Future Exploration of EuropaRonald Greeley

Arzona State Unversty

Robert T. PappalardoNASA Jet Propulson l-aboratory/Caffirna Insttute of Technology

Louise M. ProckterJohns Hopkins Unversity Applied physcs laboratory

Amanda R. Hendrix and Robert E. LockNASA Jet Propulson LaboratorylCalifuna Insttate of Technology

Reports from NASA, the National Research councir (NRC), the European Space Agency(ESA)' and science community groups identify Europa as a priority for ouier solar system ex-ploration, especially for astrobiology. From these reports, an intemational group proposed theEuropa Jupiter system Mission, involving a NASAJupiter Europa orbitei 1lo, ttre Nesaelement), which is the focus of this chapter. Current knowledge of Europa s reviewed, out-standing questions identified, and science objectives formulated. The JEO goal is to .,ExploreEuropa to Investigate its Habitability;" this goal is to be met rhrough objtives to study (inpriority-order) (l) Europa's ocean and deep interior structrre, (2) the icy shell and its struc-ture' (3) its chemistry and composition, (4) the geology, and (5) rhe general Jupiter sysrem,including the other major satellites and their atmospheres, the jovian plasma and magnetosphere,the parent planet Jupiter, and the small moons, rings, ancl clust.

1. INTRODUCTIONSince the first glimpses provided by Voyager, Europa has

been recognized as an object worthy of exploration. Asreviewed in the chapter by Alexander et al., the Galileomission confirmed the suspicions that Europa in many waysis unique in the solar system and is a primary target forastrobiology. This chapter draws on results from a studycommissioned by NASA and the European Space Agency(ESA) in 2008 for a future Europa Jupiter System Mission(EJSM) in which the Joint Jupiter Science Definition Team(JJSDT) (Table l) reviewed prcvious studies for Europa(Table 2), assessed the current state of knowledge, formu_lated the key questions for the next mission, and identifiedthe measurements that should be made to meet the explo_ration objectives. A candidate payload was also described,recognizing that the actual payload would be competed withthe selection based on the best instruments to answer thekey questions.

1.1. The Relevance of Jupiter System Exploration

Jupiter is the archetype for the giant planets of our so_lar system, and for the numerous planets known to orbitother stars. Jupiter's diverse Galilean satellites

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three of

which could harbor internal oceans -

are the key to under_standing the habirability of icy worlds. Thus, the JJSDT hasidentified "The Emergence of Habitable Worlds Around GasGiants" as the overarching theme for a combined NASA_ESA mission.

Since the first extrasolar planets were detected in the late1980s, their discovery has increased tremendously (Vogt etal., 2005) and 10Vo of all Sun-like stars may have planets.With existing discovery techniques, almost all the knownextrasolar planets are giant planets, more akin to Jupiterthan to Earth. These bodies are expected to have large icysatellites that formed in their circumplanetary disks, analo_gous to Jupiter's Galilean satellites. Europa and Ganymedeboth could be geologically active and harbor internal salt_water oceans. They are straddled by Io and Callisto, keyendmembers that tell of the origin and evolution of theJupiter system. If extrasolar planetary systems are similar,then icy satellites may be the most common habitats in theuniverse

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probably much more abundant than Earthlikehabitats, which require very specialized conditions to per_mit surface oceans.

EJSM would afford rhe opportuniry for detailed scrurinyof the archetype gas giant planet and its four diverse largesatellites. EJSM would be invaluable for the insights it couldprovide into otr solar system and into planetary architec_

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6s6 Europa

TABLE L Joint Jupiter Science Definition Team.Team Member

Co-ChairsGreeley, RonaldLebreton, Jean-Pierre

Study ScientistsLebreton, Jean-PierrePappalardo, Robert

United StatesAnbar, ArielBills, BruceBlaney, DianaBlankenship, DonChristensen, PhilDalton, BradDeming, JodyFletcher, LeighGreenberg, RickHand, KevinHendrix, AmandaKhurana, KishanMcCord, TomMcGrath, MelissaMoore, BillMoorg JeffNimmo, FrancisParanicas, ChrisProckter, LouiseSchubert, JerrySenske, David

Afliation

Arizona State UniversityESASTEC

ESA/ESTECJet Propulsion Laboratory

Arizona State UniversityNASA-Goddard/l.JCSDJet Propulsion LabortoryUniv. Texas at AustinArizona State UniversityJet Propulsion LaboratoryUniv. WashingtonJet Propulsion LaboratoryUniv. ArizonaJet Propulsion LaboratoryJet Propulsion LaboratorYUCLABear Fight CenterNASA-MarshallUCLANASA-AmesucscJHU-APLJHU-APLUCLAJet Propulsion Labortory

Team Member Affiliation

SETIUniv. ArizonaMBLSWRISWRI

cole PolytechniqueUniv. TrentoImperial College LondonParis ObservatoryUniv. NantesDLR, BerlinMax Planck Inst.Imperial College LondonINTAUniv Bretagne OccidentaleDLR, BerlinUniv. BolognaIFSIUniv. Bern

ISAS, JAXATohoku Univ.NOAJTohoku Univ.ISAS, JAXA

United States (continued)Showalter, MarkShowman, AdamSogin, MitchSpence JohnWaite, Hunter

European UnionBlanc, MichelBruzzonem, LorenzoDoughert MicheleDrossart, PiereGrasset, OlivierHuman, HaukeKrupp, NorbertMueller-Wodarg, IngoPrieto-Ballasteros, OlgaPrieur, DanielSohl, FrankTortora, PaoloTosi, FedericoWurz, Peter

JapanFujimoto, MasakiKasaba, YasumassaSasaki, ShoTakahashi, YukihiroTakashima, Tkeshi

TABLE 2. Previous studies of Europa and heritage of science objectives and investigations.

Committee Report Title Reference

Europa Orbiter Science Definition Team

Committee on Planetary and LunarExploration (COMPLEX)NASA Campaign Science WorkingGroup on Prebiotic ChemistrYin the Solar System

Solar System Exploration("Decadal") SurveyJupiter Icy Moons Orbiter (JIMO)Science Definition Team

Europa Focus Group of the NASAAstrobiology Institute

Outer Planets Assessment Group (OPAG)

NASA Solar System ExplorationStrategic Roadmap Committee

Europa Science Definition Team

Jupiter System Observer ScienceDefinition Tean

The Laplace Team

Europa Olbiter Mission and Project Description

A Science Strategy for the Exploration of Europa

Europa and Ttan: Preliminary Recommendationsof the Campaign Science Working Group on

Prebiotic Chemistry in the Outer Solar System

New Frontiers in the Solar SYstem:An Integrated Exploration Strategy

Report of the NASA Science Definition Teamfor the Jupiter Icy Moons Orbiter (JIMO)

Europa Science Objectives

Scientific Goals and PathwaYs forExploration of the Outer Solar System

2006 Solar System Exploration Roadmap forNASA's Science Mission Directorate

2007 Europa Explorer Mission Study: Final Report

Jupiter System Observer Mission Study: Final Report

Laplace: A Mission to Europa and the JupiterSystem for ESAs Cosmic Vision Programme

NASA AO 99-OSS-04 (1999)

ss8 (1999)

Chyba et al. (1999)

ss (2003)

J(MO SDT (2004)

Pappalardo (2006)

)PAG (2006)

NASA (2006)

Clark et al. (2007)

Kwok et al. (2007)

Blanc et al. (200'l)

ture and habitability throughout the universe. For these rea-sons, both NASA s Solar System Decadal Survey (SSB,2003) and ESAs Cosmic Vision (ESA, 2005) emphasize theexploration of the Jupiter system to investigate the emer-gence of habitable worlds.

EJSM would include a NASA Jupiter Europa Orbiter(JEO) and an ESA Jupiter Ganymede Orbiter (JGO); a Ju-piter Magnetospheric Orbiter (JMO) is also being consid-ered by the Japan Aerospace Exploration Agency (JAXA).While the primary focus of JEO is to orbit Europa, the sci-ence return would encompass the entire jovian system withflybys of Io, Ganymede, and Callisto, along with

-2.5 yearsobserving Jupiter's atmosphere, magnetosphere, and rings.Similarly, JGO would investigate Callisto and ultimately or-bit Ganymede, and its focused observations of the Jupitersystem would complement those of JEO. If it comes to frui-tion, JAXA's JMO has the potential to focus on particlesand fields observations of the jovian magnetosphere. WhileJEO and JGO are complementary and porentially synergis-tic, both are designed as "stand-alone" missions as a con-tingency. The remainder of this chapter focuses on JEO andthe potential for future exploration of Europa.

1,2. The Relevance of Europa Exploration

Europa's icy surface is thought to hide a global subsur-face ocean with a volume more than twice that of Earth'soceans (see chapter by McKinnon et al.). The moon's sur-face is young, with an estimated age of about 60 m.y.(Schenk et al., 2004; see chapter by Bierhaus et al.), im-plying that it is probably geologically acrive roday. The mo-lecular constituents of life have fallen onto Europa through-out solar system history, are potentially created by radiationchemistry at its surface, and may pour from vents at theocean's floor (Baross and Hofftnann, 1985; Pierazzo andChyba,2002). On Earth, microbial extremophiles take ad-vantage of environmental niches arguably as harsh as thosewithin Europa's subsurface ocean (see chapter by Hand etal.). Ifthe subsurface waters are eventually found to containlife, the discovery would spawn a revolution in our under-standing of life in the universe.

It is now recognized that oceans could exist in severalicy solar system objects. Titan could have a subsurfaceammonia-water ocean (Lorenz et al., 2008; Tobie et al.,2005) sandwiched between ice polymorphs and ice, ratherthan being in direct contact with the mantle. Enceladusshows jets of water vapor and ice grains streaming from itssurface, and emits a measurable heat flux from its southpolar region (Spencer et a1.,2006), suggesting that pock-ets of water might exist below the surface (Porco et al.,2006). If Neptune's Triton is a captured Kuiper belt object,it would have experienced tremendous tidal heating duringits capture and subsequent orbital evolution (e.g., Prockteret a1.,2005). This heating likely produced an intemal ocean;the

-100-rn.y. crater age of the surface (Stern and McKin-non,2000) suggests that a subsuface ocean might still exist.

Greeley et al.: Future Exploration of Europa 657

However, it too might not be in direct contact with themantle (Hussman et a1.,2006). Although some Kuiper beltobjects and satellites of Saturn and Uranus could have in-ternal oceans, these are expected to be cold ammonia-richoceans and energy sources for life are probably lacking.

It is tantalizing to consider whether life might exist inseas of ethane-methane or cold oceans of ammonia-water.Such environments could be fascinating places to search forlife unlike we know it. However, it is more tractable to fo-cus searches on potential icy habitats comparable to thosein which we know biology could work. Experience withEarth shows that carbon-and-water-based life functions wellover a wide range of temperature, pressure, and chemicalregimes. Thus, Europa is the natural target for the first fo-cused spacecraft investigation of the porenrial habitabilityof icy worlds. Its putative thin icy shell, candidate sourcesof chemical energy for life, and potentially active surface-ocean material exchange make it a top priority for explora-tion. The JEO would be the first critical step in understand-ing the potential of icy satellites as abodes for life.

Europa's high astrobiological potential and its complexinterrelated processes have been rccognized by rnany gloups,including the National Research Council (NRC) and NASA.The NRC's Committee on Planetary and Lunar Exploration(COMPLEX) (SSB, 1999) stated thar Europa "offers the po-tential for major new discoveries in planetary geology andgeophysics, planetary atmospheres, and, possibly, studiesof extratenestrial life. In light of these possibilities, COM-PLEX feels justifred in assigning the furure exploration ofEuropa a priority equal to that for the future exploration ofMars." The NRC's New Frontiers in the Solar System (re-ferrcd to as the "Decadal Survey") (SSB, 2003) identifieda Europa Geophysical Explorer as the rop priority "Flag-ship" mission for the decade 2003-2013, principally be-cause such a mission addresses the fundamental sciencequestion: "Where are the habitable zones for life in the solarsystem, and what are the planetary processes responsiblefor producing and sustaining habitable worlds?" This rec-ommendation was reaffirmed by the NRC's Committee onAssessing the Solar System Exploration Program (CASSE)(SSB, 2007), which recommends "NASA should select aEuropa mission concept and secure a new start for the pro-ject before 20ll:'

The NRC recommendations are in turn reflected in theNASA Science Mission Directorate's Solar System Explora-tion (SSES) Roadmap (NA., 2006), which states "Europashould be the next target for a Flagship mission." NASA'sscientific community-based Outer Planets Assessment Group(OPAG) "affirms the findings of the Decadal Survey, COM-PLEX, and SSES, that Europa is the top-priority sciencedestination in the outer solar system" (OPAG,2006).

Noting that Europa's neighbors Ganymede and Callistoare also considered to have internal oceans, the NASARoadmap frther states "It is critical to determine how thecomponents of the jovian system operate and intemct, lead-ing to potentially habitable environments within icy moons.

"r658 Europa

By studying the Jupiter system as a whole, we can betterunderstand the type example for habitable planetary sys-tems within and beyond our solar system."

NASA s 2007 Science Plan (NASA, 2007) echoes themany previous recommendations, calling Europa "an ex-tremely high-priority target for a future mission." This doc-ument acknowledges that several icy satellites are nowthought to have subsurface oceans, and states 'Althoughoceans may exist within many of the solar system's largeicy satellites, Europa's is extremely compelling for astro-biological exploration. This is because Europa's geologyprovides evidence for recent communication between theicy surface and ocean, and the ocean might be suppliedfrom above and/or below with the chemical energy neces-sary to support microbial life." The Science Plan affrrms thepriority of Europa exploration in addressing fundamentalthemes of solar system origin, evolution, processes, habit-ability, and life.

The NASA Astrobiology Roadmap (Des Marais et al',2003) includes the goal "Explore for past or present habit-able environments, prebiotic chemistry, and signs of lifeelsewhere in our solar system." A subsidiary objective is to"provide scientific guidance for outer solar system missions'Such missions should explore the Galilean moons Europa,Ganymede, and Callisto for habitable environments whereliquid water could have supported prebiotic chemical evo-lution or life." A 2007 lettet from the NASA AstrobiologyInstitute's Executive Council to the previous Europa Ex-plorer SDT reaffirms that a Europa orbiter mission "is inits highest priority mission category for advancing the as-trobiological goals of solar system exploration."

The exploration of the Jupiter system and Europa is simi-larly a high priority of ESA s Cosmic Vision strategic docu-ment (ESA, 2005). Key questions to be addressed include(l) What are the conditions for planet formation and theemergence of life? This question includes the subtopic "Lifeand habitability in the solar system," and the goal "Explorein situthe surface and subsurface of solid bodies in the so-lar system most likely to host

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or have hosted -

life."(2) How does the solar system work? This includes thesubtopic "The giant planets and their environments," andthe goal "Study Jupiter in situ, its atmosphere and internalstructure."

If an orbital mission finds that Europa contains a habit-able environment today, with active communication be-tween subsurface water and the near surface, then a EuropaAstrobiology Lander has been recommended as an impor-tant next step in the satellite's exploration (NASA, 2006).A Europa orbiter would feed forward to a future landedmission.

All the above recommendations are consistent with theNASA Vision for Space Exploration (NAS.A, 2004), whichplaces high priority on robotic exploration across the solarsystem, "In particular, to explore Jupiter's moons ' . . tosearch for evidence of life (and) to understand the historyof the solar system . . ."

There are many high-priority targets for exploration inour solar system, each offering potential for rich science

return. Europa continues to top the priority list for the outersolar system because of its scientifrc potential, especiallyrelated to habitability. The scientific foundation for a mis-sion to Europa has been clearly laid.

2. SCIENCE BACKGROUND

Although scientific studies of Europa predate the spaceage, the understanding ofthe satellite has increased greatlyin the past dozen years since the Galileo mission. The fol-lowing summarizes current knowledge for Europa, outlinesthe broad cross-cutting themes including habitability andplanetary processes, and notes the outstanding science is-sues to be addressed with new data.

2.1. Habitability

Europa's probable subsurface ocean has profound im-plications in the search for past or present life beyond Earth(see chapter by Hand et al.). Coupled with the discoveryof active microbial life in harsh terrestrial environments(Rothschild and MancineUi, 2001)' Europa takes on newimportance in searching for habitable worlds. Life as weknow it depends upon liquid water, a photo- or chemical-energy source, complex organics, and inorganic compoundsof N, R S, and Fe, and various trace elements. Europa ap-pears to meet these requirements and is distinguished bypotentially enormous volumes of liquid water and geologicactivity that promotes the exchange of surface materialswith the subice environment (see chapters by Moore andHussmann and by Vance and Goodman)'

Life on Earth occupies niches supplied by either chem-ical or solar energy. Europa's ocean has likely persistedfrom close to the origin of the jovian system to the present(Cassen et at., 1982), although its chemical characteristicslikely evolved (McKinnon and Zolens,2003; see alsochapter by Zolotov and Kargel). Inferences from its youngsurface and models suggest that an ocean and hydrother-mal system may lie beneath a sheet of ice a few to tens ofkilometers thick (Greeley et a1.,2004). Tidal deformationmay drive heating and geologic activity within Europa, andthere could be brine pockets within the ice, partial meltzones, and clathrates. Hydrothermal systems driven by tidalheating or volcanic activity could serve as a favorable en-vironment for prebiotic chemistry or sustaining microbialchemotrophic organisms.

Cycling of water through and within the icy shell, ocean,and permeable upper rocky mantle could maintain an oceanrich with oxidants and reductants necessary for life. In or-der to address this aspect of Europa's habitability a betterunderstanding of the mantle and icy shell is needed.

Radiolytic chemistry on the surface is responsible for theproduction of Or, H2Or, CO2, SO2, SOa, and other yet tobe discovered oxidants (see chapter by Johnson et al.)' Atpresent, mechanisms ancl timescales for delivery of thesetut"tiult to the subsurlace are poorly constrained. Similarly'cycling of ocean water through seaflool minerals couldrptenish the water with biologically useful reductants' If

659

much of the tidal energy dissipation occurs in the mantle(see chapter by Moore and Hussman), therl.tbgre could besignificnt cycling between the ocean water and rockymantle. Conversely, if most of the tidal dissipation occursin the icy shell, then the ocean water could be depleted inthe reductants needed for biochemistry. Chemical cyclingofenergy on Europa is arguably the greatest uncertainty inour ability to assess Europa's habitability.

Although it is not known if life existed or persists todayon Europa with available information, it is possible throughnew spacecraft data to determine if extant conditions arecapable of supporting organisms. Key to this question is theoccurence of liquid water beneath the icy surface andwhether the geologic and geophysical properties can sup-port the synthesis of organic compounds and provide theenergy and nutrients needed to sustain life.

2,2. Ocean and Interior

Europa's surface suggests recently active processes op-erating in the icy shell. Jupiter raises gravitational tides onEuropa, which contribute to thermal energy in the icy shelland roc interior (Ojakangas and Stevensor, 1989; seechapters by Sotin et al., Schubert et al., and Goodman andVance), produce near-surface stresses responsible for somesurface features, and may drive currents in the ocean. Al-though little is known about the internal structure, mostmodels include an outer icy shell underlain by liquid wa-ter, a silicate mantle, and iron-rich core (Anderson et al.,1998a). Means to constrain these models include measure-ments of the gravitational and magnetic fields, topographicshape, and rotational state of Europa, each of which in-cludes steady-state and time-dependent components. Addi-tionall the surface heat flux and local thermal anomaliesmay yield constraints on internal heat production and ac-tivity. Results can be used to characterize the ocean and theoverlying icy shell and provide constraints on the deep in-terior structure and processes.

2.2.1. Gravity. Observations of the gravitational fieldprovide information on the interior mass distribution. Fora spherically symmetric body, all points on the surfacewould have the same gravitational acceleration, while inthose regions with more mass, gravity will be greater. Lat-eral variations in the field strcngth thus indicate lateral varia-tions in density structure. Within Europa, principal sourcesof static gravity anomalies can be due to thiclaress varia-tions of the icy shell, or topography on the.ocean floor. Ifthe icy shell is isostatically compensated, it will only yieldvery small gravity signatures. Gravity anomalies that are notspatially coherent with surface topography are presumed toarise from greater depths.

One of the most diagnostic gravitational features is theamplitude and phase of the time-dependent signal due totidal deformation (Fig. l). The forcing from Jupiter is wellknown, and the rcsponse will be much larger if a fluid layerdecouples the ice from the interior, permitting unambigu-ous detection of an ocean and characterization of the oceanand icy shell. With the surface ice decoupled fiom the rocky

GreeLey et al.: Future Exploration of Europa

Apojove

PerijoveE:qxl

-1.0 -0.5 0.0 0.5 1.0Normalized

Gravitational Potential

Fig.1. See Plate 35. Europa experiences a time-varying gravi-tational potential field in its eccentric orbit (eccentricity = .6994r,with a 3.551-day (l eurosol) period. Its tidal amplitude varies pro-portionally to the gravitational potential, causing Europa to flex asit orbits. This yiew shows the north pole of Jupiter as Europa orbitscounterclockwise with its prime meridian pointed toward Jupiter.Measuring the varying gravity field and tidal amplitude simulta-neously allows the interior rigidity structure of Europa to be de-rived, revealing the properties of its ocean and icy shell (Mooreand Schubert, 2000).

interior, the amplitude of the semidiurnal tide on Europa is-30 m, vs. -l m in the absence of an ocean (Moore andSchubert,2000).

2.2.2. Topography. At long wavelengths (hemispheric-scale), topography is mainly a response to tides and thick-ness variations ofthe icy shell driven by tidal heating (O7a-kangas and Stevenson, 1989; see chapter by Nimmo andManga), and is thus diagnostic of internal tidal processes.At intermediate wavelengths (hundreds of kilometers), to-pographic amplitudes and correlation with gravity are di-agnostic of the density and thickness of the icy shell. At

T660 Europa

the shortest wavelengths (kilometer-scale), small geologicfeatures tend to have topography diagnostic of formationalprocesses.

2.2.3. Rotation. Tidal dissipation probably drives Eu-ropa's rotation into equilibrium, with implications for boththe direction and rate of rotation (see chapter by Bills etal.). The mean rotation period should nearly match the meanorbital period, so that the sub-Jupiter point will librate inlongitude, with an amplitude equal to twice the orbital ec-centricity. If the body behaves rigidly, the expected ampli-tude of this forced libration should be

-100 m (Comstockand Bills,2003), but if the icy shell is mechanically de-coupled from the silicate interior, then the libration couldbe three times larger.

Similar forced librations in latitude are due to the finiteobliquity, and are also diagnostic of internal structure. Thespin pole is expected to occupy a Cassini state (Peale, 1976),similar to that of Earth's Moon. The gravitational torqueexerted by Jupiter on Europa will cause Europa's spin poleto precess about the orbit pole, while the orbit pole in turnprecesses about Jupiter's spin pole, with all three axesremaining coplanar. The obliquity required for Europa toachieve this state is -0.1", but depends upon the momentsof inertia, and is thus diagnostic of internal density struc-ture (Bil/s, 2005).

2.2.4. Magnetic field. Magnetic fields interact withconducting matter at scales ranging from atomic to galac-tic, and are produced when currents flow in response toelectric potential differences between interacting conduct-ing fluids or solids. Many planets generate stable magneticfields in convecting cores or inner shells through dynamospowered by internal heat or gravitational settling of theinterior. Europa does not generate its own magnetic field,suggesting that its corc has either frozen or is still fluid butnot convecting.

Europa, however', responds to the lotating magnetic fieldof Jupiter through electromagnetic induction (Khurana etal., 1998; see chapter by Khurana et al.). In this process,eddy currents are generated on the surface of a conductorto shield its interior from changing external fields. The eddycurrents generate their own magnetic field (the inductionfield) external to the conductor, as measured by a magne-tometer.

The induction technique exploits the fact that the primaryalternating magnetic field at Europa is provided by Jupiterbecause its rotation and magnetic dipole axes are notaligned. It is now believed that the induction signal seen inGalileo data arises within a subsurface ocean. The measuredsignal remained in phase with the primary field of Jupiter(Kivelson et aL., 2000), thus unambiguously proving thatthe perturbation signal is a response to Jupiter's field.

Mocleling the measurecl induction signal, although in-dicative of an ocean, suffers from nonuniqueness in thederived palameters because of the limited data from Gaileo,forcing certain assumptions. Nevertheless, the analysis ofZimtner et al. (2000) reveals that the putative ocean must

have a conductivity of at least 0.06 S/m. Recently, Schillinget al. (2004) determined the ratio of induction field to pri-mary field at 0.96 + 0.3, leading Hand and Chyba (2007)to infer that the icy shell is 6 S/m.

To determine the ocean thickness and conductivity, mag-netic sounding of the ocean at multiple frequencies is re-quired. The depth to which electromagnetic waves penetrateis inversely proportional to the square root of its frequency.Thus, longer-period wave sound deeper and could provideinformation on the ocean's thickness, the mantle, and themetallic core.

For Europa, the two dominant frequencies are those ofJupiter's synodic rotation period (-l I h) and Europa's or-bital period (-85 h). Observing the induction response atthese frequencies could allow determination of both theocean thickness and the conductivity (Fig. a of chapter byKhurana et al.).

Remaining key questions to be addressed regardingEuropa's ocean, bulk properties of the icy shell, and deeperinterior include the following: (l) Does Europa undoubt-edly have a subsurface ocean? (2) What are the salinity andthickness of Europa's ocean? (3) Does Europa exhibit ki-lometer-scale variations in the thickness of its icy shell?(4) Does Europa have a nonzero obliquity and if so, whatcontrols it? (5) Does Europa possess an lo-like mantle?

2.3. Icy Shell

Understanding the intemal structure of the icy shell isessential for assessing the processes that connect the oceanto the surface (see chapters by Nimmo and Manga and byBlankenship et al.). The structure and composition of thesurface result from various geologic processes and includesmaterial transport and chemical exchange through the shell.The icy shell may have experienced one or more episodesof thickening and thinning, directly exchanging materialwith the ocean at its base. Thermal processing could alsoalter the internal structure of the shell through convectionor local melting. Exogenic processes such as cratering in-fluence the surface and deeper structure.

2.3.1. Thermal processing. The thermal structure ofthe icy shell is governed prirnarily by heat from the inte-rior (see chapters by Moore and Hussman and by Baru andShowman). Regardless of the properties of the shell or heattransport, the uppermost several kilometers is thermallyconductive, cold, and stiff. The thickness of this conductive"lid" is set by the total amount of heat that must be trans-ported, and thus measurement of the thickness of the blittleshell is a constraint on interior heat production. Convectiveinstabilities can result in thermal variations in the shell thatmay be associated with surface features with scales of I kmto hundreds of kilometers. When warm, relatively pure icediapirs from the interior approach the surface, they may befar from the pure-ice melting point, but may be above theeutectic of materials trapped in the lid. This could melt

.

parts of the shell above the flattening diapir (Fig. 2). Thehorizon associated with the melt would provide a meas-ure of the conductive layer thickness. Other sources of localheat such as friction on faults may lead to similar melting(Gaidos and Nimmo, 2000),

2.3,2. Ice-ocean exchange. Europa's icy shell haslikely experienced phases of thickening and thinning, asthe orbital evolution alters the internal heating from tides(Hussmann and Spohn,2004). For example, the shell maythicken similar to ice that accretes beneath the ice shelvesof Antarctica where ice crystals form directly from theocean (Moore et al., L994). This model is characterized byslow accretion (freezing) or ablation (melting) on the lowerside of the icy crust (Greenberg et al., 1999). Temperaturegradients are primarily a function of ice thickness, and thetemperature profile is described by a simple diffusion equa-tion for a conducting ice layer (Chyba et al., 1998).Thelow temperature gradients at any ice-water interface, com-bined with impurities, would likely lead to structural hori-zons resulting from contrasts in ice crystal fabric and com-position.

Melt-through of thin ice probably also would lead to iceaccretion beneath the melt-features on the surface. Thisprocess will result in a sharp boundary between old ice (orrapidly frozen surface ice) and the deeper accreted ice. Theamount of accreted ice would be directly related to the timesince melt-through and could be compared with the amountexpected based on the surface age. Testing various hypoth-eses of ice-ocean material exchange requires measuring thedepth of interfaces to a resolution of a few hundred meters,and horizontal resolutions of approximately kilometers.

2.3.j. Surface and subsurface structure. Europa rep-resents a unique tectonic regime in the solar system, andthe processes controlling the distribution of strain in its icyshell are uncertain. Tectonic structures could range from

Greeley et al,: Future Explorarionofuropa 661

Fig.2. See Plate 36. Europa'sice, assuming a thick shell model:Convective diapirs could causethermal perturbations and partialmelting in the overing rigid ice.Faulting driven by tidal stresses(upper surface) could result infrictional heating. Impact struc-tures might show central refrozenmelt pools, surrounded by ejecta.

subhorizontal extensional fractures to near-vertical stike-slip features, and will produce structures associated prima-rily with faulting of fractured ice (see chapter by Kattenhomand Hurford). This also involves zones of deformationalmelt, injection of water, or preferred orientation of crystal-line fabric. Some faults may show local alteration of pre-existing sffucture including fluid inclusions or juxtapositionof dissimilar regions. There are many outstanding issuesregarding tectonic features, including correlation of subsur-face structure with surface properties (length, stratigraphicposition, height and width of the ridges) to test hypothesesfor formation of the fractures and ridges.

Extensional structures observed on Europa (e.g., graybands) may be particularly important for understanding ma-terial exchange processes (see chapter by Prockter and Pat-terson). If the analogy with tenestrial spreading centers(Pappalardo and Sullivan, 1996) is correct, band materialis newly supplied from below and may have a distinct struc-ture. Horizontal resolutions of a few hundred meters areneeded to discriminate such processes, along with the abil-ity to image structures sloping more than a few degrees.Additionall tens of meters of vertical resolution are re-quired to image near-surface melt zones.

Impact structures can reflect significant disruption of theshell. At an impact site, the ice is fractured, heated, and partsare melted, and ejecta blankets the surrounding terrain. Re-bound of the crater leads to tectonism that can include fault-ing and other subsurface structures detectable by sounding.An outstanding mystery on Europa is the process by whichcraters are erased from the surface. It may be possible tofind the subsurface signature of impacts that are no longerevident on the surfacg which would constrain ideas for theresurfacing processes.

Key questions to be addressed regarding the icy shell byfuture missions include the following: (1) What is the thick-

I662 Europaness of the icy shell? (2) What is the structure within theicy shell? (3) Do pockets of liquid water and/or brine exist?(4) Is there evidence for diapiric activity, past or present?(5) Have diapirs or "melt-through" zones provided exchangeof material between the ocean and the surface?

2.4. Composition

Surface materials may be ancient, derived from the ocean,altered by radiation, or exogenic in origin. Europa's bulkdensity and solar system models suggest the presence ofboth water and silicates. It is likely that differentiation andmixing of water with silicates and carbonaceous materialsresulted in chemical alteration and redistribution, with in-terior transport by melting and/or solid-state convection anddiapirism bringing materials to the surface. High-energyparticles from Jupiter leave imprints on the surface thatprovide clues to the exogenic environment, but can alsocomplicate understanding evolution and modification of thesurface. Moreover, surface materials can be incorporatedinto the subsurface and react with the ocean, or can besputtered from the surface to form Europa's tenuous atmo-sphere. Thus, characterizing surface composition and chem-istry provides fundamental information on the propertiesand habitability of Europa (see chapters by Carlson et al.and Zolotov and Kargel).

2.4.1. Ice and non-ice composition. Telescopic obser-vations and spacecraft data (e.g., Kuipe4 1957; Moroa,1965; Clark and McCord, 1980; Dalton 2000; McCord,2000; Spencer et a1.,2005) show that Europa's surface iscomposed primarily of crystalline and amorphous water ice(Pitcher et al., 19'72; Clark and McCord, 1980; Hansen andMcCord,2004). The dark, non-icy materials on the surfacehelp unravel the geological history, and determining theircomposition is the key to understanding their origin. Spa-tial distributions and context provide clues to surface pro-cesses and the connections to the interior, Understandingthis linkage provides constraints on the nature of the inte-rior, potential habitability, and processes and timescalesthrough which interior materials reach the surface. Com-positional variations in surface materials may reflect agedifferences indicative of recent activity, while the discoveryof active vents or plumes would demonstrate current con-nections with the subsurface.

Non-ice components include CO2, SO2, H2O2, and 02based on comparison with laboratory spectra of the rclevantcompounds (Lane et aI., l98l; Noll et al., 1995; Smythe etaI., 1998; Carlson, 1999,2001; Carlson et aI., 1999a,b;Spencer and Calvin, 2002; Hansen and McCord,2008).Spectral observations fi'om the Galileo Near Infrared Map-ping Spectrometer (NIMS) of disrupted dark and chaotictenain indicate water bound in non-ice hydrates. Hydratedmaterials observed in regions of surface disruption couldbe magnesium and sodium sulfates that originate from sub-surface ocean brines (McCord et al., 1998b, 1999). Alter-natively, they may be sulfuric acid hydrates created byradiolysis of sulfur from Io, processing of endogenic SO2,

or from ocean-derived sulfates or other S-bearing species(Carlson et al., 1999b,2002,2005).It is also possible thatthese surfaces have a combination of hydrated sulfate saltsand sulfuric acid (Da\ton,2000,2007; McCord et a1.,2001,2002; Carlson et a1.,2005; Orlando et a1.,2005; Dalton eta1.,2005). Thus, an important objective is to resolve thecompositions and origins of the hydrated materials.

Earth-based telescopes detected sulfur species thoughtto be linked to effects of Jupiter's magnetosphere (e.g., NoIlet al., 1995). Brown and HiIl (1996) first reported a cloudof sodium around Europa, and B'own (2001) found a cloudof potassium and reported that the Na/K ratio could reflectendogenic sputtering.

A broad suite of additional compounds is predicted forEuropa based on observations of other icy satellites, as wellas from experiments with irradiated ices, theoretical simu-lations, and geochemical and cosmochemical arguments.Organic molecular groups, such as CH and CN, occur onthe other icy satellites (McCord et aI., 1997,1998a), andtheir presence or absence on Europa is important to under-standing potential habitability. Other compounds that maybe detected by high-resolution spectroscopy include HrS,OCS, 03, HCHO, H2CO3, SOr, MgSOo, H2SO4, H3O+,NaSOo, HCOOH, CH3OH, CH3COOH, and more complexspecies (Moore, 1984; Delits and Lane, 1997, 1998;Hudson and Moore, 1998; Moore and Hudson,2003; Bru-netto et a1.,2005; see also chapter by Zolotov and Kargel).

As molecules become more complex their radiationcross-section increases and they are more susceptible toalteration by radiation. Radiolysis and photolysis can alterthe original materials and ptoduce highly oxidized speciesthat react with other non-ice materials to form a wide ar-ray of compounds. Given the extreme radiation environmentof Europa (see chapter by Paranicas et al.), organic mol-ecules or molecular fragments are not expected in olderdeposits nor in those exposed to greater radiation (Johnsonand Quickenden, 1997 Cooper et a1.,2001). They might,however, survive in younger deposits or in regions of lesserradiation.

Improved spectral observations over broad ranges andhigh spectral and spatial resolution, together with labora-tory studies, are needed to understand Europa's surfacechemistry. These data will provide major improvements inthe identification of the original and derived compounds,radiation environment, and associated reaction pathways.

2.4.2. Relationship of composition to processes. Gali-leo's instruments wele designed to study surface composi-tions on regional scales. The association of hydrated anddark materials with certain geologic terrains suggests anendogenic source for the emplaced materials, although thesemay have been altered by radiolysis. Many surface featureswith compositionally distinct materials appeil to have beenformed by tectonic processes, suggesting that the associ-ated materials are derived from the stbsurface.

Major open questions include the links between surfacecomposition and the undellying ocean and rocky interior(Fanale et al., 1999; Kargel et al., 2000 McKinnon and

TI

I

Fig.3. Europa's diverse surface shows different styles ofdefor-mation, which provide clues to its geology, possible connectonsto tidal processes, and the subsurface ocean. The Galileo NIMSfootprint (box) sampled and "mixed" multiple tenain types.

Zolens,2003; see chapter by Zolotov and Kargel), andthe relative significance of radiolytic processing (Johnsonand Quickenden, 1997; Cooper et al.,200lt Carlson et al.,2002,2005). To test these hypotheses, compositional dataare required at scales suffrcient to resolve geologic features.One of the critical limitations of NIMS data is the low spa-tial resolution of the high-quality spectra and the limitedspatial coverage. The spectra to identify hydrated materialswere typically averaged from areas

-75 x 75 km (McCordet al., 1998b' Carlson et al,, 1999b) (although a few higher-resolution "postage stamp" datasets were obtained). Thistypical footprint is shown in Fig. 3, illustrating the problemof "mixing" of terrains. Future observations must resolvenon-ice materials at

-100-m scales. In addition, sampling awide range of latitudes and longitudes is needed to under-stand global effects such as implantation, temperature de-pendence, and surface ages. Ultraviolet to IR spectroscopyis needed to identify organic, ice, non-ice, and radiolyticallygenerated materials. Such data, together with images, canprovide the spatial conelations necessary to develop modelsfor the origin and history of the surface.

In addition to compositional differences associated withrecent geological activity, changes related to exposure agewill also provide evidence for sites of recent or curentactivity. The composition of even the icy parts of Europais variable in space and time. Polar fine-grained depositssuggest frosts formed from ice sputtered or sublimated fromother areas (Clark et al., 1983; Dalton,2000; Hansen andMcCord,2004). Equatorial ice regions are more amorphousthan crystalline, perhaps due to radiation damage, and map-ping ice crystallinity might be used to assess relative age orradiation dose. Venting or transient gaseous activity on Eu-ropa would indicate present-day surface activity, and couldbe detected by UV IR, or millimeter spectroscopy, similarto those on Enceladus (Porco et a1.,2006; Spencer et al.,2006; Hansen et a1.,2006;Waite et a1.,2006).If a subsur-

Greeley et al.: Future Exploration of Europa 663

face ocean is present and outgases through fissures, it mightresult in transient activity, and its composition could pro-vide clues to ocean composition.

Exogenic processes are also important, and much isunknown on the chemistry and sources of implnted mate-rials. Magnetic field measurements of ion-cyclotron wavesin the wake of Europa provide evidence of sputtered andrecently ionized Cl, 02, SO2, and Na ions (Volwerk et al.,2001). Medium-energy ions (tens to hundreds of keV) de-posit energy in the upper tens of micrometers; heavier ions,such as those of oxygen and sulfur, have an even shorterdepth of penetration, while MeV electrons can penetrate andaffect the ice to a depth of more than I m (see chapters byJohnson et al. and Paranicas et al.). The energy ofthese par-ticles breaks bonds to sputter water molecules, molecularoxygen, and impurities within the ice (Cheng et at., 1986),producing the observed atmosphere and contributing to sur-face erosion.

A major issue is the exogenic vs. endogenic origin ofvolatiles such as CO, and their behavior in time and space.CO, was reported on Callisto and Ganymede, with hints ofCOr(McCord et al., 1998a), SOr(Smythe et al., l99B), andH2O2 (Carlson et al., 1999b). Recent analyses of NIMSspectra indicate the concentration of CO, and other non-ice compounds on the antijovian and trailing sides of Eu-ropa (Hansen and McCord,2008), suggesting an endogenicorigin. Radiolysis of CO2 and HrO ices is expected to pro-duce additional compounds (Moore, l9B4; Delits andLane, 1997,1998; Moore and Hudson,2003; Brunetto etal., 2005). Determining the presence and source of organiccompounds, such as CH and CN groups detected by IRspectroscopy at Callisto and Ganymede (McCord et aI.,1997, 1998b) and tentatively idenrified on Phoebe (Ctarket al., 2005), would be important for evaluating the astro-biological potential of Europa, especially if there is demon-strable association with the ocean.

Some surface constituents are directly exogenic. Forexample, Io's volcanos release SO, that is dissociated andionized, accelerated by Jupiter's magnetic field, and im-planted in Europa's ice. Once there it can form new mol-ecules and some of the dark surface components. It is im-portant to separate surface materials formed by implantationfrom those that are endogenic. For example, the detectedNa/K ratio is supportive of an endogenic origin

-

andperhaps an ocean source

-

for Na and K (Brown, 2001;Johnson et a1.,2002; McCord et a1.,2002: Orlando et al.,2005).

The relative importance of endogenic vs. exogenicsources of non-ice constituents depends on factors such asthe radiation environment. As a result, detailed analysis ofspectral observations of disrupted tenain on the leading andtrailing hemisphercs, which encounter far different radi-olytic fluxes, would help to determine radiation effects andunravel the endogenic history.

Some key outstanding questions to be addressed regard-ing Europa's chemistry and composition include the follow-ing: (l) Are endogenic organic materials on the surface?

-.

T664 Europa

(2) Is chemical material from depth carried to the surface?(3) Is iradiation the principal cause of alteration of Eu-ropa's surface materials? (4) Do materials formed from ionimplantation play a major role in surface chemistry?

2,5. Geology

Europa's surface is geologically young, and parts maybe active today (see chapter by Bierhaus et al.). This youthis inherently linked to the ocean and the effects of gravita-tional tides, which trigger processes that include fracturingof the icy shell, resurfacing, and possibly release of mate-rials from the interior. Clues to these and other processesare provided by features such as linear fractures and ridges(see chapters by Kattenhorn and Hurford and by Prockterand Patterson), chaotic tenain (see chapter by Collins andNimmo), and impact craters (see chapter by Schenk andTurtle).

2.5.1. Linear features. Europa's unusual surface isdominated by tectonic features in the form of linear ridges,bands, and fractures (Fig. a), Ridges are common and ap-pear to have formed throughout the visible history. Theyrange from 0.1 to >500 km long, are as wide as 2 km, andcan be several hundred meters high. Ridges include simplestructures, double ridges separated by a trough, and inter-twining ridge-complexes. Whether these represent differentprocesses or stages of the same process is unknown. Cyc-loidal ridges are similar to double ridges, but form chainsof linked arcs.

Fig. 4. Volcanic plumes on Io imaged by New Horizons in2007.The 29O-km-high plume from the polar volcano Tvashtar is seenat the top, while the plume from Prometheus is on the left. Be-ginning with Voyager discoveries, Prometheus has been activeduring all spacecraft flybys. Long-term observations and flybyswith JEO will provide unprecedented detail on Io's active volca-nism.

Most models of linear feature formation include fractur-ing in response to processes within the icy shell (Greeleyet a1.,2004). Some models suggest that liquid oceanicmaterial or warm mobile subsurface ice squeezes throughfractures to form the ridge, while others suggest that ridgesform by frictional heating and possibly melting along frac-ture shear zones. Thus, ridges might represent regions ofcommunication among the surface, icy shell, and ocean,plausibly providing a means for surface oxidants to enterthe ocean. Some features, such as cycloidal ridges, appearto be a direct result of Europa's tidal cycle (Hoppa et al.,1999).

Bands reflect fracturing and lithospheric separation,much like seafloor spreading on Earth, and most displaybilateral symmetry (e.g. Sullivan et al., 1998). The young-est bands tend to be dark, while older bands are bright,suggesting brightening with time. Geometric reconstructionof bands suggests that a spreading model is appropriate,indicating extension in these areas and possible contact withthe ocean (Tufts et a1.,2000l, Prockter et a1.,2002).

Fractures are narrow (hundreds of meters to the -10-mlimit of image resolution) and can exceed 1000 km inlength. Some fractures cut across nearly all surface features,indicating that the icy shell is subject to deformation on themost recent timescales. The youngest ridges and fracturescould be active today in response to tidal flexing. Subsur-face sounding could help identify zones of warm ice coin-ciding with current or recent activity. Young ridges may beplaces where the ocean has recently exchanged materialwith the surface, and would be prime targets as potentialhabitable niches.

2.5.2. Chaotic terrain. Europa's surface has been dis-rupted into circular lenticulae and inegularly shaped chaoszones (see chapter by Collins and Nimmo). Lenticulae in-clude pits, spots of dark material, and domes where thesurface is upwarped and commonly broken. Pappalardo etal. (1998) argued that these features are typically -10 kmacross, and possibly formed by upwelling of composition-ally or thermally buoyant ice diapirs through the icy shell.In such a case, their size distribution would imply the thick-ness of the icy shell to be at least 10-20 km at the time offormation (McKinnon, I 999). An alternative model suggeststhat there is no dominant size and that lenticulae are smallmembers of chaos (Greenberg et aI., 1999), formed througheither direct (melting) or indirect (convection) communi-cation between the ocean and surface (e.9., Carr et al.,1998a).

Chaos is characterized as fractured plates of ice shiftedinto new positions within a matrix. Much like a jigsaw puz-zle, many plates can be fit back together. Some ice blocksappear to have disaggregated and foundered into the sur-rounding finer-textured matrix, while other chaos areasstand higher than the surounding terain. Models of chaosformation suggest whole or partial melting of the icy shell,perhaps enhanced by local pockets of brine (Head and Pap-palardo, 1999). Chaos and lenticulae commonly have as-sociated dark reddish material thought to be derived fromthe subsurface, possibly from the ocean. However, these and

related models are poorly constrained because the total en-ergy partitioning within Europa is not known, nor are de-tails of the composition of non-ice components. Imaging,subsurface sounding, and topographic mapping are requiredto understand the formation of chaotic tenain and its im-plications for habitability.

2.5.3. Impactfeatures. Only 24 impact craters >10 kmhave been identified on Europa (Schenk et a1.,2004; seechapter by Schenk and Tirrtle), reflecting the young surface.This is remarkable in comparison to Earth's Moon, whichis only slightly larger but far more heavily cratered' Theyoungest known europan crater is 24-km-diameter Pwyll,which rctains bright rays and likely formed less than 5 m.y.ago (Zahnle et al., 1998; see chapter by Bierhaus et al.).Complete global imaging will allow a more comprehensivedetermination of Europa's surface age and help identify thevery youngest areas.

Crater morphology provides insight into ice thickness atthe time of impact. Morphologies vary from bowl-shapeddepressions with crisp rims, to shallow depressions withsmaller depth-to-diameter ratios. Craters up to 25-30 kmin diameter have morphologies consistent with formationin a warm but solid icy shell, while the two largest impacts(Tlre and Callanish) might have punched through brittle iceabout 20 km deep into a liquid zone (Moore et al., 2001;Schenk et al.,2004).

2.5.4. Geologic history. Determining the geologic his-tories of planetary surfaces requires identifying and map-ping surface units and stluctules and placing them into atime sequence. In the absence ofabsolute ages derived fromrock samples, planetary surface ages are assessed from im-pact crater distributions, with more heavily cratered regionsreflecting greater ages. The paucity of impact craters onEuropa precludes this technique. Thus, superposition (i.e.,younger materials seen "on top" of older materials) andcross-cutting relations are used to assess sequences of for-mation (Figueredo and Greeley, 2004; see chapter byDoggett et al.). Unfortunately, only I}Vo of Europa has beenimaged at sufficient resolution to understand relationshipsamong surface features. For most of Europa, data are bothincomplete and disconnected from region to region, makingthe global surface history difficult to decipher. Where im-ages of suffrcient resolution (better than 200 m/pixel) ex-ist, it appears that the style ofdeformation evolved throughtime from ridge and band formation to chaotic terrain(Greeley et aL,2004), although there are huge areas of thesurface where this sequence is uncertain (e.g., Riley et aL.,2000). Europa's surface features generally brighten andbecome less rcd through time, so albedo and color can selveas a proxy for age (Geissler et al., 1998)'

Quantitative topographic data can provide informationon the origin of geologic featules and may show trends withage. Profiles across ridges, bands, and disrupted terrains willaid in constraining rnodels of origin. Moreover, flexuralsignatures rc expected to be indicative of local elastic litho-sphere thickness at the time of their formation, and mayprovide evidence of topographic relaxation (e'g., Nimmo eta1., 2003; Billings and Kattenhorn, 2005).

Greeley et al.: Future Exploration of Europa 665

2.5.5. Innding site characterization, Landers are iden-tified as priority missions if Europa has habitable environ-ments. Landed missions would require high-resolution im-ages (approximately a few meters per pixel or better) forlanding site selection. The roughness and overall safety oflanding sites can also be characterized through radar data,photometry, thermal inertia, and detailed altimetry. Suchdata will also illuminate fine-scale regolith and other sur-face processes (see chapter by Moore et al.). Along withcorresponding high-resolution subsurface sounding, thesedata would help assess likely sites of recent communica-tion with the ocean.

Some outstanding questions for Europa's geology include(l) Do Europa's ridges, bands, chaos, and/or multiringedstructures require the near-surface liquid water to form?(2) V/here are the youngest regions? (3) Is cunent geologicactivity sufficiently intense that heat flow from the interioris measurable? (4) What is the overall history of the surface?

2.6. Jupiter System

Europa cannot be understood in isolation, but must beconsidered in the context of the entire jovian system' Europaformed from the jovian nebula and evolved through com-plex interactions with the other satellites, Jupiter, and Ju-piter's magnetosphere (e.g., see chapters by Canup andWard and Estrada et al.). To understand the developmentof potential habital environments, knowledge is needed forthe origin and evolution of the jovian system, and how thesystem curently operates. This requires observations ofJu-piter and the satellites' magnetosphere and ring system'

2.6.I. Satellite surfaces and interiors, The present en-vironment of Europa depends partly on how it formed andevolved. Europa itselfdoes not record its early surface his-tory, but its neighboring satellites

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Io, Ganymede, andCallisto

-

provide clues to Europa's origin, evolution, andpotential habitability, and are interesting on their own.

2.6.1 .1. Io. The innermost of the Galilean satellites ex-periences intense tidally driven volcanism (Fig. a) and shedslight on Europa's tidal heat engine. Io also provides cluesto Europa's silicate interior and could be a major source ofcontamination on Europa. Io's density suggests a primarilysilicate interior (Mcwen et al., 2004) while the 4:2:l La-place resonance among Io, Europa, and Ganymede as theyorbit Jupiter leads to tidal flexing and generation of the heatfor global volcanism (Yoder and Peale, 1981; see chapterby Sotin et al.).

Galileo data indicate extensive moon-plasma interactionsnear Io but appear to rule out a magnetic field. Io's mo-ment of inertia suggests that it is differentiated into a me-tallic core and silicate mantle (Anderson et a1.,2001).Theinfered Fe-FeS core has a radius slightly less than half ofIo and a mass 20Vo of the satellite. The apparent lack of amagnetic field suggests that the silicate mantle experiencessufficient tidal heating to prevent cooling and a convectivedynamo in the core (Weinbruch and Spohn, 1995).

Io's mantle may undetgo partial meltin g (Moore, 2001)that produces mafic to ultrarnafic lavas, suggesting an un-

666 Europa

differentiated mantle. Silicate volcanism appers to be dom-inant, although secondary sulfur volcanism may occur lo-cally (Greeley et al., 1984). The heat flux infened fromlong-term thermal monitoring exceeds 2W/nP, making Ioby far the most volcanically active body in the solar sys-tem (Nasir et al., 1986i Veeder et al., 2004; McEwen et al.,2004; Lopes and Spence4 2007).

Despite the high heat flux, mountains as high as l8 kmindicate that the lithosphere is at least 20-30 km thick, rigid,and composed mostly of silicates (e.g., Carr et al., 1998b;Schenk and Bulmen 7998;Turtle et a1.,2001; Jaeger et al.,2003). The thick lithosphere can only conduct a small frac-tion of lo's total heat flux, suggesting magmatic transportof heat through the lithosphere (O'ReiIly and Davies, l98l;Carr et al., 1998b; Moore,200l).

Silicate lavas, sulfur, and sulfur dioxide on Io interactin complex and intimate ways, with volcanism that includesmassive lava eruptions, high-temperature explosions, andoverturning lava lakes. Volcanic plumes erupt from centralvents and along lava flow fronts where surface volatiles aremobilized. Volcanism and sputtering on Io feed a uniquepatchy and variable atmosphere, in which S, O, and Na be-come ionized to form Io's plasma torus, neutral clouds, andaurorae. Sublimation of SO2 frost is also a source of lo'sthin atmosphere but the relative contributions to the atmos-phere are not well understood. Electrical currents flow be-tween Io and Jupiter and produces auroral "footprints" inthe jovian atmosphere. Near the ionospheric end of the Ioflux tube, accelerated electrons interact with the jovian mag-netic field and generate decametric radio emissions (Lopesand Williams. 2005).

There is an apparent paradox between Io's putative ul-tramafic volcanism and the widespread intensity of thevolcanism on Io. At the current rate, Io would have pro-duced a volume of lava

-40 times the volume of Io overthe last 4.5 G.y., resulting in differentiation and consequenteruption of more silicic materials. The resolution of thisparadox requires either that Io only recently entered the tidall'esonance and became volcanically active, or that whole-scale recycling of Io's lithosphere is sufficient to preventextreme differentiation (McEw en et al., 2004).

JEO could improve knowledge of Io in several rcspects.For example, Galileo studies of Io's dynamic processes werehampered by the low data rate and major volcanic eventswere missed entirely or seen only in disconnected snap-shots. JEO would provide a 100-fold increase in data returnper Io flyby compared to Galileo, and much mote long-termmonitoring, which is likely to rcveal phenomena not seenpreviously. Moreover, JEO's superior instruments would al-low new investigations, such as high-spatial-resolution spec-troscopy of lava flows and in situ sampling of its upper at-mosphere and plumes.

JEO objectives fol lo include (1) understanding lo's heatbalance and tidal dissipation, and their relationship to Eu-ropa's tidal evolution; (2) monitoring active volcanos andtheir effect on the surface and atmosphere; (3) determiningrelationships among volcanism, tectonism, erosion, and dep-

osition; and (4) understanding the silicate and volatile com-ponents of lo's crust. Because Io is a dominant source ofplasma for the jovian magnetosphere, measurements oftrace ion composition in the Io torus and throughout themagnetosphere may reveal details of the internal composi-tion. Knowledge of the composition of material escapingfrom Io will help distinguish endogenic from Io-derivedmaterials on Europa.

Additional gravity data during flybys would place morestringent constraints n interior structure. New discoveriesare likely, such as gravity anomalies similar to those de-tected by Galileo for Ganymede from a flyby (Palguta eta\.,2006). Determination of Io's pole position and changesin the location of the pole would be valuable as constraintson the satellite's shape and thus internal structure. Heat flowdeterminations would place important constraints on theo-ries of tidal dissipation, internal structure, and thermal andorbital evolution.

2.6.1.2. Ganymede. Ganymede is our largest satellite,exceeding Mercury in diameter, and is the only satelliteknown to have an intrinsic magnetic field. Its surface isbroadly separated into bright and dark terains (Shoemakeret al., 1982; McKinnon and Parmentier 1986; Pappalardoet al., 2004). Dark temain overs one-third of the surfaceand is dominated by impact craters. It is ancient, and ap-pears grossly similar to the surface of Callisto (Prockter etal., 1998). Dark terrain also displays hemisphere-scale con-centric funows, which are probably remnants of vast multi-ring impact basins.

Bright terrain forms a global network of intelconnectedlanes, separating dark tenain into polygons, and has a patch-work of smooth surfaces and closely spaced parallel ridgesand grooves (Fig. 5). The grooves are extensional tectonicfeatures, and have much in common with terrestrial riftzones (Parmentier et al., 1982; Pappalardo et al., 1998).

Ganymede's surface is dominated by water ice (McKin-non and Parmentier, 1986). The polar "caps" appear to fol-low the magnetospheric boundary between open and closedfield lines (Khurana et a1.,2007), which provides an oppor-tunity to examine differences in space weathering underdifferent conditions. Dark non-ice materials at lower lati-tudes could be hydrated brines similar to those infened forEuropa; other minor constituents include CO2, SO2, andsome sort of tholin material exhibiting CH and CN bonds(McCord et al., 1998b). There is also evidence for trappedO, and O, in the surface, as well as a thin molecular oxy-gen atmosphere, and auroral emissions are concentratednear the polar cap boundaries (McGrath et al., 2004),butthere are no ionospheric indications frorn Galileo radio oc-cultation data of an equatorial atmosphere.

Galileo data indicate that Ganymede's moment of iner-tia is 0.31 MR2, which is the smallest measuled for any solidbody in the solar system (Anderson et a1.,1996). Three-layer models, constrained by plausible compositions, in-dicate that Ganymede is differentiated into an outermost-800-km ice layer and an underlying silicate mantle of den-sity 3000-4000 kg/m:. A central iron core is allowed, but

I

i

I

Fig.5. High-resolution (20 m/pixel) Galileo image of Gany-mede's surface; showing an area about 15 x 16 km. Figure cour-tesy of NASA/JPL/Brown University.

not required, by the gravity data. Ganymede's magneticfield, however, supports the presence of such a metalliccore. Galileo gravity data also indicate that Ganymede hasinternal mass anomalies, possibly related to topography onthe ice-rock interface or internal density contrasts (Ander-son et a1.,2004; Palguta et a1.,2006).

Galileo magnetometer data provide tentative evidence foran inductive response at Ganymede, which suggests thepresence of a salty internal ocean within 100-200 km ofthe surface. However, the inference is less robust than forEuropa and Callisto, because the data can also be explainedby an intrinsic quadrupole magnetic field (superposed onthe intrinsic dipole), whose orientation remains fixed in time(Kivelson et al., 2002).

Galileo data show that Ganymede has an intrinsic fieldstrong enough to generate a mini-magnetosphere embed-ded within the jovian magnetosphere (Fig. 6) (Kivelson etal., 1996).4 model with a fixed Ganymede-centered dipolesuperposed on the ambient jovian field provides a goodfirst-order match to the data and suggests equatoril andpolar field strengths of -7L9 and 1438 nT, respectively;these values are 6-10 times the 120-nT ambient jovian eldat Ganymede's orbit. The most plausible mechanism forgeneration of the intrinsic field is a dynamo in a liquid-ironcore (Schubert et al., 1996).

Multiple flybys of JEO would provide topographic data,subsurface sounding, and high-resolution imaging and spec-troscopy for understanding surface formation and evolution.For example, the role of volcanism in modifying the sur-faces of icy satellites is poorly understood. Like many othericy satellites, vidence is ambiguous for cryovolcanic proc-

Greeleyetal.: FutureExplorationofEuropa 667

esses on Ganymede. Given the physical constraints in erup-tion of cryovolcanic melt onto the surface (Showman et al.,2004), such deposits would give insight into the interior.Thus, it is critical to leam whether cryovolcanism is wide-spread or rare on Ganymede, with implications for its roleon other icy satellites.

With its mix of old and young terain, ancient impactbasins and fresh craters, and landscapes dominated by tec-tonism, volcanism, and degradation by space weathering,Ganymede serves as a type example for understanding icysatellites in the outer solar system and would provide in-sight into how this entire class of worlds evolves differentlyfrom terrestrial planets.

2.6.1.3. Callisto. Of the Galilean satellites, Callisto isleast affected by tidal heating, thus offering an endmemberof icy satellite evolution (McKinnon and Parmentier 1986;Showman and Malhotra, 1999: Moore et a1.,2004).

Gravity data and the assumption of hydrostatic equilib-rium suggest that Callisto's moment of inertia is 0.355 MR2,suggesting partial differentiation with an ice-rich outer layer

668 Europa

tiated, its formation time must have exceeded 106 yr (Canupand Ward, 2002; Mosqueira and Estada' 2003)'

Galileo magnetometer data indicate that Callisto has an

inductive magnetic response best explained by a salty ocean

within 200 km of the surface (Khurana et qt'' 1998; Kivel-son et aI., 1999;Zimmer et al',2000)' Maintaining an oceantoday either requires a suffrciently stiff ice rheology to stifleconvection und h"ut loss or existence of "antifreeze" (am-monia or salts) in the ocean (McKinnon,2006)' However'reconciling partial differentiation with the existence of theocean is difficult; some part of the uppermost ice layer mustremain at the melting temperature today' while the mixedice-rock layer must never have attained the melting'

Along with the discovery of Callisto's probable ocean'major dcoveries include the absence of cryovolcanic re-ruriu"ing and the inference of surface erosion by sublima-tion. Callisto's landscape at decameter scales is uniqueamong the Galilean ,ut"llit"t, and might be akin to that ofcometary nuclei.

Calliito's surface composition is bimodal (water ice andan unidentified non-ice material), with trace constituents inthe non-ice material. The color of the non-ice material issimilar to C-type asteroids and carbonaceous chondrites'Trace materials detected in the non-ice material includeCOr, C-H, CN, SO2, and possibly SH (Carlson et al" 1999al,Mc'Cord et al., 1998a)' Carbon dioxide is detected as an at-mosphere and is nonuniformly dispersed over the surface'beini concentrated on the trailing hemisphere and moreabunant in fresh impact craters (Hbbitts et al'' 2002)'Thisasymmetry is similai to that for sulfate hydrates on Europaun i, ut. suggestive of externally induced effects by co-rtating magniospir"ric plasma (Cooper et^al'' 2001)'

fhe lgOmission would enable many of the key issuesforCallistotobeaddressed,includingdeterminingthedis-tribution of impact craters; studying mass wasting; charac-terizing the jovian and Callisto magnetospheres to under-stand te

"*t.nt und depth of the globally conducting layer;

urr"ttlng the internal itttibution of mass to gain insightinto differentiation on large icy satellites; measuring ener-getic particle fluxes and energies over a long duration andiff"r"n, time periods to understand radiolysis of icy satel-lites; and tupping surface compositions to understand therelative influerrces of primordial composition, geologicalpro."tttt, and radiolysis. Results would shed light on simi-iu, pto""tt"t on the other icy satellites'

).6.2. Satettte atmospheres' Europa's tenuous atmos-phere (Figs. 5-12 in chapter by McGrath et al') is the in-ierface' between Jupiter's magnetosphere and the satellite's

surface. Composed principally of O, with a surface pres-sure of only -2x 10-lz bar (McGrath et aL' 2004; see alsochapter by McGrattr et al')' there is no widely acceptedexpianation for the nonuniform nature of the atmosphericemissions, and only a single attempt has been rnade to ad-dress this issue (Ccrssldy et al',2008)' The atmosphere ismaintained principally by ion sputtering of the surface' with

molecules subsequently dissociated and ionized by electron

impact, charge exchange, and solar photons' The abundance

and distribution of atmospheric constituents provide cluesio tu.fu"" processes and links to composition' Once releasedfrom the surface, some constituents such as Na and K aremore readily observed in their gas phase' Their abundancerelative to It provides a discriminator between endogenicunJ

"*og"ni" rigin for these species (Johnson.et.al" 2002)'

Europai atmosfhere could be in part supplied by activeg"Vt.t (Nimmi et at.,2007),the discovery of which wouldlrviOe clues to subsurfacp processes and interior structure'' B".uur" material from Io is implanted on Europa' it isimportant to understand lo's atmosphere' Ganymede andCalisto also have tenuous atmospheres, which shed lighton the evolutionary paths these satellites have followed' The

atmospheric emissions of Ganymede, for example' are remi-

nircerrt ofclassic polar auroral emissions, very different than

the case for Europa. Callisto is thought -

like Europa and

Ganymede -

to huu" a predominantly O, atmosphere' butlacks oxygen emissions as seen on Europa' Io' and Gany-nede 6rel et a1.,2002). Instead, Callisto has CO, emis-sion above the limb, detected by Galileo (Carlson' 1999)'Although IR limb scans at Europa were not performed' smallu.ou,ri, of CO, may be present in its atmospherc' by anal-ogy with Callito. iallisto's atmosphere may be thickertan either Europa's or Ganymede's (McGrath et a\" 2004;Liang et at.,2005, which is reflected by its relatively denseionospherc (Ktiore et a1.,2002)'

2.6.3. plasma and magnetospheres' The plasma ofJupiter's rapidly rotating magnetosphere overtakes the sat-elfites in tnir orbits with flow of charged particles predorni-nantly onto the trailing hemispheres' Energetic ions sput-i"t n"uouf particles from the surfaces' Many of the liberatedparri"t", immediately return to the surface' but some be-

"o*" purt of the satellite atmospheres, and so,me escape to

,pu.".^n fraction of the neutrals that are no longer boundt a moon can form a circumplanetary neutral torus (Mauket a1.,2003).

Io is the dominant source of particles in Jupiter's mag-netosphere (Thomas et a1.,2004; Nozawa et a\" 2005)'butother'moons contribute water products and minor speciesthrough atmospheric and surface interactions (Johnson etat, z0; forixample, Europa is a source of Na (Brown'2001 Leblanc et a1.,2005)'

Perturbations of the magnetospheric plasma and electro-

magnetic fields near the satellites are diagnostic of the sat-

elliLs themselves. Through such analysis, satellite-inducedmagnetic fields were detected (e'g', Kivelson et al'' 2004)'which is the key evidence for subsurface oceans' In turn'*ugn"torptt"ric particle interactions produce changes insuriu." chemistry (Johnson et al',2004)'

2.6.4. Jupiter atmosphere' Jupiter contains most of themass in the jovian ,yri"t and is the largest object in thesolar system after te Sun' Its atmospheric compositionreflects the initial nebula conclitions, albeit with significantreprocessing, from which the satellites formed (Ingersoll etoi., ZOO|; Wrrt ,t at., 2004; Taylor et aI', 2004; Moses etat., 2004; Yelle and Milten 2004)' The jovian system pro-vicles the best analog for the formation of both our own sol

system and the hundreds of exoplanetary systems beingdiscovered around other stars. JEO investigations focusedon Jupiter's atmosphere are discussed in section 3.5.4.

2.6.5. Rings, dust, and smallmoons. A systemofsmallmoons and faint rings encircles Jupiter within Io's orbit.Although Saturn's rings are more familiar, faint and dustyrings are more common in the outer solar system. Suchrings may represent the evolution of a much denser ringsystem such as Satum's. Dusty rings reveal a variety of non-gravitational processes that are masked within more mas-sive disks. For example, fine dust grains become electricallycharged by solar photons and interactions with Jupiter'splasma. Their orbits are perturbed by solar radiation pres-sure and Jupiter's magnetic field (e.g., Burns et a\.,2004).Thus, a better description of dust dynamics and proper-ties might provide information on Jupiter's plasma and mag-netic field within regions that cannot be probed easily byspacecraft.

Jupiter's rings share many of their properties with pro-toplanetary disks. In both systems, dust and larger bodiesco-mingle and interact through various processes. Thus, thering system provides a dynamic laboratory for understand-ing the formation of the broader jovian system. JEO inves-tigations of rings, dust, and small moons are described insection 3.5.5.

lVith rcgard to the Jupiter system as a whole, some re-maining key questions to be addressed include the follow-ing: (l) What factors control the different styles of volca-nism on Io? (2) Are plasma processes responsible forGanymede's bright polar caps and if so, how? (3) HasGanymede experienced cryovolcanism, or does intense tec-tonism create smooth terrains; what is the distribution andthickness of Callisto's dark component? (4) How does Eu-ropa's sputter-produced atmospherc vary? (5) Are Gany-mede's and Callisto's atmospheres produced mainly by sput-tering or sublimation? (6) How do the sources and dynamicsof the fields and plasma in the jovian magnetosphere vary,especially as correlated with Io's activity? (7) How doesjovian local atmospheric convection contribute to largerstorms?

3. JUPITER EUROPA ORBITER SCIENCEGOAL, OBJECTIVES, AND INVESTIGATIONS

The scientific objectives for JEO were formulated basedon previous strdies (Table 2) and the science as reviewedin section 2. The goal for JEO is to explore Europa to in-vestigate its habitability, which implies undersranding theorigin, evolution, and cunpnt state of the satellite. This in-cludes addressing the questions outlined above, while alsoallowing discovery science

-

unpredicted findings of thetype that have often reshaped the very foundations ofplan-etary science. "Habitability" includes confirming the exist-ence of water below Europa's icy shell and determining itscharacteristics, understanding the possible sources and cy-cling of chemical and thermal energy, investigating theevolution and composition of the surface and ocean, and

Greeley et al.: Future Exploration of Europa 669

evaluating the processes that have affected Europa throughtime.

Understanding Europa's habitability is intimately tied toinvestigating the Jupiter system as a whole. Both Ganymedeand Callisto may possess subsurface oceans, while Io holdsclues to the fundamentals of tidal heating and interactionswith the jovian environment, Jupiter can shed light on theinitial conditions of the planerary system. Each Galileansatellite can be related to the others, and is intimately tiedto Jupiter and the jovian magnetospheric environment. Asstated in the2006 Solar System Exploration Roadmap, .,Bystudying the Jupiter system as a whole, we can better un-derstand the type example for habitable planetary systemswithin and beyond our Solar System."

Within this context, hve primary objectives have beendefined for the proposed JEO mission; in priority orderthese relate to (l) Europa's ocean, (2) Europa's icy shell,(3) Europa's chemistr (4) Europa's geology, and (5) Jupi-ter system science.

In the following sections, each objective is described,along with the scientific investigations that are needed tomeet the objectives.

3.1. Ocean Objective: Characterize theOcean and Deeper Interior

The first step in characterizing Europa's ocean is to con-frm its existence and extent. If Europa has no ocean andits icy shell is coupled to its rocky mantle, then as it orbitsJupiter the measurable radial tide will vary by only a fewmeters. On the other hand, if Europa has liquid water be-neath a relatively thin icy shell, the ride will vary by

-30 m.Thus, measuring the tides provides a simple and definitivetest of the existence of a subice ocean.

In the likely instance that an ocean exists, several geo-physical measurements (Fig. 7) will place constraints on itsdepth, extent, and physical state, as well as provide informa-tion on the internal structure of Europa, including the man-tle and core. In priority order, investigations would be to(1) determine the amplitude and phase of the gravitationaltides, (2) determine the induction response fiom the oceanover multiple frequencies, (3) characterize surface motionover the tidal cycle, (4) determine the satellite's dynamicalrotation state, and (5) investigate the corc and rocky mantle.

The gravitational tidal potential from Jupiter varies pe-riodically as Europa orbits (Fig.8), applying stress thatdeforms the satellite. The amplitude and phase of the gravi-tational and topographic tidal rsponses are determined bythe mechanical strength and density of the layered interior.Love numbers are the dimensionless scale factor.s that pa-rameterize these effects, wherc k, rpresents effects on thegravitational potential and h, represents radial topographiceffects. A homogeneous fluid body would have values ofkz = 1.5 and h, = 2.5. If present, a liquid ocean would domi-nate the tidal response, while the product of icy shell thick-ness times icy shell rigidity has a lesser but important ef-fect (Fig.9).

-.

670 Europa

Ur* = 10 GPa

Static gravity(density structure)

Tidal deformation(Love numbers)

Magnetic inductionsignature

Radar penetration(lower bound)

10 100lce Shell Thickness (km)

Fig. 7. See Plate 38. The combination of (hypothetical) JEO measurements can constrain the thickness of the icy shell. Based on thebulk density and moment of inertia (from future flybys by JEO and other spacecraft), the thickness of the water + ice layer may beobtained (gray shading) (Anderson et aL, l998a,b); uncertainties arise mainly from lack of knowledge of the rocky interior density(bulk density is already known). Measuring time-variable gravity and topography gives the k2 and h2 Love numbers, respectively;hypothetical Love number constraints (red shading) assume observed h, and k, of 1.202 and 0.245, respectivel and constrain shellthickness as a function of rigidity tt (Moore and Schubert,2000). The hypothetical values assumed here are characteristics of a mod-erately thick icy shell. In the example shown, the icy shell deformation is sufficiently large that a shell thickness in excess of 40 kmis prohibited. Determining both k2 and h2 provides additional information. A lower bound on the icy shell thicknesses may be derivedfrom radar data. Here, a tectonic model of icy shell properties is assumed (Moore,2000), resulting in a radar penetration depth (andlower bound on shell thickness) of 15 km (green shading). Multiple frequency (hypothetical) set of observations results in a range ofacceptable icy shell thickness (15-40 km) and a range of acceptable ocean thicknesses (45-70 km). A different set of observationswould result in different constraints, but the combined constraints ar more rigorous than could be achieved by any one techniquealone. JEO would be able to provide those constraints to determine the thickness of Europa's icy shell.

150E

..Y(t,aoc 1oo

!t-ooooso

Based on plausible internal structures, measurement un-certainties of t0.0005 for and t0.01 for h, will permitthe actual k, and h, to be inferred with sufficient accuracytht the combination characterizes the depth of the oceanand constrains the thickness of the icy shell (Wu et aL, Z0fl;Wahr et a1.,2006).In turn, icy shell thickness is an impor-tant constraint on geologic processes, astrobiolog and heatflux from the silicate interior.

The Love number is estimated from the time-variablegravitational field of Europa, which is measured by pertur-bations in the paths of orbiting spacecraft. The componentof the velocity change that is in the direction to Earth ismeasured by a Doppler shift in the radio-frequency com-munication with the satellite. Because the perturbations aremeasured only by a single projected component at anygiven time, a complete resolution of the gravity field re-quires multiple orbits; moreover, a single profile is diffi-cult to interpret because the same data must be used todetermine the spacecraft orbit itself.

At X-band frequencies, velocity measurement accuraciesof 0.1 mm/s are typically attained for 60-s averages. At Ka-band the performance is somewhat better and, used together,the two frequencies help reduce interplanetary plasma-in-

duced noise. Figure 8 illustrates the estimated gravitationalspectrum for Europa, \ryith separate contributions from anicy shell and a silicate interio along with simulated errorspectra for 30 days of tracking at each of three representa-tive orbital altitudes (cf . Wu et a1.,2001), using the X-band-only enor estimate. The recovered gravity erors are smallerat lower altitudes because the spacecraft is closer to theanomalies, and thus experiences larger perturbations.

Improving accuracy in the measurements allows betterdetermination of long wavelength features and initial dis-crimination of some shorter wavelength features. Variationsin gravitational signal amplitude and correlation with to-pography are diagnostic ofinternal structures. For the modelparameters depicted in Fig. 8, the lowest-altitude orbit er-rors are small enough to resolve part of the transition fromthe long-wavelength, silicate-dominated part of the spec-trum (in which conelation with topography would be poor)into the shorter-wavelength, ice-dominated regime, wheretopography and gravity should be spatially coherent (Luf-trell and Sandwell,2006). This would permit detection ofisostatic anomalies in response to topographic variations(such as volcanos) at the silicate-ocean interface. Radio fre-quency tracking data will provide initial spacecraft orbit

oE ro-8c3 io-lc()'

1o-8o

Error l 300 km

0 10 20 30 40 50 60 70 80 90 100Harmonic Degree

Fig.8. Models of Europa's gravity spectrum, assuming an icyshell l0 km thick with isostatically compensated topography abovean ocean, and a silicate interior with a mean surface 100 km be-low the ice surface. The variance spectra of the ice topographyand silicate gravity are assumed similar to those seen on tenes-trial planets (Bills and lmoine,1995). The signal has contribu-tions from the silicate mantle and icy shell. The error spectrarepresent 30 days at fixed altitude, and reflect variations in sensi-tivity with altitude. The error spectra at different orbital altitudesdo not have the same shape because the longer wavelength anoma-lies are attenuated less at higher altitudes. During a few days atthese altitudes, the improvement is linear with time; for longertimes, repeat sampling leads to improvement proportional tosquare root of time.

Greeley er a!.: Future Explorarion of Europa 671

estimates. As the gravity field knowledge improves duringthe orbital mission, near-real-time orbit position knowledgewill also improve. The tracking data will be used, togetherwith spacecraft attitude and altitude information, to estimatesimultaneously the static and tidal components of gravityand topography, and the forced rotational variations includ-ing libration.

The Love number ht is derived by measuring the time-variable topography of Europa, specifically by measuringtopography,at crossover points (Fig. l0), a technique thathas been demonstrated for Earth (Luthcke et a1.,2002,2005)and Mars (Rowlands et aI., 1999; Neumann et a1.,2001).After

-60 days in orbit about Europa the subspacecraft trackwill form a reasonably dense grid, comprised of N (-700)great circle segments over the surface of Europa. Each ofthe N arcs intersects each of the remaining N I arcs at tworoughly antipodal locations, and at these crossover loca-tions, the static components of gravity and topographyshould agree. Differences in the measured values at cross-over points are equal to a sum of actual change in radiuscaused by tides and libration, combined with the differencein orbital altitude, along with any enors in range to thecenter of the body or orbital position (Fig. 10). The erorsare dominated by long-wavelength effects and can be rep-resented by four sine and cosine terms in each orbital com-ponent (radial, along track, and cross track). The tidal ef-fects in gravity and topography have known spatial andtemporal patterns and can each be reprcsented globally bytwo parameters, an amplitude and phase. The librations are

15

14

13ErP

J..ntz

N-:

0.25

0.15a '...

N-c

40 60 80 100 120lce Shell Thickness (km)

40 60 80 100 120lce Shell Thickness (km)

Fig.9. Sensitivity of Love numbers k2 (left) and h2 (right) to thickness and ligidity of the icy shell (assuming a subsurface ocean).For the same curves that depict hr, the righthand axis shows the amplitude (tidal (which is half of the total neasurable tide) as afunction ofthickness ofthe icy shell. For a relatively thin icy shell above an ocean, the tidal amplitude is (tidal - l5 m (total measureableti

Orbil

672 Europa

Fig. 10. Illustration of the crossover technique. Actual changein radius of Europa due to tidal and librational motions is deter-mined by measuring altitude from the spacecraft to the surface,and by accounting for the distance of the spacecraft from thecenter of mass by means of Doppler tracking (lVahr et a1.,2006).

effectively periodic rigid rotations with specified axes andperiods, and again an amplitude and phase parameter suf-fices to describe each axis. Thus, there are l2N + 10 param-eters to be estimated (12N orbital, 4 tidal, and 6 librational),from 2N*(N l) crossover points. The accuracy with whichthe altimetric profiles can be interpolated to the crossoverlocations depends on range accuracy, surface spot size overwhich altitude is sampled, and along-track sampling rate.In an ideal case, the surface spots would be small (to mini-mize topographic variation within spots), and near-contigu-ous or even overlapping. Those considerations need to beassessed against power and data-rate constraints of an in-strument, and the desire to interrogate topography for asmuch of the surface as possible.

The magnetic induction signal from an ocean withinEuropa is sensitive to the product of the electrical conduc-tivity and thickness of the ocean (Fig. of chapter by Khu-rana et al.). Determining the induction response at both thesynodic frequency with respect to Jupiter's rotation (T =l1.l h) and the orbital frequency of Europa (T= 85.2h)can allow for ocean thickness and conductivity to be de-termined uniquely. In turn, ocean conductivity constrainsits salinity. It is possible that additional longer-period sig-nals, caused by the background fluctuations of the magneticfield (e.g., associated with lo's torus reorganizations), couldbe used to sound the ocean. This requires the sensitivity ofthe