives and nine investigations defined to address this goal.

Table 5-3 Goal Two - Assessment of Roadmap Platforms for addressing GOI Investigations

5.3.1 Objective A. Global Atmospheric Dynamics

There are five investigations required to address this objective. Here we describe how the different platforms contribute to each investigation

Investigation II. A. I - Lower atmosphere below 75 km

Characterize the dynamics of Venus’ lower atmosphere (below about 75km), including: the nature of the retrograde zonal super-rotation, the magnitude of the meridional circulation, radiative balances, generation of mountain waves, and interactions at Venus’ surface that affect the planet’s rotation rate.

This investigation focuses on characterizing the current state of the deep atmosphere of Venus, semi-arbitrarily defined as that portion of the atmosphere lying beneath the cloud tops around about 70 km. The reason for this is both theoretical and practical. At this altitude, the zonal wind speed reaches a maximum, before beginning to transition to the Subsolar-Antisolar flow. This altitude also exhibits a minimum in the vertical temperature profile. Each of these observations suggests a significant transition


Understand atmospheric dynamics, composition and climate history on Venus

What processes drive the global atmospheric dynamics of Venus?

occurring at or near these altitudes. In a practical sense, 70 km represents an altitude above which it is difficult to obtain long-term and repeated measurement of atmospheric properties from an in situ platform. Above this altitude, the primary means of data acquisition must likely come from an orbital platform; while below this altitude, capability of measurement is expected to be possible from both in situ and remote means.

This investigation can benefit from coordinated and contemporaneous in situ measurements with an aerial platform and orbital observations. Such a two-component investigation would require precise tracking and positional data for the aerial platform(s) so that the in situ measurements made can be placed in global and regional context.

Orbiter – Atmosphere and Ionosphere: Observations from the orbiter would include measurements of motions of atmospheric constituents at a variety of altitudes, utilizing the complexity of the Venusian emission and reflection spectra, similar to the manner in which Akatsuki currently operates. Radio occultation measurements from a constellation of CubeSats and SmallSats may also be used to give an independent measurements of the thermal profiles that drive the general circulation.

Aerial Platform: Measurements from the aerial platform include tracking the platform from Earth and from the orbiter. Additional tracking data from CubeSats deployed from the orbiter could provide coverage at times when the platform is in view from Earth and when ground-based antennas are not available. Measurements on the platform to provide upward and downward welling radiance are needed because energy deposition within the cloud layers is an important driver for the general circulation and are needed for the models.

An orbiting platform has the capability, pending data management capabilities, of making a comprehensive contribution to this investigation, especially when coupled with a substantial amount of data assimilation of the observational data into general circulation models. An Atmospheric Platform will make substantial contributions by itself.

Investigation II.A.2 – Upper Atmosphere and Thermosphere

In Venus’ upper atmosphere and thermosphere, determine the role of solar-atmosphere interactions by characterizing global atmospheric dynamics and the space environment, including the effects of the space weather (fields and charged particles) environment. The ionosphere and upper thermosphere of Venus is a highly dynamic region, influenced by the retrograde zonal super rotation, subsolar to antisolar flow, and the solar wind. Venus' thick atmosphere, lack of an intrinsic magnetic field, and close proximity to the Sun provides a unique laboratory for the study of solar-atmosphere interactions. Without a magnetic field to protect the planet, the solar wind is able to penetrate the upper atmosphere, interacting with the atmosphere in a multitude of ways. These interactions can appear over very short timescales, coinciding with major solar events, and an be studied through a variety of techniques and observations. These studies, while critical for understanding atmospheric processes on Venus, also improves our understanding of exoplanets.

Observational Approaches: To address this investigation, measurements are needed of hot outflowing ions an neutrals (escaping atmosphere), auroral emission (ionospheric emission), and nightglow emission (upper thermosphere) as well as solar wind conditions in order to understand processes driving these events. Remote sensing observations, using a variety of imaging and spectroscopic instruments, could be made with one or more orbiter spacecraft. Observations at different spatial scales willallow investigations of small-scale and planet-scale processes. Some of these synoptic observations are complementary with observations of the infrasound signatures of Venus quakes discussed later under Investigation III.A.3.


Orbiter - Atmosphere and Ionosphere: Particle and field measurements of both the space weather environment upstream of Venus as well as within the Venusian environment are desirable as it would allow for the direct connection between solar wind properties and Venusian atmospheric response. Instruments include, but are not limited to, magnetometer, Langmuir probe, electron spectrometer, ion spectrometer, and SEP detector. Detecting and isolating times of solar storm impacts and comparing them to atmospheric responses (including outflows of ions and electrons, auroral and nightglow emission, and electron temperature and density measurement) will place better constraints on properties and loss processes of the Venusian atmosphere. For many of these observations, simultaneous measurements at multiple locations within the space weather environment, although with a more limited set of instruments, provides an alternative approach (e.g. smallsats).

Overall Assessment: Measurements from a single spacecraft with an extensive instrument set or one or more smallsats as described in the Venus Bridge study (Grimm & Gilmore, 2018)) are deemed as providing comprehensive alternative solutions to this investigation.

Investigation II.A.3 - Role of Mesospheric Dynamics

Determine the role of the modes of mesoscale dynamics in redistributing energy and momentum throughout the four-dimensional Venus atmosphere system.

Mesoscale processes, including the behavior and evolution of convective cells, horizontal and vertical wave propagation, and other structures, can be of small enough scale, whether spatially or temporally, to be an impractical target for an investigation focused on characterizing global scale processes. Nevertheless, such features and processes can be observed from orbit, as demonstrated by the discovery of numerous mesoscale features in both Venus Express and Akatsuki data. Moreover, direct in situ measurement of the local dynamics of isolated convective structures and/or wave propagation as demonstrated with the VeGa mission. However, as the VeGa mission demonstrated, without global or regional contextual information, spatio-temporal degeneracies will remain. Accordingly, to address this objective, we require simultaneous in situ measurements from an aerial platform and remote sensing measurements from an orbital platform.

Orbiter – Atmosphere and ionosphere: The most important data types here are imaging data at a range of wavelengths that probe to different depths within the cloud layers accessible to the aerial platform.

Aerial Platform: The critical data here are measurements of the dynamics of the aerial platform at a variety of scales accomplished by tracking the platform from Earth and from the orbiter. Additional tracking data from CubeSats deployed from the orbiter could provide coverage at times when the platform is of view from Earth and when ground-based antennas are not available. Inertial measurement units (IMUs) on the platform would be used to determine small scale dynamics including accelerations and rotations and motion relative to the atmosphere. Observations should be obtained at different latitudes, times of day and over regions of varying topography. Observations from a fixed altitude platform would be valuable but the ability to make observations at different altitudes would be transformative.

Orbiter and Atmospheric Platform make substantial contributions when taken separately. Taken together in simultaneous observations they make a comprehensive contribution to the investigation.

5.3.2 Objective B Atmospheric Composition and Radiative BalanceWhat processes determine the baseline and variations in Venus atmospheric composition and global and local radiative balance?


There are formally five investigations under this objective. However, one of them is so complex we have subdivided it into two parts so that we can effectively address it.

Investigation II.B.1. Atmospheric Radiative balance and physical, chemical and biological interactions

Characterize Venus’ atmospheric radiative balance, and the nature of the physical, chemical, and possible biological interactions among the constituents of the Venus atmosphere, the associated radiative

interactions, and the atmospheric dynamics

This investigation has been subdivided into two components. This is the first of these:

Sub Investigation II.B.1.a - Venus Atmospheric radiative balance and atmospheric dynamics

Measurement of the atmospheric radiative balance requires in situ measurements of the upward and downward flux of radiation in several spectral bands with a net flux radiometer coupled with measurements of the atmospheric dynamics using the methods described in Investigation II.A.1. We recommend consolidating the measurement of net flux into that objective.

Orbiter Atmospheric and ionosphere: Synoptic orbital observations using imaging systems at a range of wavelengths is necessary to provide a regional and global context for in situ observations. In addition, it is important to be able to locate the in situ observations in relation to the cloud features observed from orbit with an accuracy commensurate with the feature size of cloud features.

Aerial Platform: In order to develop a complete picture of the radiative balance, it will be necessary to measure the net flux at different latitudes, time of day and as a function of altitude. The variable altitude balloon with a range of 10 km (50 to 60km) will enable substantial progress. The enhance platform with a range of 20 km will enable the base of the clouds to be accessed.

Probe and Short Duration Surface Platform: These platforms enable radiative balance and wind velocity to be measure for one profile but all the way to the surface. These data complete similar measurements made on earlier missions and play a supporting role. Multiple sondes which could sample several times of day and latitudes may make a larger contribution.

Long Duration Lander: These measurements, extending through an entire Venus day at one location, can also play a substantial role in characterizing the radiation throughout the day or if such a measurement is not possible, the surface temperature.

Assessment: Collectively measurements at even a subset of the platforms identified can make a comprehensive contribution to this investigation.

Sub Investigation II.B.1.b - Nature of physical, chemical and possible biological interactions among atmospheric constituents

The sulfur-carbon-based chemistry ongoing in the Venus atmosphere is the most complex and highly coupled system that is known, after the organic chemistry of the Titan atmosphere. Not only does it involve multiple cycles of exchange of oxygen among water vapor, sulfur-based, and carbon-based molecules, but it also involves condensation of the sulfuric acid clouds, and the aqueous chemistry that necessarily results. Several of the players in this chemical system also have substantial roles in determining the radiative balance of the Venus atmosphere (greenhouse gasses and albedo-enhancing aerosols). Adding further to the complexity and intrigue is the occasionally floated possibility of microbial life persisting in the habitable environment within the clouds of Venus.Atmospheric and ionosphere:


Spectroscopy or Hyperspectral imaging of the atmosphere, coupled with sophisticated radiative transfer models and chemical equilibrium models involving microphysics can potentially address this investigation. However, returning the volume of data required to do so can be prohibitive. Furthermore, until the advent of sophisticated “big data” methods to handle such enormous data products and their connection to highly coupled physical systems, even the data analysis may be prohibitive for the near term.

Aerial Platform: From a chemistry standpoint, other than perhaps the near surface environment, where supercritical chemistry must be taken into account, the most complex (hence the most interesting) region of the atmosphere of Venus is the cloud layer, where multi-phase physical chemistry must be considered for potentially several species (sulfuric acid, sulfur allotropes, several others have been postulated). The Aerial platforms considered here all are well-suited to making the in situ observations necessary to characterize the physical chemistry of this region, by assaying the changes in the composition of trace, chemically active, atmospheric species, and potentially identifying biological activity.

Probe and Short Duration Surface Platform: These platforms enable trace species to be measured for one profile but all the way to the surface. These data complete similar measurements made on earlier missions and play a supporting role. Multiple sondes which could sample several times of day and latitudes may make a larger contribution.

Long Duration Lander: N/A

Assessment: Both an Orbital Platform and an Aerial Platform within the clouds can make substantial contributions to this Investigation. Coupled and coordinated Orbital and Aerial Platforms can make a Comprehensive contribution.

Sub Investigation II.B.2 – Physical and Chemical Nature of Aerosols

Determine the physical characteristics and chemical compositions of aerosols in Venus atmosphere as they vary with elevation, including discrimination of aerosol types/components

The Venusian aerosols have a major impact on the Venus greenhouse effect and its remotely observable properties. While the primary constituent of the upper clouds has long been known to be spherical particles of highly concentrated sulfuric acid with typical radii of one micron, the exact nature of the Venusian aerosols is incompletely known. In particular, the UV absorption properties, which appear to be associated with aerosols in the upper cloud, have received many different explanations none of which are unique. Laser based counters of individual particles such as LOAC (Light Optical Aerosol Counter can be used to characterize the size and the albedo but can provide no information on the composition. Determining the composition requires either a filter or an aerodynamic separator to remove the aerosol particles from the gas and a means of analyzing the chemical composition of particles themselves with a mass spectrometer or other analysis approach.

Aerial Platform: Measurements from an aerial platform provides the time necessary to perform the analysis. Measurements over the diurnal cycle, which can be completed in six days in the superrotating flow at 55 km altitude, and altitude excursions can be used to develop a more complete description of the aerosol spatial and temporal behavior. In addition, the analysis of the composition of aerosols,


which will take many hours of analysis to complete because of the low concentrations, is enabled by the extended lifetime of an aerial platform.

Atmospheric Probe: the only existing data on aerosol properties were obtained with a Particle Size Spectrometer on the large Pioneer Venus probe, and nephelometers on the small Pioneer Venus probes. A repeat of this measurement with more sensitive instruments could help characterize the structure of the cloud layer along one profile. It would be particularly important to characterize the aerosol populations at the cloud base which are quite uncertain and not only affect the dynamics of the atmosphere but also the ability to image the surface from the cloud base.

Orbiter – Atmospheric and Ionosphere: While an orbiter is not necessary for addressing the investigation as defined, it could provide a useful support role.

Sub Investigation II.B.3 – Ultraviolet absorber investigation

Determine the identity of the unknown shortwave absorber in Venus’ upper atmosphere, and the nature and magnitude of its influence on both local and global environments.

Short-wavelength visible and near-ultraviolet light is unaccountably absorbed in Venus’ upper atmosphere. The effects of this unknown absorber are strongest in the near-ultraviolet, but are apparent well into the wavelengths of visible light. The unknown absorber varies in strength over space and over a wide range of timescales. Its importance, besides that of an enduring curiosity, is that it controls much of the deposition of solar insolation into the atmosphere, and thus the thermal structure of the atmosphere. It is possible that the unknown absorber is the molecule OSSO and its isomers, that iron chlorides are important absorbers, or even that the unknown absorber in the Venus atmosphere is a product of biological processes. The candidate processes involves either gases or aerosols or some combination and a definitive measurement would require measurements in situ with a suitable instrument. The approach can involve some combination of the techniques used to address investigations II.B.1 and II.B.3.

Aerial Platform: Measurements on the platform should be performed over as wide an altitude range as can be achieved since the occurrence of the UV absorber has been inferred to vary significantly with altitude. The ability to separately measure the composition of the gas and the aerosols would be highly beneficial. Use of an aerodynamic separator rather than a mechanical filter can enable many repetitive operations. Altitude control will enable the platform to move between air masses containing different amounts of the UV absorber.

Orbiter – Atmosphere and Ionosphere: Since the UV absorber has been identified in spectral imaging and spectroscopic data, complementary remote sensing observations are required in which the aerial platform sampling location can be identified with respect to the locations of more intense UV absorption. Improved spectroscopic techniques may help strengthen the identifications of both gas and aerosol species made in situ. In addition, correlating in situ and orbital measurements will help refine estimates of absorption as a function of altitude, latitude and time of day.

Investigation II.B.4 – Surface Atmosphere Chemical Interaction

Assess Venus’ surface-atmosphere chemical interactions by determining the composition of, and chemical gradients in, Venus' atmosphere from the ground surface up to the cloud base, both at selected locations and in global perspective


Venus’ rock and its atmosphere intersect at the surface, where temperatures of ~470°C and pressures ~90 bars virtually ensure geologically rapid chemical reactions. These reactions will impart significant changes to both atmosphere and surface: effects on the surface are considered in Investigations IIIB3 and IIIB4.

Away from sites of active volcanic outgassing (see IIB2), Venus’ rocky surface should interact chemically with the ambient atmosphere, and affect it in several known or suspected ways.

Production of reduced gas species, like CO from CO2, and SO2 or S2 from SO3. Reaction of SO3 with Ca-bearing silicates to form CaSO4, anhydrite, thus reducing the proportion

of atmospheric sulfate. Exchange of atmospheric halogens with the surface, perhaps reducing the Cl/F ratio by

formation of Cl-bearing phosphate phases. Decomposition of rocks containing hydroxyl-bearing igneous minerals (like amphibole or

biotite), should release hydrogen (with D/H values of the interior) to the atmosphere.

To determine gradients in atmospheric chemistry on a global scale, to a greater fidelity than is now known, will require detailed spectroscopic observations and interpretations. Now, the chemistry of the deep atmosphere is known mostly from modeling of high-resolution infrared spectra taken from Earth, specifically modeling the pressure broadening of specific spectral lines of compounds of interest. Acquiring better global spectra will be useful, but especially in the context of continued improvement in modeling the effects of pressure on the specific line widths and strengths.

Determining regional variations in near surface gradients would be enabled by orbital or balloon platforms carrying high-spectral-resolution spectrometers, so that one could apply the pressure effect models to specific locations – e.g., is Venus’ near-surface atmosphere at the equator the same as that over Maxwell Montes. Determining gradients at a local scale would be accomplished best by probe platforms, with suitable mass spectrometers, designed to sample Venus’ lower atmosphere at frequently as the probe descended to the surface. Such measurements could then provide a ‘ground truth’ to help interpret spectra taken from balloon, orbit or Earth.

Assessment: This investigation is extremely challenging because no roadmap platform available in this time frame is able to make repeated measurements in the region from the cloud base to the surface or measurements over a protracted period. Accordingly, we consider that Roadmap platforms can make a substantial contribution but not a comprehensive answer to this investigation.

Investigation II.B.5 – Products and Rates of Outgassing

Determine the products of volcanic outgassing on Venus, and rats of outgassing to constrain its effects on atmospheric composition.

On Venus, transient and high concentrations of SO2 in the atmosphere and thermal anomalies on the surface have pointed to currently-active volcanism, supported by a host of volcanic surface features and a young cratering-based surface age of 500–800 Ma. The strategy for implementing this investigation would focus on monitoring the planet for signs of activity and supplementing those observations with spectroscopic investigations of gas plumes.

Orbiter – Surface and Interior: Measurements of infrared anomalies near 1 um have already been associated with potentially recent volcanic features but evidence for temporal variations is


controversial. This situation can be resolved by observations from this platform which would enable changes to be detected from overlapping coverage on several successive orbits hours apart as well as observations one half Venus day later when the ground tracks once again come into coincidence. These observations will be limited in spatial resolution to 50 km but the thermal discriminations are such that a hot spot a small fraction of this size could still be detected.

Orbiter- Synoptic: This class of orbiter can provide complementary observations of hot spots from a a high altitude. The advantage of this vantage point is the ability to monitor at all time scales from hours to months for evidence of change. This would also enable transient events arising in the atmosphere to be characterized. However, these measurements are also limited to a spatial resolution of 50 km.

Aerial Platform – Subcloud: A major advance in the ability to investigate possible volcanic events would be offered by an aerial platform with a spatial resolution of 10 m or better in the same infrared bands. However, this would not be useful as a global or even regional survey instrument because only a small portion of the planet can be observed and repeat observations outside the 30 minutes or so the event was visible from the platform would be infrequent or impractical. Nevertheless, the spatial resolution gain would be transformative and would be a necessary step in executing and follow up in situ measurements.

Probe or sonde – targeted: For determining the products of outgassing and even the rates of outgassing, measurements as close as possible to the source identified here with the largest hot spots would be needed otherwise emission may be so diluted by the atmosphere as to be unidentifiable. Concepts for targeted sondes for this kind of mission have been devised ( (Cutts, et al., 2014) and would be a natural step to completing this investigation.

Assessment: Given the spatially distributed and temporally variable nature of outgassing, it is difficult to conclude that an entirely satisfactory answer to this investigation can be completed and so the overall contribution is considered substantial but not comprehensive.


5.4 Goal Three - Surface (and Interior).

Table 5-4 describes the three objectives and ten investigations formulated to address this goal.

Table 5-4 Goal Three - Assessment of Roadmap Platforms for addressing Roadmap Objectives

5.4.1 Objective A - Current Geologic Processes

There are five investigations required to address this objective. Here we describe how Roadmap platforms contribute to each investigation

Investigation III.A.1 - Volcanic Tectonic Sedimentary Activity

Search for current volcanic, tectonic, and sedimentary activity on Venus, including: active deformations; eruptions and thermal anomalies; and sediment deposition and erosion. Compare current levels and rates of activity with evidence of that in the past, and evaluate from them current resurfacing rates.

Questions of current geological activity are best addressed by time-resolved observations, of which several types are possible. Another approach is to make observations of the seismic or infrasound signatures of tectonic and volcanic events.

Orbiter – Surface and Interior: Comparison of radar and infrared images of the surface taken at different times, e.g. comparing Magellan images with those of a mission 30 years later.

Comparing radar images taken from this mission with those from Magellan, with much poorer spatial resolution 30 years earlier

Comparing infrared and radar (with SAR interferometry) for images on successive passes about 90 minutes apart

Comparing infrared and radar images one Venus day apart when the orbital coverage begins to repeat.


Understand how physical and chemical processes interact to shape the surface of Venus

What geologic processes are currently acting on Venus?

The higher spatial resolution from this SAR, the more subtle the signs of activity that could be detected. Accordingly, the second-generation orbiter mission with targeted higher resolution coverage would build on the capability developed earlier. Infrared imagery is limited in spatial resolution by the atmosphere so there would be no gain there.

Aerial Platforms: Aerial platforms can be used to monitor infrasound which is generated efficiently on Venus by both tectonic and volcanic activity. Aerial platforms able to reach the cloud base can view thermal signatures of volcanic activity at high resolution but because of the limited coverage feasible the features would first have to be identified in orbital images.

Surface Platform: Surface morphology viewed with lander images, including eolian and volcanic features is one approach to identifying processes. Observations of temporal change would be pursued but will be of limited valued given the limited lifetime of imaging systems on surface platforms. If the advanced platform can be targeted to a location defined in the orbital images this would have major advantages.

Assessment: A comprehensive approach to this investigations would be provided by beginning with a search with orbital infrared data and using this to guide an investigation with an orbital platform including thermal and infrasound investigations. An advanced lander with precision targeting capability would enhance science return using images acquired during descent and after landing. Probes and sondes with surface imaging capability would provide useful support information but this would be of less value because they will not include the extreme close up imaging obtained by a lander.

Investigation III.A.2 – Mineral Composition – Oxidation State

Determine elemental chemistry, mineralogy, and rock types at localities representative of global geologic units to constrain the compositional diversity and origin of the crust.

Completing this investigation requires a series of measurements with different platforms. Sequential missions with orbital, aerial and surface platforms are needed in order to collect the necessary data.

Orbiter Surface and Interior: The first step is to characterize the surface with infrared imaging data in order to characterize global geologic units. The spatial resolution that is achievable from orbit is about 50 km.

Aerial Platform and Targeted Sonde: Data from an aerial and sonde mission can characterize the distribution of mineralogy and rock type with much greater spatial resolution than in possible from orbit. The Aerial Platform operating beneath the clouds (<40km) can provide infrared data with resolution better than 10 m for targeted areas. Targetted sondes could do even better but the number of sites would be limited to the number of sondes.

Surface Platform: Acquiring information on elemental chemistry and specific information on mineralogy and rock type will require measurements on the surface. In order to tie the specific locality sampled by the lander to the global and regional units defined by orbital and aerial platform observations it may be necessary to precisely locate the lander relative to the orbital and aerial maps. This will be practical for future landers using descent imaging

Assessment: Measurements from orbit, aerial and surface systems will provide a comprehensive response to the intent of this investigation.

Investigation III.A.3 – Structure of Crust

Determine the structure and thickness of Venus’ crust, in three dimensions, to constrain lithospheric structure and processes and crustal volume.


Investigations of the structure and thickness of Venus’ crust require complementary techniques with differing degrees of specificity. They include different approaches to the detection of seismic signals, electromagnetic probing of the crust, ground penetrating radar and gravity-altitmetry data.

Orbiter-Atmosphere-Ionosphere: The most powerful techniques for characterizing the structure and thickness of the crust is by imaging the infrared signature of seismic surface waves from orbit ( (Lognonne, 2018). Functionally, this observation is like a very large network of surface stations occupying an entire hemisphere of Venus. In principle, it should be possible to map crustal thickness variations radially for up to 1000km from the epicenter of a large Venus quake. The optimal orbital vantage point would be a dedicated smallsat mission in a high circular orbit. The technique is not affected by the background environmental noise that affects surface platform and balloon missions. However, it is ultimately limited by the shot noise in the optical signals to a Venus quake magnitude of > Mv -5.5.

Orbiter-Surface-Interior: Orbital measurements from a tight circular orbit can also help constrain crustal properties

Gravity and altimetry data have provided strong constraints on the thicknesses and distributions of Venus’ crust (in concert, of course, with theoretical investigations and experimental analyses of materials properties). Gravity and altimetry data could be improved significantly by, for instance, a GRAIL-like gravimeter system, and global stereo DEM topography.

Shallow crustal structure might be investigated via ground-penetrating radar (based either on an orbiter or on aerial assets). A ground penetrating radar is including in the baseline payload of the proposed ESA M5 mission.

Aerial Platforms: Aerial platforms offer several methods of probing crustal structure:

Monitoring of infrasound is a promising technique for detecting seismic activity in Venus’ atmosphere. Venus’ high atmospheric density and hence strong coupling to surface motions makes these signals 60 times larger than for comparable events on Earth. Techniques are being developed that would discriminate seismic events from signals originating in the atmosphere.

Passive electromagnetic sounding exploits signals produced by lightning on Venus which excites the Schumann resonance. Balloons borne techniques are being evaluated in Earth analog experiments (See Appendix A)

Active electromagnetic sounding and gravity measurements are also possible and could offer a sensitivity gain over measurements from orbit but the feasibility but instruments may be too large for aerial platforms and the sensitivity gain has not yet been evaluated.

Advanced Surface Platform: Seismology is the nominal method for determining crustal structure on the Earth. There is progress in high-temperature electronics, power systems, and motion sensors, and implementation of a seismometer is believed to feasible in time for the advanced lander platform. Since a global seismic network is beyond the scope of missions in the roadmap the best opportunity for using landed measurements would be in concert with orbital and aerial observations. The ability to measure surface motion in all three dimensions would complement the orbital and aerial measurements and make it possible to be much more definitive about quake mechanisms.

Assessment: The many techniques that are available for probing techniques from orbital, aerial and surface platforms enable a comprehensive solution to this investigation. The orbital techniques alone will enable global and regional characterization of crustal structure with aerial and surface techniques allowing the crust to be explored in more detail by exploiting different properties.

Investigation II.A.4 – Crustal Recycling

Search for structural, geomorphic, and chemical evidence of crustal recycling.


Many types of observations could bear on this investigation, but none by itself can necessarily prove the existence or define the importance of crustal recycling on Venus. Geomorphic evidence has suggested the presence of subduction or underthrusting in limited areas – these ideas could be tested with high-spatial-resolution SAR images and altimetry, the latter especially useful when coupled with a high-resolution gravity field. Although suggestive, these data and models based on them may (or may not) indicate that crustal material descends to depths at which ‘recycling’ can occur. Another sort of data would be the chemical compositions of lavas associated with potential recycling zones (e.g., like arc volcanics on Earth). These lavas will bear chemical signatures associated with the recycled material, and could be expected to be (by analogy with Earth arc lavas) enriched in volatile components, and enriched in elements that are soluble in those volatiles. For Venus, recycled water would not be significant so one can look for analogies to the compositions of CO2-rich lavas on Earth, like carbonatites. Those are typically enriched in light rare earth elements, phosphorus, and niobium – one might expect similar enrichments in Venus’ lavas affected by crustal recycling.

Orbiter – Surface and Interior: High spatial resolution and gravity data will be the first step in pursuing this investigation. Maps of subsurface structures may also be helpful.

Aerial Platform: Aerial platforms may provide indications of crustal recycling in targeted high-resolution infrared compositional maps and in remanent magnetism which could indicate areas of new crust formed as a result of crustal recycling.

Surface Lander: Confirming the occurrence of recycling will require the ability to characterize the elemental abundance of the rocks. This will required precursor orbital and aerial observations to identify candidate areas where the rocks are exposed and not covered with sediment and may require a precision landing capability to reach those areas.

Assessment: If crustal recycling has occurred the global radar investigations should find evidence of it. Targeted orbital radar imaging and sounding will enable it to be explored in more detail. Aerial and surface platforms will help establish confirmation from geochemical measurements.

Investigation II.A.5 – Interior Mantle and Core

Constrain Venus’ interior (i.e., mantle & core) processes that drive current and recent geologic activity.

Information based on several lines of evidence can be applied to partially address this investigation. However, the platforms defined for this roadmap will not provide complete answers.

Orbital – Surface and Interior – Global: The presence and distribution of hot spots (thermal anomalies) on the Venus surface may be distinctive for different mantle tectonic regimes. For instance, a ‘heat pipe’ regime should have few but very hot anomalies. Detecting thermal anomalies could involve comparison of maps high-resolution altimetry with that of thermal emission. High-resolution gravity maps, again combined with altimetry, may define areas of deep density anomalies, some of which would represent thermal anomalies. High-resolution SAR may reveal surface topographic features that can be modeled to infer deep processes. Seismic experiments, involving a limited number of stations are unlikely to provide direct information on the crustal structure.

Aerial Platform: Detection of remanent crustal magnetism would provide an important constraint on the spin and evolution of Venus’ core. However, neither infrasound nor EM sounding are likely to provide information on such deep structures.

Surface Platform: The Roadmap Focus Group is still studying this.


5.4.2 Objective B – Age of Surfaces

Investigation III.B.1 – Crater populations- Morphology

Constrain Venus’ average surface age, and relative ages of surface units, by evaluating impact cratering rates & distributions, the relationship between impactor properties and crater morphology, and the

processes that modify craters and their extended ejecta.

Magellan SAR radar images have permitted development of a framework of relative for surface units on Venus, but better constraints would be important. In one instance, recognition of partially filled impact craters, filled with basalts of Venus’ plains, have helped constrain the nature of the impact record. However, Magellan SAR lacks the spatial resolution to recognize fill in smaller craters, and even to detect smaller craters on rough terrain. In another instance, ejecta from some impact craters is recognizable in SAR, and is inferred to be modified and removed over time. However, the details of that modification and removal are not apparent in Magellan SAR. In Magellan SAR, few details are visible within smooth plains lava units – flow fronts are rarely observed, and they would be important for understanding volume and repetition rates of the effusions.

To address this investigation, we require imaging that bridges the gap between the radar images that we have today and the visual images from the landed platforms. That would included improved radar data and infrared and visual data from probes, and aerial and surface platforms.

Orbiter- Surface and Interior: To address this investigation, one would want surface imaging at much higher spatial resolution, with better control on small elevation changes or slope breaks (e.g., to detect flow fronts), improved signal/noise for flat (i.e., radar-dark) surfaces, and data about the surface materials characteristics (e.g., porosity). Global imaging coverage at 30m resolution which is about 7 time the Magellan resolution would be transformative. In addition, global topographic maps with an even greater gain over Magellan would be vital for not only these geological investigations but complementary geophysical studies.

A subsequent step would be imaging at 6 m resolution or even 1 m resolution as proposed for the ESA EnVision mission, which would enable the processes modifying craters to be examined with even greater fidelity but for areas limited by the overall mission data rate. A sounding radar with the ability to probe to depths up to several hundred meters would be valuable in characterizing the way in wheich sedimentary and volcanic processes have modified crater populations.

Descent Probes and Sondes: Targeted data will be needed to focus on features of particular significance for understanding the sequence of geological events.

Surface Platforms: The surface platform would provide visual imaging data during the last few kilometer before landing bridging the gap between orbital radar data and panoramic imaging obtained after landing.

Investigation III.B.2 – Age dating on rocks

Determine absolute (radiometric) ages for Venus rocks at locations that are key to understanding the planet’s geologic history.


What are the ages of Venus’ surfaces?

Knowing absolute ages would provide a key constraint on global stratigraphy, to geological and geophysical history, and thus to understanding the dynamics of Venus in comparison to Earth and Mars. Measurement of absolute ages for Venus surface rocks would depend on radio-isotope chronometers – analyzing the abundances of parent and daughter isotopes in suitable radiochronometric systems. Suitable chronometers could include the Rb-Sr and Sm-Nd systems (and potentially others), all of which require high-resolution mass spectrometric analyses of relatively rare elements and their isotopes. The K-Ar radiochronometer is not suitable for Venus, as its surface temperature is so high that radiogenic Ar is rapidly lost to the atmosphere.

Advanced Surface Platform: Mass spectrometry in a landed element on Venus may be possible – spacecraft mass spectrometers designed for Rb-Sr are under development and have been proposed for planetary landers. However, model-independent age determinations require preparation of sample separates with different Rb-Sr ratios – this level of sample preparation and handling has not been demonstrated in a spacecraft setting. Moreover, the time available for carrying out this analysis for even an advanced surface platform that may be up to 12 hrs is prohibitively short. Accordingly, radiometric age dating is deemed impractical. This means that we need a more intensive assessment of stratigraphic and other relative age methods that might be feasible on Venus with technologies that are achievable in the time frame of the Roadmap.

5.4.3 Objective C – Atmosphere-Surface Interaction

III.C.1 – Mineralogy Oxidation State In situ

Evaluate the mineralogy, oxidation state, and changes in chemistry of Venus’ surface-weathered rock exteriors, (including thicknesses of rock weathering rinds), at localities representative of global geologic units.

Venus’ rock and its atmosphere intersect at the surface, where temperatures of ~470°C and pressures ~90 bars virtually ensure geologically rapid chemical reactions. These reactions will impart significant changes to both atmosphere and surface: effects on the surface are considered in Investigation IIB4. The atmosphere is moderately oxidizing (being mostly CO2), and the basalt rock of Venus’ volcanoes and plains is reduced. Their reaction should coat the basalt with a weathering rind of oxidized iron minerals (hematite or magnetite). Hematite (Fe2O3) is interpreted to be the product at low elevations, based on lander and orbiter VNIR data, and the formation rates of these rinds could provide a way to obtain ages of young volcanic units.

Surface Platform: The vehicles must be equipped for obtaining samples from a range of depths and for analyzing the differences in composition as a function of depth. Mineralogical data could come (as above) from Raman or near-infrared spectrometry, or X-ray diffraction; chemical data could come (as above) from LIBS, or X-ray fluorescence. Analysis for these at various depths obviously requires a tool for drilling, cutting, or scraping surface material. These analyses would only be fully interpretable when supported by laboratory experiments and theoretical models. A comprehensive description of the status of instruments and drilling technologies appears in a companion Venus technology plan ( (Hunter & VEXAG Technology Focus Group, 2018)


How do Venus’ atmosphere, surface, and interior interact?

Orbiter – Surface and Interior: Although orbital data are not required as a precursor for these observations they are desirable for site selection and in order to interpret the data and place it in context. The ability to locate the surface platform precisely relative to the orbital imagery is key and could be done either through the use of descent imaging or radar detection of the lander.

III.C.2 – Weathering Causes and Extent

Determine the causes and spatial extents of global weathering regimes on Venus.

Studies of global patterns, such as weathering products, require orbital platforms but other platforms with regional and local investigative capability will be needed to unravel the nature of the phenomena. orbiters.

Orbiter –Surface and Interior – Global: Orbital radar, with higher spatial resolution and better resolution of surface electrical properties, would help explain the altitude (and latitude) effects on radar emissivity and reflectance. Orbital NIR emittance (as in the VIRTIS instrument on Venus Express) could map the extent of hematite (as a weathering product) across Venus, in addition to its other mineralogical uses.

Aerial Platform- Variable Altitude (40 to 60km): These broad-scale remote sensing measurements would be augmented and improved by comparable measurements at greater resolution to better than 10m (but smaller extent) as from balloon assets.

Advanced Surface Platform: A lander with the targeting precision to reach areas of interest will be needed to confirm the chemistry and mineralogy of the weathering species. This will require precision landing for targeting of the lander to sites at which›h these confirmations can be made. and ultimately by landed assets to provide ground truth. The roadmap only envisages one advanced lander mission in the decade and so it will be challenging to find a site that satisfies all of the landed objectives.

Investigation III.C.3 – Coupling of Mantle, Crust and Atmosphere

Characterize the coupling of spin states (angular momentum) among Venus’ core, mantle, crust, and atmosphere, and how this coupling has affected Venus’ evolution.

The GOI document specifies the measurement requirements for defining different types of coupling and discusses what can be accomplished by measurements made by Earth based radar including Goldstone and the Greenbank telescope. We need to define what additional levels of coverage and accuracy could be achieved by a spacecraft mission. We also need to define the extent to which these measurements could be used to arrive at definitive information about the planet’s interior such as the moment of inertia, the size of the core, the traction on the planet exerted by the atmosphere etc.

5.5 Mapping to Goals-Objectives-Investigations - Summary CommentsThis assessment indicates that the recommended Roadmap platforms can address almost all GOI investigations effectively. As we show in the next section, many of these platforms are technically mature now enabling an early start on addressing GOI objectives. In Section 6, we describe a strategy extending over the next 25 years for addressing almost all of the GOI investigations within a projected environment of NASA competitive missions, Flagship missions and missions implemented in collaboration with international partners in ESA, Russia, India and Japan.


6 Venus Exploration RoadmapThe Roadmap considers mission in three time periods phased with respect to the next Planetary Science Decadal Survey (PSDS). Near Term or Pre Decadal refers to the period of four years (2019-2022) before the implementation period for next PSDS begins. Mid Term or Decadal refers to the period of ten years (2023-2032), which is the period for which the PSDS will make its primary recommendations. Far Term or Post decadal refers to the subsequent decade (2033 to 2042). In addition, we will discuss objectives that we believe are important for Venus but must be delayed beyond 2042. In formulating the Roadmap a key factor, but not the only one, is the technical maturity of the mission concept. Drawing on analyses presented in the Technology Roadmap ( (Hunter & VEXAG Technology Focus Group, 2018)) as well as the results of NASA reviews of past NASA proposals, we have characterized the technology readiness of concepts on a five level scale which is presented as color codes in Figure 6-2.

Figure 6-2 Roadmap for Venus Exploration in which exploration platforms are ordered in time based in large part on technical maturity but also reflecting programmatic factors and scientific and technical feedforward. The color code indicates the overall maturity of the concepts as determined from a joint assessment with the Venus Technology Focus Group.


The manner in which these are formulated as missions will depend on the nature of the opportunities. Here we consider how these platforms might fit with existing NASA competitive opportunities, NASA Flagship missions, international collaborations and potential future opportunities such as Venus Bridge. Elements of a possible strategy are illustrated in Table 6-5.

Table 6-5 Mission Opportunities for the Scientific Platforms identifies in the Roadmap

6.1 Near Term – Pre Decadal -2019 to 2022In this timeframe, we do not anticipate the launch of any new Venus missions but there will be up to three Venus proposal opportunities in NASA programs, decisions may be made on missions being studied by other space agencies and technology development is takes place relevant to Roadmap missions.

6.1.1 Ongoing MissionsThe only ongoing mission is Akatsuki which is now in an extended operations. Akatsuki was launched with four imaging cameras: UVI (near 300nm), IR1 (near one micron), IR2 (near 2 microns), and LIR (near 10 micron), an Ultra Stable Oscillator (USO) for radio occultations, and LAC, an imager capable of thousands of frames per second, to investigate lightning and airglow processes. Today, the two near infrared cameras (IR1, IR2) have failed, but the remaining instruments are operating reliably. The USO is requiring longer to build up a complete map of atmospheric profile as a function of latitude, longitude, and local solar time; several remaining gaps in that domain will be filled over the next few years. The LAC continues to build statistics on the frequency and distribution of lightning at Venus; in 2019, it may become possible to make a statistically significant assessment. UVI and LIR will continue to investigate the upper clouds of Venus (both are restricted to observations largely above about 65 km), including a characterization of the newly discovered bow wave, first seen in the first ever image taken by LIR in Venus orbit), and a characterization of upper cloud aerosols, through analysis of phase angle variations.


6.1.2 New Mission StartsThe only Venus mission that is likely to complete development in this period is ISRO’s Venus mission that includes an orbiter, scheduled for launch in 2023, and is reported to include a balloon. The model payload identified in ISRO’s December 2018 announcement includes instruments that have capabilities for surface as well as atmospheric observations but the primary emphasis appears to be the atmosphere and this is the discipline where it is most likely that ISRO can build on prior missions.

6.1.3 New Mission OpportunitiesA Discovery AO is scheduled for release in March 2019 and another AO could be released in 2021 or 2022. The New Frontiers NF-5 AO is likely to be issued before 2022. A Small Innovative Missions for Planetary Exploration (SIMPLEx) AO was issued in July 2018 and others can be expected on annual intervals. With the possible exception of SIMPLEx, candidates for mission proposals would need to be already at either a high to very high level of technical maturity to be credible.

Both Orbiter Surface and Interior concepts and Probe concepts have proved their competitiveness in recent Discovery or New Frontiers competitions are deemed to be at a very high level of technical maturity (Figure 6-2) (Orbiters with an Atmospheric/Ionospheric is also at a very high level of technical maturity and would be credible candidate in this time frame provided that they represent new science not addressed effectively by Venus Express, Akatsuki and the new ISRO Venus orbiter. A SmallSat implementation of these missions is not as mature but would still be a credible candidate.

Fixed altitude aerial platforms and short- lived landers are also candidates for competitive missions in the Pre Decadal timeframe but at present they are deemed to be of moderate-to-high maturity and may require some technology development only. Some of this is underway in the New Frontiers program.

6.1.4 Technology DevelopmentNASA has recognized the vital importance of technology development for implementing these missions as well as more ambitious missions that are candidates for implementation in the Decadal time frame.

High Energy Environment Entry Technology (HEEET): This new thermal protection system technology has been under development for a number of years and is expected to reach TRL 6 in 2019 which would make it available for the probe, short duration lander and aerial platform missions described above. The status of the technology is described in details in the companion report ( (Hunter & VEXAG Technology Focus Group, 2018)

Long Lived In Situ Exploration Systems: NASA has been conducting an initiative for extending the lifetime of in situ missions for periods up to one Venus year. These includes technologies developed under the Hot Tech program ( (Nguyen, 2017) as well as the HotTech Integration Program, which is a structured effort to integrate these technologies into systems capable of realizing mission objectives in time for a mission demonstration – the Long Lived In Situ Surface Explorer during the Decadal time frame.

Aerial Platform Technology: Technologies for aerial platforms are also required. The technologies for fixed altitude aerial platforms are of moderate to high maturity (Figure 6-2). According to the aerial platform report ( (Venus_Aerial_Platform_Study_Team, 2018), the technology for 5.5m diameter balloons is ready for targeted missions today. However, technology development is needed to enable missions requiring larger balloons and for flight duration beyond a few weeks. For variable latitude


platforms, NASA has identified several technologies that should be developed (Mercer, 2018) including altitude control technology, science instrumentation, power and navigation techniques.

6.2 Mid Term – Decadal - 2023 to 2032)The candidate mission modalities for this time-frame include the concepts that we have already described as ready for the Near Term in section 6.1. In addition, there are two additional concepts that require more significant technology development to a level such that that they would not be ready for a science proposal in 2023 or later for a launch in 2028 or later. Both are indicated as being currently of moderate technical maturity.

6.2.1 Orbiter for Surface and Interior- Targeted InvestigationsThis mission is envisaged as a follow on to a mission in the prior decade that would provide a global radar imaging at 30m resolution, global topography and global infrared imaging for compositional characterization in the 1 um region of the spectrum. This mission would perform targeted observations at still higher resolution. Based on currently projected technology, imaging with spatial resolution of 6 m with moderate S/N or 1 m resolution but very low S/N seems achievable (Ghail, 2018).

In addition to imaging, radio sounding would be carried out using techniques used on Mars with MARSIS and SHARAD and planned for Europa and Ganymede in the same period (JUICE enters Ganymede orbit in 2033). The European Venus community is backing the EnVision mission concept that has many of these capabilities. There would be significant feedforward from an earlier NASA-led radar mission to obtain global coverage. Those global maps at 30 m resolution would be used to select locations of targeted coverage. The topographic data at 1 km spatial resolution would be used to remove surface clutter from radar sounding profiles. EnVision also carries a suite of spectrometers, including a near infrared spectrometer to study surface composition and search for possible thermal anomalies related to volcanic eruptions and both infrared and ultraviolet spectrometers to study atmospheric composition.

6.2.2 Long Lived LanderThis concept is identified here with the Long Lived In Situ Surface Explorer (LLISSE) described in Section 4.4.2. The Surface Platform Study team met in November 2018 and reviewed the status of this concept and in particular the technologies that need to be completed. There are many new technologies that are needed to implement LLISSE- high temperature batteries, communications and sensors – to name just a few and so a long-lived lander is judged as not ready for adoption as a science mission until 2023 or later.

6.2.3 Aerial Platform Variable Altitude (50 to 60 km) This concept is described in Section 4.5.2 of this document and in more detail in (Venus_Aerial_Platform_Study_Team, 2018)). As a concept, it is in some respects less mature than the LLISSE although the component technologies are much less challenging that those used in the long lived lander. Accordingly, as with the long lived lander, we recommend a period of development in this case perhaps of up to five years in order to bring the component technologies to readiness and to demonstrate some of them at a system level.

6.2.4 Platform synergiesThe platforms depicted in Figure 6-2 can, in principle, be launched on separate launch vehicles and arrive at Venus at different times and with operational lifetimes that do not overlap. However, it is also


possible to combine them sometime as a larger NASA mission or in an international collaboration. This section describes some of the potential synergies that are provided by combining these platforms in various ways. The midterm provides the opportunity for a Venus Flagship mission including several of these platforms. The Venus Climate Mission recommended to the last decadal was an example of a Flagship mission of this type, which included Orbiter, Probe, Sonde and Aerial Platform. The technical and scientific syneriges that result from this combination have been described previously. A combination of a lander-orbiter and aerial platform is currently under study jointly with Russia and is another example.

Orbiter and atmospheric probe/ lander: In this example, these two vehicles are launched with the same launch vehicle. The orbiter releasing the probe during the passage to Venus and from there the two vehicles are guided to their destinations. Such an approach was adopted for the New Frontiers VOX proposal. The orbiter may also function as a communications relay.

Lander/Probe and Aerial Platform: In this example, the two scientific platforms not only share a launch vehicle but also use a common entry and descent system. This approach was followed in the Soviet era VEGA mission where the entry system contained both the lander and the balloon and in the Venus Climate Mission concept developed for the last Decadal Survey where Aerial Platform and probes shares a common entry system. The probe may also relay data through the aerial platform and sondes may be released from the aerial platform at various times during the mission permitting wide longitudinal dispersion in sampling locations.

Orbiter Aerial Platform/Long Lived Lander: In this example, the orbiter delivers either of the other two platforms to Venus where they enter the atmosphere and then provides a relay communications function for the duration of the mission. In addition, the orbiter may provide context imaging and tracking data that will be of most significance for the aerial platform.

6.2.5 Mission Interrelationships – FeedforwardIn addition to synergies that depend on the vehicles being either launched together or operating at Venus at the same time, there are also benefits where the platforms are deployed sequentially.

Orbiter Surface and Interior and Lander: When the orbiter occurs first or orbital data are available before the arrival of the lander, those data can be used to select safe landing sites or sites of particular scientific significance. Given the accuracy of landing on Venus with existing technologies, refined targeting is not possible. However, if there is significant progress in targeting, this could be extremely important. Measurements of the elemental composition, mineralogy and lithology of the surface will be important for characterizing the nature of global geological units identified from orbital spectroscopic data.

Orbiter Atmospheric and Aerial Platform: Orbiter Atmospheric missions can play a similar role for aerial platforms as Orbiter Surface missions do for landers: they characterize the environment in which the vehicles must operate. Key issues are the temperature profile, the solar radiance distribution and the zonal and meridional velocities as a function of location and time of day. To be useful, these data must be consolidated into predictive models and this is already occurring based on Venus Express and Akatsuki data.

Orbiter/Aerial Platform and Advanced Lander: A prime objective for the Post Decadal period is to carry out a seismic investigation from an Advanced Lander with the capability to observe the seismic activity


at the surface with the sensitivity approaching that achieved at Mars with InSight. Important precursor observations conducted from orbit, from an aerial platform operating for months in the middle atmosphere and from the surface:

Seismic events greater than Mv 5.5 have been predicted to generate transient infrared signals at 1.28 and 2.2 um that can be observed from an orbital monitoring system ( (Lognonne, 2018). A prior orbital survey would be useful to guide the design and placement of an Advanced Lander.

Seismic events as small as Mv 3, as well as excitations originating in the atmosphere, are predicted to be detectable from a Venus aerial platform. These data would help guide the selection of landing sites for the Advanced Lander. They would also help determine the feasibility of ambient noise seismology on Venus if the planet proves to be aseismic ( (Komjathy, 2018).

Measurements from the LLISSE using a short period seismometer can help characterize the background seismic noise environment at the surface of Venus (Kiefer, 2019)

Platform synergies and interrelationships are discussed further in Appendix B.

6.3 Far Term – Post Decadal 2033-2042

In this decade, capabilities developed and deployed in the 2023 to 2032 would be enhance with the addition of further advances in technology ( (Hunter & VEXAG Technology Focus Group, 2018). The platform concepts and their maturity is indicated in Figure 6-2 and the colors indicate the wide range in technology maturity of these concepts. As with missions in the prior decade, we anticipate a significant role for both leadership and partnership with other space agencies in this time frame. Missions in this time frame would extend knowledge of Venus through the acquisition of imaging of progressively higher resolution making it possible to bridge the enormous gap that currently exists between the view from orbit and the view from the surface.

6.3.1 Aerial Platform – Variable Altitude (40 to 60 km)An aerial platform capable of spanning this altitude range would provide significantly more science. It would cover the entire lower and middle cloud layers and would be able to investigate the complex cloud chemistry in these regions. It would also provide additional data on atmospheric dynamics. Perhaps most significantly, by reaching the base of the clouds, it would be able to obtain nighttime infrared images of dramatically higher resolution than is possible from orbit. There are also significant technical challenges for this kind of platform, which is why most of the elements are of low to moderate maturity. The greater altitude range is part of this. The other challenge is coping with the high temperature environment at the lower part of the range. The approaches to this are described in the VEXAG Technology Plan (Hunter & VEXAG Technology Focus Group, 2018)

6.3.2 Advanced Lander – Hybrid with short and long lived elementsThe advanced lander envisaged for the Post Decadal period integrates features of the short lived and long-lived lander. Precision landing would be an important capability for both improving science return and reducing landing hazard. Following landing, and during the period in which the thermally controlled lander systems are operating, rapid compositional measurement techniques are deployed. A seismometer could also be delivered to Venus on the payload deck of the short duration lander. After landing, the seismometer would be placed on the surface of Venus with an arm - a similar approach to that used for SEIS on InSight. However, on Venus this would be accomplished autonomously in a period


of a few hours and in a manner that interfered minimally with other science conducted by the lander. Another important science measurement relying on long-lived high temperature electronics would be heat flow measurements. These would require precise measurements of surface temperature through a diurnal cycle. 6.3.3 Decision Point on Future DirectionsWith the completion of this set of missions, the scientific and technical data necessary to formulate a future direction for subsequent exploration would be brought to bear on deciding the appropriate direction for the next phase of Venus exploration. The candidate here would likely be Venus Surface Sample Return (4.6) and Surface or Near Surface Mobile Exploration (4.7). The resource requirements for each of these capabilities and their technical needs are so different that a decision would be needed on which pathway to pursue. Whichever choice were made in this time frame only a precursor mission could be contemplated as indicated in Figure 6-3

Figure 6-3 Roadmap for the decade 2033 to 2042.. The color indicates the overall maturity of the concepts as determined in a joint assessment with the VEXAG Technology Focus Group

6.3.4 Surface Sample Return PrecursorsThe extreme technical challenges of VSSR described in Section 4 mean that implementing a mission would extend far beyond the time frame of this Roadmap. However, it is useful to consider what precursor missions might be included although specifying what that mission would be is premature.

Many of the missions already described would pave the way to a VSSR by contributing important science results:

Orbiter – surface and interior mission will tell us much more about the nature of the surface and the types of samples that might be acquired – ancient crust, sedimentary deposits, recent volcanic rocks.


Surface Platforms will clarify the nature of the immediate surface layers and will determine whether pristine rocks can be accessed.

Aerial Platforms may reveal organic materials in the cloud layer that might motivate a Venus Cloud Sample Return mission.

Aerial Platforms and Surface Platforms will demonstrate some of the technologies needed to execute a VSSR mission.

One possible VSSR precursor mission would obtain a Venus cloud sample as described at the Planetary Science Visions 2050 workshop (Shibata, 2017). If in situ analysis of cloud samples by an aerial platform mission indicated organic materials, a Venus Cloud Sample Return (VCSR) mission would be logical follow on and would exercise elements of the VSSR architecture. An alternative would be to demonstrate sample acquisition and balloon ascent from the surface but with in situ analysis rather than sample return. This is actually the mission concept originally recommended as a New Frontier mission candidate by the 2003 Planetary Science Decadal Survey (Belton, 2003).

6.3.5 Surface or near Surface Platform with Regional MobilityMany of the missions already described that would pave the way to a VSSR by contributing important science results would also be applicable to this mission. One possible precursor mission would be a metal bellows or buoyant metal sphere with high temperature sensors that could float in the lower atmosphere and acquire temperature, pressure and compositional information on the atmosphere. If instruments capable of imaging or analyzing the surface are developed they may be candidates for such a precursor mission.

7 Summary This Venus Exploration Roadmap lays out a framework for the future exploration of Venus, encompassing observations of the atmosphere, surface, and interior using a variety of mission modes ranging from orbiters and probes to aerial platforms, and long duration landers. It was developed for the planetary science community by the Venus Exploration Analysis Group (VEXAG) to provide guidance to the Planetary Science Division and the Planetary Science Decadal Survey of 2021 which is charged with framing a strategy for all of planetary exploration for the next decade and beyond.

With the prolonged hiatus in U.S. led Venus exploration, the science and the technology are now ready for not just a single mission. In planetary exploration, the next decade can and should be the Decade of Venus.


Appendix A: Science from Aerial PlatformsAtmospheric Science from Aerial Platforms

Observations from within the atmosphere make it possible to make key measurements of the atmosphere that are not feasible from orbit. The in situ vantage point makes it possible to employ techniques where physical contact with the atmosphere is required including powerful mass spectrometers for determining the abundances of different atomic and molecular species and their isotopes. It also make it possible to isolate measurements to a particular region of the atmosphere. This is particularly important deep within the atmosphere where remote sensing measurements will be blocked by absorption and scattering in the overlying atmosphere. In situ measurements can be synergistic with orbital measurements that can provide a regional and global perspective..

Atmospheric measurements from aerial platforms have been grouped into three categories (Table A-6) : Atmospheric Gas Composition; Cloud and Haze Particles and Atmospheric Structure. The instruments used to make these measurements and the measurements and objectives are also listed in Table A-6.

Atmospheric Gas Composition: Measurements of atmospheric gas composition made with remote sensing techniques both from ground-based telescopes and from orbit. The in situ techniques can uniquely measure the composition of noble gases and their isotopes that requires mass spectrometric techniques. The aerial platform approach permits measurements of unprecedented sensitivity and precision because of the ability to integrate over period of hours days or weeks. The other strength of the in situ measurement is the ability to measure variations of composition with altitude in the clouds which are expected on Venus but cannot be measured remotely with the required altitude resolution.

Table A-6 Atmospheric Measurements that can be made from Aerial Platforms. Most of these measurements can be made from the Aerial Platform itself. Some can also be performed by probes and sondes

Instruments Abbreviation Measurement Type/ObjectivesATMOSPHERIC GAS COMPOSITION

Mass Spectrometer MS or GCMS Atmospheric species including noble gases and their isotopes. Survey instrumentTunable Diode Laser Spectrometer TDL Trace species including isotopic abundances. Targeted on a few speciesUV/IR Spectrometer UVS, IRS Atmospheric species from their spectal signatures. Survey instrumentChemical Sensors (MEMS based) ChemSens Chemical species. Small low power instrument targeted on a few species

CLOUD AND HAZE PARTICLESNephelometer Neph Size, scattering properties and abundance of cloud and haze particles in bulkLight Optical Amospheric Counter LOAC Size, scattering properties and abundance of cloud and haze particles individualImaging Microscope Mic Images larger cloud particles captured on a filter.Aerosol Mass Spectrometer AMS Chemical composition or biological nature of aerosols (individual or bulk)

ATMOSPHERIC STRUCTUREAtmospheric Structure Instrument ASI Temperature, pressure and vertical wind speed. Net Flux Radiometer NFR Upward and downward flux of radiation in multiple spectral bandsUltra Stable Oscillator USO Wind velocity from Doppler signatures from DSN and orbit

2. Cloud and Haze Particles

Remote sensing measurements from the ground and orbit have indicated the presence of cloud and haze particles but do not provide unambiguous information about their nature. In situ measurements from the Pioneer Venus probe identified three distinct haze layers and characterized particle sizes. Measurements from an aerial platform can indicate the chemical nature of those particles, their spatial


and temporal variation, and to expand the size distribution to smaller sizes. These measurements can help determine what creates and sustains the haze layer and what determines the absorption and scattering processes that are involved in the Venusian greenhouse effect.

c. Atmospheric Structure

Location on an aerial platform makes it possible to make direct measurements of temperature and pressure and the vertical wind speed relative to the platform. Of additional value is to measure the upward and downward welling radiation at different locations within the atmosphere, which is critical to measuring the energy deposition. Tracking of the aerial platform from the DSN or from orbit will provide measurements of wind velocity since the platform tracks the motion of the atmosphere. Measurements from the Earth will be primarily valuable for tracking the zonal component. Tracking from a high inclination orbiter will be needed to determined meridional velocities. Measurements with an inertial measurement unit will help determined the effects of turbulence and also contribute to the geophysical measurements.

Surface and Interior Science from Aerial PlatformsAerial platforms make it possible to makes measurements of the surface and interior of Venus that are not practical from orbit. These measurements complement orbital measurements and in some cases may be highly synergistic with themThe primary reasons for the advantages of aerial platforms over orbital measurements are proximity to the surface and physical contact with the atmosphere. The investigations have been grouped into two categories in Table A-7: studies of the Interior and a investigations of the surface

Table A-7 Techniques for studying surface and interior for aerial platforms

Geophysics - Interior Investigations: The techniques draw in part on methods used in airborne geophysics. In general, proximity to the surface enables measurements at higher spatial resolution. Investigations of remanent magnetic fields from aerial platforms also benefit from shielding by the Venus ionosphere. The infrasound techniques exploit physical contact with the Venus atmosphere. Infrasound from Venus quakes is linked to the atmosphere 60 X more efficiently than on Earth.

Surface Investigations: Observations for aerial platforms can enable data types in terms of spectral range and spatial resolution that are not feasible from orbit. Nighttime Infrared images wavelengths has already been demonstrated () and can be extended with future orbital missions but spatial resolutions is fundamentally limited by scattering in the Venus clouds. An aerial platform that can access the base of the clouds (40 km) can generate infrared imaging with spatial resolution of few meters. However, there are technical challenges for sustained observations from this altitude. Observations of the surface in


reflected light are only feasible within 10 km of the surface and quality observations will require access below 5 km.

Appendix B: Platform InterrelationshipsVenus exploration platforms have important synergies and interdependencies that must be taken into account what constructing a detailed roadmap. Figure B. 1, which has been adapted from Fig 6.1 illustrates some, but by no means all, of these dependencies.

Data on the Venus surface acquired from a global mapper would be used for subsequent targeted observations at higher resolution which in turn would be used for site selection for an Advanced Lander

Orbiters play a vital role in data relay from aerial and surface platforms particularly for surface platforms with extended lifetime. Orbiters are also important for locating aerial platforms and providing context imaging

Aerial platforms can deploy sondes potentially with much higher precision than probes and can capture and relay data from sondes at much higher rates than using orbiters alone.


Figure B-4Interrelationships between Venus exploration platforms. The relationships shown as arrows connecting the baxes include both operational synergies such as deployment and data relay and scientific and technical feedforward where data acquired from one mission is used to influence targeting, analysis and even design of future missions


Acronyms and AbbreviationsAO announcement of opportunityEM electromagneticGOI (Venus) Goals, Objectives, and InvestigationsH/D hydrogen/deuteriumPDR preliminary design reviewSAR synthetic aperture radarTRL technology readiness levelVCM Venus Climate MissionVEXAG Venus Exploration Analysis GroupVIRTIS (Venus Express) Visible and Infrared Thermal Imaging SpectrometerVISE Venus In Situ ExplorerVSSR Venus Surface Sample Return


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AcknowledgmentsThis document was supported in part through research carried out at the Jet Propulsion Laboratory,California Institute of Technology, under a contract with the National Aeronautics and SpaceAdministration.

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