Science options and priorities for the exploration of Phobos and Deimos

Scott MurchieThe Johns Hopkins University/Applied Physics LaboratoryNancy Chabot and Andy Rivkin (JHU/APL)

Abigail Fraeman and Ray Arvidson (Washington University)

David Beaty, Deborah Bass, Julie Castillo-Rogez (NASA/JPL)

Paul Abell (NASA/JSC)

Ruthan Lewis (NASA/GSFC)

Tony Colaprete (NASA//ARC)

Compiled from:

Scientific Objectives for the MPD Mission, Beaty et al., 2012

Phobos and Deimos: Achieving scientific goals and objectives with robotic and human exploration, Murchie et al., 2014

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Phobos DeimosSize 27 x 21 x 19 km 15 x 12 x 10 km

Orbital Period 7.66 hrs 30.3 hrs

Density 1.9 g/cm3 1.5 g/cm3

Normal albedo, 0.55 µm

7% 7%

Phobos and Deimos …

• Are the only terrestrial planet satellites besides the Moon and provide insights into terrestrial planet formation.

• Reconnaissance by several missions gives us working knowledge of the moons’ outstanding science issues

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Red Unit

Blue Unit

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• The moons resemble outer main belt and Trojan bodies, and could provide insights into primitive bodies and sources of volatiles and organics

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Phobos – the better known moon

• Properties well known globally- Mass- Global morphology at moderate (~10s m) scale- Volume from imaging- Density (1870±20 kg/m3)

Murray et al. 2007

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Phobos – the better known moon

• Known only regionally or at low resolution- Morphology at meter scale- VISIR spectral reflectance

Weak absorptions, D-type OH is present at optical surface Best analog desiccated clay +

carbon or opaques- Relationship of color to morphology

Hints of subsurface structure- Thermal emission / thermal inertia

Nearly black-body spectrum Fine grained regolith

500 m

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Deimos – the lesser known moon

• Properties well known globally- Mass

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Deimos – the lesser known moon

• Poorly known globally- Global morphology at ~10-m scale- Volume- Density (1490±190 kg/m3)

• Known only locally or at low resolution (lower than Phobos)- Morphology at meter scale- VISIR spectral reflectance- Relationship of color to morphology

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Bright regolith streamers

South polar concavity may be impact scar

• First-order differences: shape, surface morphology, density- Deimos’s lower density and smooth surface may result from

fragmentation/ejecta blanketing by south polar impact- Basic Deimos measurements are needed: volume to improve density

determination, global/spectral imaging at lighting geometries to assess morphology and subsurface structure

Stickney

Grooves

Morphologically Distinct, Spectrally Similar

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• Deimos is spectrally similar to Phobos’ red unit – but we can’t tell if composition/origin is the same or different- Different compositions may hide in bland spectrum – issue for D bodies- An origin outside the Mars system does not require any relation- Moons’ relation requires at least in situ measurement of both moons,

though returned samples would provide greater insight

Fraeman et al. (2014)

Morphologically Distinct, Spectrally Similar

OH

Fe-clay?

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Relevance to Decadal Survey GoalsBuilding new worlds: Understanding solar system beginnings. • The composition of the moons indicates their origin and elucidates accretion and

moon formation

Planetary habitats: Searching for the requirements for life: • If the moons are captured primitive bodies they sample the carbon and volatiles

brought by incoming impactors

Workings of solar systems: Revealing planetary processes through time. • The moons’ distinct regolith evolutions, and possibly different internal

structures suggested by density, provide a laboratory for divergent small body regolith characteristics.

• Groove formation is a widespread but controversial process and Phobos is a type example.

• Understanding microphysical effects of space weathering calibrates the interpretability of bland D-type spectra

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1) Study Phobos and Deimos as planetary bodies that provide insight into small bodies and terrestrial planet evolution- Some investigations can be accomplished robotically; others are assisted

by a human presence- Initial robotic results would help plan human exploration

2) Close strategic knowledge gaps for future human exploration- Very high overlap with highest-priority robotic objectives- Effectively same as #1

3) Use as a support area for human exploration of Mars- Teleoperation, weather monitoring, etc.- ISRU

Top-Level Phobos and Deimos Exploration Objectives

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Objective 1: Origin and evolution of Phobos and Deimos, providing insight into Mars & small bodies1) Determine origin of moons evidenced by composition

- Proposed origins have radically differing implications for composition If the moons originate outside the Mars system, they may provide

insights into how volatiles and/or organics were delivered If they originate in the Mars system, they may sample Mars’ earliest

crust- Can be determined by basic mineral and elemental abundance- Solvable with a precursor mission – frames human science objectives

Origin Plausible Composition

Capture of organic- and water-rich outer solar system body Primitive composition; like CI or CM

Capture of organic and water-poor outer solar system body Anhydrous silicates plus elemental C

Capture of inner solar system body, or co-accretion with Mars Space weathered ordinary chondrite

Giant impact on Mars Space-weathered Mars crust or mantle

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- Major elements easily separate a chondritic origin from a differentiated composition

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- Minor elements separate ordinary chondrite-like compositions from different carbonaceous compositions

- Mn, Zn, C, and S are particularly diagnostic

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- Major and mineral minerals are complementary information and distinguish subtypes

- Phyllosilicates and carbonates are critical to distinguish primitive compositions

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Objective 1: Origin and evolution of Phobos and Deimos, with insight into Mars and small bodies2) Conduct detailed studies of surface

chemistry Determine absolute ages of materials Constrain conditions (P, T, redox) of formation Quantify the amount, compositions of material

from Deimos, possible extinct moons, Mars• Mars contribution est. as ~150 ppm

Inventory & characterize organic & volatile phases3) Determine the microphysical effects of

space weathering- These require analysis of returned samples in

Earth’s laboratories- Stickney ejecta, non-Sitckney ejecta, fresh and

mature regolith- Human participation invaluable

Stickney Ejecta (Thomas)

Secondary Impacts from Mars (Murray)Stickney Rolling Boulders (Head & Wilson)

Tidal Stress (Dobrovolskis)

Stickney Fracturing (Fujiwara & Asada, Thomas) Map of Phobos’ Grooves (Murray)

Murray (2007)

4) Determine regolith & internal structure, processes affecting them – if either moon is a rubble pile, there is a fuzzy line

Test models for formation of grooves

Objective 1: Origin and evolution of Phobos and Deimos, with insight into Mars and small bodies

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Fracture with Drainage• No raised rims• Loose fracture fill

matching composition of surrounding regolith

Fracture with Degassing • Raised rims• Loose fracture fill with

composition from depth

Secondary Cratering• Raised rims• Compacted regolith inside

grooves

- Key surface properties can be determined robotically- Cross-sectional variations in composition, competence assisted by human

participation: sampling transects, seismic studies and/or GPR

Objective 1: Origin and evolution of Phobos and Deimos, with insight into Mars and small bodies

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Determine the moons’ internal structure- Phobos’ and Deimos’ differing densities suggest differing internal structure

• Orbital radio science, spectral mapping• Radar and/or seismic sounding• Sampling transects, chemical/isotopic analysis of returned samples

- Human participation in sampling / sounding could elevate data quality

Notional internal structural model from initial analyses of Phobos 2 data(Murchie et al. 1991)

Objective 1: Origin and evolution of Phobos and Deimos, with insight into Mars and small bodies

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Quantify regolith processes- Some features resemble more S-asteroids like Eros, e.g. streamers- Albedo variegation is distinct - suggests processes in “primitive” composition

Objective 1: Origin and evolution of Phobos and Deimos, with insight into Mars and small bodies

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“A human mission to the Phobos/Deimos surface would require a precursor mission that would land on one or both moons.” (Finding #2 of P-SAG Report, 2012)

Objectives of a precursor- Hazards- In situ resources- Tactical data to plan surface

operations- Science to frame human

scientific exploration

Phobos and Deimos dust belts from Hamilton (1996)

Objective 2: Determine the moon’s properties that are important to planning human exploration

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Strategic Knowledge Gap (P-SAG, 2012) Relevant Measurements

A3-1. Orbital particulate environment • Particle size-frequency distribution of dust belts

C1-1. Surface composition and potential for ISRU (C, H)

• Elemental composition, including C and H• Mineral composition, including hydrous phases• Global spectral imaging or elemental abundance

mapping, for context

C2-1. Charged particle environment • Near-moon total dose and energy measurements

C2-2. Gravitational fields • Mass, mass distribution from radio science• Global shape through stereo imaging or lidar

C2-3. Regolith geotechnical properties

• Thickness, rock abundance from imaging, radar• µm to cm scale structure, particle size• Regolith mechanical properties experiment

Objective 2: Determine the moon’s properties that are important to planning human exploration

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• A platform to support Mars science - somewhat redundant to lower-cost robotic exploration- Teleoperation of rovers

- Mars weather / atmosphere monitoring

- Retrieve Mars samples from Mars orbit

• In situ resource utilization – powerful but yet to be proven (by robotic precursor?)- Amount of C, OH still speculative

Objective 3: Use the moons as a staging area to support human exploration of Mars

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Take-away Messages - Science

• Phobos and Deimos are relevant to important science questions- “Exploring the outer solar system from Mars orbit”

• Some questions can be addressed robotically- Would close SKGs

• More challenging questions benefit from human participation• The science is distinct & complementary to Mars surface science

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Take-away Messages - Tactical• Science value in visiting both moons• Phobos appears to be of greater value• Depending on timeline, visit both but with more time at Phobos• Precursor data are needed to formulate detailed objectives• Robotic mission to Phobos and/or Deimos is required prior to a human

mission. • The precursor should interact with the surface of Phobos and/or

Deimos to understand geotechnical properties• Robotic Phobos or Deimos sample return from prior to a human

mission is judged NOT to be necessary

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Precursor Concepts

MERLIN: Deimos flybys Phobos rendezvous and land

PANDORA: Deimos and Phobos rendezvous

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BACKUP

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Relevance to SBAG Themes and ObjectivesSolar System Origins: May be remnant Mars building blocks & contain key info on Mars’ accretional environment. May be captured asteroidal materials linked to early planetesimal formation.

Solar System Dynamics: May be related to population of late-accreting planetesimals involved in early bombardment and offer insights into exchange of material from Mars to moons, to each other, and also from outside Mars system.

Current State of the Solar System: Present potential relationship with D-type asteroids, Tagish Lake chondrite, other primitive carbonaceous meteorites, and as well as Mars’ surface dust. Also understanding of regolith processing.

In Situ Resource Utilization: OH and/or water suggested by spectroscopy

Hazards: Do not present an impact hazard; physical studies can help better understand relationships to near-Earth asteroids (i.e., contribution to their near-surface geotechnical properties and internal structure).

Astrobiology: May represent pre-Noachian Mars ejecta or water-rich asteroids; may be repository for Mars’ meteorites ejected through time; may offer insights/comparisons into delivery of organics/volatiles to early Earth.