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Large Satellites. Active WorIcts anct Extreme Environments Of ~e six large outer-plmet sa~lli~s Io' Europa' C~yme~' Callisto' Titm' md Triton all are larger ~m Pluto md two are larger ~m Mercury; in addition, Bare are Il medium-si~d satellites Figure Set; Tabs Shy. Each plmet-sized satellite is unique: Io is infamy volcanically active' Europa may have ~ layer of subsurface wear greater in volume than all of Earthts ocems combined C~ymede has ~ intrinsic magnetic field' Callisto is largely undifferentiated' Tim has ~ wick Exosphere rich in organic compounds' md Triton has active, geyserlike eruptions. The large satellites have bizarre life eyeles' ir~ueneed by orbital evolution md tidal heating, revolutionizing concepts based on the terrestrial plme~. They are rich in volatile species such as H2O, SO2, N2' CH4, Cal' md perhaps NH, creating ~ rich diversify of processes md environment. The ~ ~ medium-sized satellites are also unique worlds' md they may provide essential information about the origin md evolution of satellite systems. FIGURE 5.l rfacz~g page j The 17 Urge and m~ium-sim Bellies of the outer pawns, shown to ~1~, are worlds in their own right. The Oalilean Bellies of Jupiter Mop raw) are (from lep) Io. who' surfed is constantly renewed by active volcanoes tinged with sulfur allotropes; Europe which pronely possums ~ liquid wear omen beneath its rustily in skin; Oanym~, ~ moon bigger On the planet Mercury, possessing ~ rump surfed of dire id and an internally generated magnetic field; and Cellists, ~ moon with an ancient cratered surfed who' interior is only weakly differentiate~l. Saturnine family of bright icy moons Qeca~d raw) consists of Mimas, En~ladus, Tethys, Dione, and Rhea; cloud-shrou~ Titan has an atmosphere rich in organics and possibly ~s of mound; and two-ton~1 Iapetus shows one few as bright as snow and the other as black as ~1. The five major uranian Bellies jb~M raw) are Miranda Aricl. Umbricl, Timnia. and Oberon. Each displays ~ dirW-i~ surfed and some Bionic Livid but the Pierre world of Miring with its exotic jumble of surfed terrains suggesting 0t it may have On tomlly disrupt in the pan and put Wok together ~ random sows the show. Neptunc,~ sole large Pallid ~~h raw), Triton. is ~ - with exotic ids tinge<1 pink by organic molecules; nitrogen geysers spew high into its tenuous atmosphere. Courted of NASA/JPL.

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Ado TABLE S.l Large- arid Medium-Si~d Sa~lli~s of the Outer Solar System HEW FR0~ IN =E SOLAR HEM Semimajor Axis Rotation Period Diameter Mass Density Planet Satellite ~ ~ 03 km) (days) (km) ~ ~ o 20 k ~ (kgim3) Jupiter Io 422 ~ .77 3) 643 ~ ~ 3 3) 50 0 Europa 67 ~ 3. 55 3) ~ 20 48 0 3) 00 0 G~yme~ 1~070 7 15 S)~6 1)~2 1)90O Callisto 1)~83 16 .~? 4)~20 1)076 1)800 Saturn Climax ~ 86 O.~4 394 0.375 1)200 Er~mladus 238 ~.37 502 0.7 1)100 Tethys 295 1.~? 1)048 6.27 1)000 Dione 377 2.74 1)~20 ~ 1.0 1)500 Rhea S27 4.~2 aim 23.1 1)200 Titan 1)~22 15 .?S S) 150 1)346 1)900 Iapetus 3)~1 79.33 1)435 ~ ~ 1)000 Uranus h] irar~da ~ 29 ~ .4 ~ 47 ~ O . ~ ~ ~ ) 20 0 Ariel ~ ~ ~ 2. S? ~ ) ~ ~ ~ ~ 3 .S ~ )70 0 Umbriel 266 4.~4 1)~? ~1.7 1)400 Titar~ia 43 ~ ~ .7 ~ ~ ) 57 ~ 3 ~ .3 ~ )70 0 Oteror~ SS~ 13.~6 1)~23 30.1 1)600 Neptur~e Tritest 3 ~ ~ ~ .~S 2)70 ~ ~ ~ ~ 2) ~ O O WHY DO WE CARE ABOI5T LARGE SATELLITES' Why are these large sa~lli~s worthy of nations md incarnations exploration md research: One good reason is ~~ advancing basic research about physical processes in fields such as volcanology md meteorology may eventually provide benefits that will improve our lives. Another is ~~ such interesting worlds inspire our your md students to excel in ma~em~ies md science. But He most compelling motivation is to understand He origin md destiny of life. Water is essential to life as we know it, md He large icy satellites may contain He largest reservoirs of liquid water in the solar system. Outside Earth Europa may be the best place in the solar system to search for extant life. Titan provides ~ natural laboratory for He study of organic ehemisby over temporal md spatial scales unattainable in terreshi~ laboratories. Perhaps teeming with life or perhaps sterile today' these worlds do rennin the basie ingredient for life. Wowing whether they do or do not harbor life is equally important. The origin md evolution of satellite systems also provide analogs for underfunding exhasolar plm- etary md satellite systems' some of which may be Modes for life. a Origim and Orhi~l Dyn~ni= The accretion process that led to the formation of He solar system also led to the formation of satellite systems around the gist plme~. The result of four a~itiona1 accretion Experiment within He solar system are therefore available for deviled study. The fundamental process of accretion leading to the formation of satellite systems is directly analogous to that leading to the planets' but over processes for example' gas drag md tidal interactions may have had more or less important roles in the protoplmetary nebulae. Since the satellites are much too small to capture hydrogen or helium' they provide ~ record of He inventory of condensable species in He protoplmet~ nebulae. The size' distribution' md compositions of the satellites within ~ system also inform us about the physical md d~amiea1 conditions during accretion. The Calilem satellites, for example' apparently contain ~ record of the temperature gradient in the nebula in which they formed Trough their decreasing densily with dimple from Jupiter (see Table S.~. Such ~ Rend is not obvious in He other satellite systems. The formation of four large satellites in the Jovian system while other systems have ~ most one is perhaps indicative of ~ denser nebula around the young Jupiter.

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LAT~E SA=~S The periodic driving forces of orbital resonar~es have played art important role in the formation of ply - ar~d sa~lli~ systems. This is evident in ~e dynamics of ~e ou~r-plmet sa~lli~s, Marty of which are currently involved in orbital resonances. The imported of tidal dissipation in ~e origin arid evolution of resonar~t configurations is apparent in the Jovian system' where Io' Europa' arid Gar~ymede interact through multiple resonates, arid where tidal dissipation drives Ions volcar~ism arid may maintain art ocem within Europa. At Saturn, resonances currently exist between ~e sa~lli~ pairs Mimas-Tethys' En~ladus-Dione, arid Titar~-H~rion; arid ~ Urmus, paired resonar~es likely once existed among the sa~lli~s Mirar~da' Ariel, arid Umbriel. Resonar~t configurations are set up by orbital evolution driven by tidal interactions' arid the process of evolution into arid out of resonance may involve periods of extremely large tidal dissipation, which may significar~tly affwt the sa~lli~' thermal histories md interior structures. Tidal dissipation cart be ~ long-lived hem source, complexly independent of seller radiations arid it might allow habitable Clarets or sa~lli~s to exit ~ ~ much wider range of dietaries from ~ much wider range of central stars Bars previously imagined. Europa, with its plentiful supply of whorl may ~ one of these habitats, art environment thy may ~ far more common in the universe chart Earth-like planets orbiting Sun-like Ears. Tidal dissipation was probably imports to mmy large sa~lli~s, md to ~e PlutofGharon system. 1 Interiors For the majority of Me sa~lli~s of the outer solar system' our knowledge of their interiors is limited to Me mem density of Me sa~lli~ (see Table S.~' but the Calilem sa~lli~s, which have men visited by the Galileo spacecraft, are now much Aver understood. By measuring the tidal md rotational distortion of the satellites' Me normalized moments of inertia about the rotation axes have been well constrained' leading to the following conclusions regarding the interiors of the Calilem satelli~s:~-4 ~ Io is differentiated into ~ large metallic core, roughly half the sullies radius, surrounded by ~ silicon mmile. ~ Europa has ~ 100 + 25-km-~iek HERO layer, which is frozen ~ the surface md may be liquid beneath. The remainder of Europats interior likely tonsils of ~ silicas mmile of density ~3~300 kg m-~' surrounding ~ metallic eorewitharadius of 600+150km. ~ C~ymedets metallic eve was detected by the gravi~ measurements ~ the same time thy id magnetic field was dim overed. A model for C~ymede~s interior consisting of ~ Io-sized eve md mmile surrounded by 800 km of fee fin He gravity dam md account for He metallic eve required by He magnetic field. ~ Callisto is not differentiated like C~ymede' despite He similarity in size md density. A significant metallic eve em be ruled out as em ~ completely undifferentiated structure. The intermediate value of Callistots moment of inertia requires ~ layer of mixed fee md rock, which may extend all the way to He center. These conclusions are based on He reasonable assumption ~~ Callisto is in hydrofoil equilibrium. The very different fates of Calli~o md C~ymede surged thy tidal hewing is probably ~ important favor in satellite differentiation. Titmhas undergone ~ lead ~ partial differentiation resulting in ~ dense atmosphere of N2 md other volatiles ~~ are extremely rare or absent in He Jovian satellites. Triton is currently degassing volatile species via geysers; moreover, Triton, s surface displays evidence for vigorous eryovolemie md Estonia processes' perhaps reflecting intense tidal heating md differentiation of id deep interior during capture into Neptune orbit. The surface evolution of the smaller satellites offers intriguing clues about Heir interiors. Despite their relatively small sizes' Eneeladus' Tethys, Ariel, md Titmia all seem to have experienced some internally driven surface aetivily, indicating ~~ interns evolution has occurred. Tiny Miranda has ~ complex Estonia history' which has likely been modulated by differentiation mdior tidal hewing. The Berms stays of He interiors of the outer-plmet sullies are coupled to Heir differentiation. Tidal hewing is driving He continuing magm~ie activity of Io md the ongoing loss of volatile elements (S. 0' Na, K) from Ions surface, which affects the plasma environment throughout He Jovian system. C~ymede~s differentiated interior md actively convecting e ore (required to general id magnetic field) may be ~ consequence of id passage

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HEW FR0~ IN =E 50~R HIM into resonar~, while Callisto has not experienced this history. The origin md persistence of liquid-w~r layers in icy sa~lli~s depend directly on their thermal histories. Galileo magnetometer observations of induced elechica1 current in Europe Gary md Calli~o imply ~~ liquid-wa~r layers exit in all three icy Jovian sa~lli~s.~6 While the layers in Callisto arid Car~ymede are bounded by ice on both sides (high-pressure phases of ice are denser thm liquid wa~r' resulting in art ice-liquid-ice sandwich), European liquid wear ar~alogous to Earthy deep occurs is most likely in direct contact with id silicon marble. Tidal hewing in European ice is probably sufficient to shill its liquid layer for long periods' but other icy sa~lli~s may have only transient liquid layers. 0~10~c~1 Promos= Cats Impact orders serve as proms of sa~lli~ crusts, indictors of surface age, arid records of ~e impactor Population Grouch timed Large impacts cart penetrate complexly through ~e brittle outer crust of art icy sa~lli~ to excavate pep Perhaps ocear~ic) ma~ria1 md may form ~ multiringed structure. Very large impacts may fracture ~ satellites interior or potentially disrupt ~ large satellite. Relaxation of crater topography (or ~e absence of relaxation) earl be indicative of the past thermal gradient. High-resolution imaging of the Galilear~ satellites suggest thy Me number of small impaetors in He outer solar system may be much less thm estimates ex~apola~d from the lunar flung One implication is ~~ impact gardening md regalia generation are less effective on outer- plmet s~elli~s ~m on He ~rrestria1 planets. Sun-orbiting (heliocentric) impostors are expected to produce markedly more craters on the leading hemi- sphere of ~ synchronously rotating satellite than on its trailing hemisphere. For the saturnim satellites md Triton' eraser size-frequency dam show complexities attributable in pay to plmet-orbiting (plmetocentrie) impaetor populations.~ lament flux estimates md d~amiea1 simulations thy include He newly recognized effects of Kuiper Belt md Oort cloud biometry impostors indigen higher fluxes md therefore younger satellite surface ages than previously estimated. For example' by these estimates, Tritonts plains are on average only ~100 million years old' md Europe surface is just ~50 million years old.i id The mounting evidence indices that some large outer- plmet s~elli~s have been active worlds for much of solar system history. Tecton`~s The large Bellies display ~ broad array of Estonia features interpreted as He mmifes~tion of extensional' eompressional' md s~ike-slip deform ion. Ex~nsiona1 structures are espeei~ly prevalent on mmy of He midsized icy satellites of Uranus md Saturn md on Triton' potentially He mmifes~tion of global expansion during freezing of interior wear or differential cooling of Heir surfaces md interiors. Lmes of subpar~lel ridges md troughs on hlir~da, Eneeladus, md C~ymede may share analogous origins as regions of eoneentra~d extension md icy volemism' analogous to some terrestrial rift zones. Individual ridges on saturnim satellites md sew of ridges on Eneeladus may be due to compression' perhaps from global cooling md eonLaetion or from convection. Galileo imaging of the large Jovian satellites has revolutionized our understanding of large-satellite Ebonies. Io has mountains that soar to 17 km ~11, probably formed as volemie materials piled onto the surface, placing He entire lithosphere into compression.> C~listo shows enormous multiringed structures' which ~ high resolution consist of normal fault sharps md graben.~7 These md similar concentric structures on C~ymede md Europa probably formed when large impacts penetrated through the smelliest bridle lithospheres to mobile material below plausibly liquid wear. C=ymede displays ~ array of ex~nsiona1 tectonic structures' no~bly lmes of bright ` OCR for page 118
LATHE SA=~S FIGURE 5.2 Europa displays ~ win varicose of curfew forms, including the' so-=ll~1 ridged plains. Them features Connie of many parallel. crosmuning ridges. often arranged in pairs. Dark mamria1 appears to ~ low - primarily in the valleys between the ridges, sulfating - t the dark malaria may ~ moving down the flake of the ridges and collecting along their Ado. This image shows ~ region some 20 km across and reveals features as small as 2b m. North is ~ the top, and the Sun illuminams the surfed from the upper left. Toured of NASAlIPL. tures to wider md more complex ones. The origin of these ridges is uncertain' but suggestions include diapirie infusions shear heating, diking, wa~r-rich extrusion, md compression along preexisting tectonic structures. Wider pull-apart buds may represent complete separation of the icy lithosphere' in ~ mower broadly malogous to terrestrial seafloor spreading. The global pattern of linesmen mushes stress predictions if gravi~tiona1 torques from Jupiter have induced nons~ehronous rotation of Europa~s icy shell, implying decoupling of the surface from the interior, likely by ~ liquid-water ocem. Systematically varying stress directions md magnitudes induced by diurnal orbital flexing of Europats icy shell em elegantly explain Europats eyeloidal-shaped ridge md fracture patterns md may drive strike-slip faulting along ridges md buds. Signif~e~t tidy amplitude is necessary to produce large diurnal stresses' md this argues strongly for ~ subsurface liquid Dyer but does not contain id dep~.24 Large-seale fobs have been recognized on Europa' but these em eompensa~ for only ~ small fraction of Europats ubiquitous extension.25

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~4 loom ~d Oe~s HEW FR0~ IN =E 50~R HIM The discoveries of current eruptive activity on Io arid Triton were highlights of the Voyager ~ arid 2 mis- sions.26~ In the inner solar system' geologic activity is driven primarily by early accretion arid differentiation arid the slow decay of radioactive nuclides' with ~e result thy continuing geologic wlivity was only expand on ply such ~ Earth arid Venus win sufficient silicon mass. By analogy' no current geologic activity was expected on ou~r-plar~t sa~lli~. This paradigm was fired by Voyager arid by our new understar~ding of ~e effects of orbits evolution' tidy heating, arid highly volatile crusty species. Io has several hundred currently active' high-~mperature silicate eruptions (Figure S.3~28 arid ~ global average hey flow ~20 times greater thm ~~ of Earth.~9 Marty of these 1~ have extremely high temperatures md may be rich in hey similar to Archem kom~ii~s md lunar mare basalts.~ Voluminous flood volcanism, which has had pronounced effects on Ens climate' is ongoing ~ Io. The high hey flow' Mg-rich md flood volcar~ism, arid rapid Platonism' which we cart directly observe on Io' provide insight into ar~cimt processes on ~e ~rres~ia1 ply. In addition, the gimt (up to 500 km) volcar~ic plumes of Io arid the smaller geyserlike eruptions on Triton provide fundamental experiments in fluid dummies. Marty over icy satellites exhibit evidence for past icy volcanism, expressed as smooth plains' ridges' lobed deposing arid marbling deposit. Active volcanism on some icy s~elli~s is plausible today' bred on the lightly era~red surfaces of Europa arid Eneeladus arid models of ahnospherie processes on Tim. Although Galileo yielded no evidence for active volemism on Europa'32 continued searches are warranted. D~ap'r'~m Interior material also em be brought to Me surface of ~ satellite though diapirism, in which buoyancy forces due to ~ density inversion cause mobile material to pierce md rise Trough ~ higher-density overburden. ~ the icy satellites, Tritonts piked ``em~loupe,' terrain offers the most dramatic example of ~ surface apparently turned inside out by diapirism' perhaps owing to compositions layering of various frozen volatiles. Tritonts record of infuse diapirism may reflect capture by Neptune md consequent tidal hewing. Diapirism may also explain Me unusual rounded ``eoronae,' of Miranda ~ satellite potentially frozen during the aet of differenti~io~perhaps induced by tidal hewing. Europa also may exhibit evidence of diapirism.34 Pity domes' md spots on Europa have been interpreted as the surface manifestations of Alertly induced diapirism' where warm ice' probably in donut win ~ subsurface ocem, has risen Trough colder md denser fee above. Larger Chaos', regions on Europa consist of disrupted crush blocks situated in ~ hummoeky matrix Figure S.4~. These also have been inferred to be the mmife~ion of diapirism md ~soeia~d p~ia1 melting of the fee crust, though complex melting of ~ Fin fee shell is ~ alternative hypothesis. Diapirs may be able to Disport nutrients mdior organisms between He surface md subsurface ocem of Europa md over icy satellites. Atmospheres' Surface Chemistry, and Interactions The Win Ionospheres md volemism of Io md Triton serve to redistribute md modify volatile deposits on their surfaces. However' the Cassini-Huygens mission may reveal much more dramatic effects on Tim from active < Times liquid eyele, with clouds' rain, md perhaps seas, may resemble our ~rreshia1 eoun~rpart, win several key

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LAT~E SA=~S ~5 1 km~;;`O~6~mile}~:: :~: of: : : ~ ~ :~ ~::~ _:: :::: FIGURE 5.3 The margin of the 1~a flow field associated with the Prometheus volcanic plume on Jupi~rs, moon To. This entire arm is under Prometheus,~ active plume, which is constantly raining bright ma~ria1 onto the surfa=. The darken regions, having margins similar to thou formed by fluid Ma flows on Earth, are ~lic~e<1 to ~ relatively young l~u' they are not yet covere<1 with plume fallout and are, perhaps No warm for bright gas rich in sulfur dioxin to coning. The old brigh~r plains to the upper right are co~7er~1 6y ridges formals possibly, by the folding of the surfed or by deposition or erosion. The bright streaks emoting from the 1~a flow margins may arid where hot 1~a vaporizes sulfur dioxin. This image has ~ resolution of 12 m and was Akin by the Galileo spacecraft on February 22~ 2000. Courted of NASA/JPL.

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HEW FR0~ IN =E SOLAR MOM FIGURE 5.4 This image from the Galileo spacecraft is ~ very high resolution view of the Co~mara Chaos region of Jupi~r~s moon Europa. It shows an area where icy prams have On broken apart and moved around laterally in ~ hummocky matrix. Corrugated plenum end in icy cliffs more On 100 m high; Doris piled ~ the ~= of the cliffs On ~ resolved down to blocks the sim of ~ hound. The fracture running horizontally just above the bonom of the image is about the width of freeway. Courted of NASA/JPL. differences. Titans main condensable is methane rather ~m wear. Titans atmosphere is more massive md cooler than that of Earth. Tim receives ~100 times less solar insolation' Me energy that fuels terrestrial weather. In Contact, Tim has roughly 100 times more latent hem available for fueling weaner than does Earn. lament observations indigen the sparse presence of daily clouds ~~ uniformly lie ~ We tropopause.37 In Editions ground-based observations, recorded in the past two decades' show evidence for the unique occurrence of ~ hurrieme-sized cloud system.38 The formation meehmisms of clouds' the origin of Me large md rare storm, md the effect of latent hem on cloud evolution md eireul~ion are urn own, because only limited measurement of Me lower atmosphere have been possible. Current md future investigations aim to understand Times coupled atmosphere md surface, which may provide analogs for processes important on Earth. Improved understanding of Times evolution depends on knowledge of the depths md extent of id liquid reservoirs ~ md near Me surface. The main atmospheric constituent nitrogen, dissolves in me~me. Therefore' the size md composition of Me reservoirs reflect not only the total inventory of org~ies but also Me amount of

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LATHE SA=~S ~7 FIGURE 5.5 A Rhombic of the dominant prowess effacing the volatile inventory on Titan and the formation of prebiotic molecules. Course of Ralph Lores. University of Arizona. nitrogen on Titm. The rapid md irreversible destruction of methane by solar ultraviolet photolysis indiea~s He need for ~ recent supply. Two extreme scenarios are possible: Current geologic aetivi~ may directly supply atmospheric methane And lead to ~ atmosphere that varies in size with supply)' or large near-surface reservoirs of mended such as seam may exi~.~>40 Organic chemist on Tim occurs bow in the stratosphere md on the surface. In Times stratosphere' Me photolysis of methane coupled with electron dissoei~ion of nitrogen in~iga~s ~ rich organic chemistry, for which over ~ dozen organic species have been identified. The end-product of this chemist' Times ubiquitous h~e' consists of complex organic material win ~ elemental composition that has not yet been directly measured. Even the ratio of nitrogen to carbon in Times he is unclear. Laboratory simulations of this satellites photoehemis~ produce solid residues having optical properties similar to Hose of Titans hue. Their elements composition hints ~~ alkalies' aromatic compounds' heteropolymers, md amino acids, key initial compounds in lifers chemist' are constituent of Titans h~e.4 ~ Chemical reactions ~ Times surface proceed very slowly' potentially in cold 194 K) organic liquids. In this environment' organic ehemisLy evolves in ~ solvent over ~ long time period' well shielded from ultraviolet radiation' as on Earn. Yet Times Exosphere md surface are more reduced ~m Earths (similar to Urey-Miller models of early Earths, conditions are cool, md He solvent is mainly hydrocarbon (methane md embed. It is possible ~~ He solids are not soluble in the surface liquids. At present' however, the composition of Titans surface organdies is poorly known md is inferred primarily from our understanding of He atmosphere. The paw md exit of long-term organic evolution in ~ largely non~queous solvent are up own. Titan provides us win laboratory for this chemist.

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Ads HEW FR0~ IN =E 50~R HIM Tim has, on brief occasions' experienced chemical conditions more like Lose on Earth. Episodic heating' due to impacts arid possibly volcar~ism' probably exposed organic myriad on Titans surfed to aqueous solutions. Liquid-wa~r ponds ~0.S km deep would survive on Titans surfwe for as long as 1~000 years. Considerations of reaction rams relevar~t to Base brief events indices the ready production of compounds (such as purines' pyrimidines' aldehydes) imports to prebiotic chemistry.42 At present, our understanding of organic chemistry is too poor to estimate how quickly life arose on Earn. Tim provides us with snapshot of this chemistry ~ 100- to 10000-year intervals, longer Bars possible in laboratories md shorter Shari cart be deciphered from our ~rres~ia1 record. Titans natural laboratory may uniquely hold growers to ~ evolution of prebiotic chemistry on Dicier Em. Four repark ices have ~en identified spec~oscopic~ly on Triton~s surface: N2, CHIC CO' arid CO2.43~44 The lair Area species Except perhaps CO2) exit partially in solid solution win N2, ~e main constituent. More complex orgar~ic molecules are also expected to be present as ~ result of photolysis arid radiolysis. Triton~s surface temperature of approximately 38 K creates ~ atmosphere in vapor pressure equilibrium with the ices, which is highly responsive to heating charades associated win solar insolation arid the variable photometric arid composi- tiona1 properties of the surface. As ~ result, the atmosphere experiences large-scale sublimation, transport' arid recondensation of N2' CO, arid CH4. Another unique characteristic is Triton~s geyserlike plumes ~~ enLain dark dust md rise ~ km above the surface.45 A diffuse hue pervades the abnosphere; it probably consists of He condensation of hydrocarbons created by photoehemis~. Discrete clouds' likely condensed N2' are present near the poles. lo Ions sulfur-rich chemistry reflects He moons active volemism.46 Ions infrared spectrum is dominated by He signature of solid SO2. The albedo' continuum spectrum, md atmospheric measurement India however' that other sulfurous materials are present. The surface topography md hot-spot temperatures require the presence of silicas, which are largely covered by He sulfur-rich veneer. Ions abnosphere is arguably the least underwood in He solar system. It is uniquely affected by ubiquitous md time-variable volemism' which adds to the atmospheric inventory~rough plumes md affects He surface ~mpera- ture md composition. Ground-based speetromopy identified He primary constituent, SO2, md two of the minor component' SO md S~.47~43 The surface pressure is around ~ nmobar md varies spatially by orders of magnitude. The vertical profile is poorly characterized. Two limiting' although relends origins are postulated: ~ Ionosphere produced by sublimation of SOIL md one produced by volemie outclassing. The atmospheric structure is unclear md may be determined by several processes: hydrostatic equilibrium' plume dynamics' md general eireul~ion driven by large pressure gradient. The roles of these processes are not well known md require knowledge of He surface properties (porosity, composition' md temperature), He atmospheric ~mper~ure md composition, atmo- spherie escape processes' md the composition md energeties of the plumes. Icy samd] ~s In addition to wear ice' which by the I970s had been identified on most of the icy satellites by ground-based spee~oseopy, the surfaces of these bodies contain non-iee material, which may be composed of mixtures of silicas md carbonaceous material as well as component produced by eharged-p~iele bombardment of their surfaces. Calileo~s spectral measurement have also identified features due to CO2' C-HP S-H' md C-N on several of the Calilem satelli~s.49 Similar marries have been identified in spectra of interstellar fee grains. This non-iee component presumably represents ~ mixture of material originally secreted with He smelliest subsequent comet md Steroid impacts, md components implanted mdior modified by magnetospherie environments. On Europa' the presence of heavily hydrated sulfurs has been inferred, including sulfuric acid md sulfate Salk. Charged-

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749 .~ Con /10~109~] 892008 DIAL Pass 8-~H I9)T 410 SOUBIn 19~1410 ~UR) /~51d oUR)~N I AT 410 up ALUM ~ LIMO PI l~pUB] /~01Ol4Ol~ B6010 lOpUB] lOpUl]~Bd B6010 I9I0I6X~ 1~olSAq509D B60103 lolol~xa U6llL suo$~H-Iulss~: _, o EN 4, 4~ ~ -A Of ~ ~ 1 1 1 1 ~ I1 1 1 ~ ~ ~ ~ I1 1 1 1 ~ ~ 1 I1 1 1 1 1 1 1 I1 1 1 1 pi: pi: is: Pi: Pit 1 Pi 1 ~ ~ ~ ~ ~ ~ 1 Pi: Pi: Pit 1 Pi CAM CAM A C) C, ~ ~ BAD ~ _ ~ ~ _ To A 4. _~_ ~ To ~ _~ ~ O ~ ~ ' ski. 3, a - - is: O D 3 v ~ ~ ~ ~ ~ ~ ~ v ~ ;,, ~ ~ v ~ A ~ ~ ~ A O 0-> ~ Ho ~ ~~ ~ ~ ~0 -A ~ m a ~ ~ ~ =~ s ~ ~ b^ ~

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744 HEW FR0~ IN =E 50~R HIM The first question directly addresses the major exploration theme ``Are we Lion md the second question directly addresses the theme ``Where did we come from92' The third arid fourth questions address h~itabiliW, in this md other perry systems, which is relevar~t to all Area overriding themes, including ``Wh~ is our destiny92' Anion Targets What is the best strategy to address these questions: Europa Ad Tim stared out as ~e highest-priori~ Urged. Each is the key to one of the high-priori questions limed above, Ad each addresses one major exploration theme arid is imports for others (Table S.3~. Europa is ~e sa~lli~ thy holds the most promise for under~ar~ding the po~ntia1 habitability of icy sallies. Convincing evident exists for ~e presence of wear within just ~ few to fens of kilometers from the surface, arid there is evidence for the r=ent or ongoing trar~sfer of myriad ~twem ~e surface Ad ~e wear layer. European occur is probably in direct contact win ~ rocky marble below arid so potentially with hydrothermal systems, arid surface Ad intra-ice oxide Respond to the ocem may ~ able to nourish oceanic organisms. The first sup in understanding the po~tia1 for icy sallies as abodes for life in ~e univerm is to send ~ spacecraft to Europa, in order to confirm the prisms of art interior ocear~' to charac~ri~ the sullies ice shell, Ad to understar~d id geological history. Europa is also key to addressing high-priority questions 3 arid 4' Gove. It is the best target for theme ~ origin Ad evolution of wa~r-rich environments in icy sullied is imports to Demos A (~e Table S.3) Ad possibly Demos ~ Ad ~ . Given ~e high cost of the Europa ~ophysica1 Explorer, the pme1 TABLE S.3 Targets Ad Missions for Future Exploration Be~ Targets h] issior~s Them ~ A. Origin arid evolution of satellite Items ~ . Origin arid evolution of water-rich Europa er~vironmerJ~ ire icy satellites C. Exp Lorinda orgar~ic-rich er~viror~me~s Titan Satellite systems D. Urlder~arlding d~amic planetary Io' Tit~'Triton pro ~ sses High-Priority Equation 1. Is there extant life ire the outer solar Europa systems 2. How far toward life does organic Titan chemistry proceed ir1 extreme erlvirorlmertts 3. How common are liquid-w~er lairs within icy sa~ llite s~ 4. How does tidal heating affect the evolution of worldly Cassini-Huygerls, Europa Geophysical Explorer' Nepturle Orbiter' Uranus Orbiter Europa Geophysical Explorer' Europa Pathfirl~r Lo dry Europa Astrobiology Lear Cassini-Huygerls, Titan Explorer Cassini-Huygerls' lo Observer' Titan Explorer' Nepturle Orbiter Europa Astrobiology Lo der Titan Explorer Tritor~' Titan Er~mladus' Callisto' G~ymede' Europa Io' Europa' G~yme~' Tritorl' Erlm ladu s' h] irarlda Cassini-Huygerls, Europa Geophysical Explorer' Nepturle Orbiter' G~ymede Orbiter To Observer' Europa Geophysical Explorer' Nepturle Orbiter' G~ymede Orbiter' Cassini-Huygerls, Uranus Orbiter

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LAT~E SA=~S 745 considers it essential ~~ ~e mission address both ~e Group ~ md Group 2 science objectives described by ~e Europa Orbiter Science Definition Team arid ~~ it contribute to Jupiter system science (theme A) during ~e Midyear Calileo-like tour prior to capture into Europe orbit. Tim is ~ unique natural laboratory for orgar~ic chemistry, unlike my over environment in ~e solar system' arid clearly the prime Argot for theme ~ exploring orgar~ic-rich environments arid high-priority question 2' How far toward life does orgar~ic chemistry proceed in extreme environmental Titar~'s atmosphere not only crews this scientifically interesting environment, but also filings future exploration via aerocapture arid airborne mobility. Titm may also have ~ subsurface wear layer md could prove to ~ ~ promising location to search for past or exit life or its precursor chemistry, md it is importmt to several over Demos md questions (s~ Table Sib. Itcarmof now be predicted whiner Europa or Titar~ will ultimately prove to be the mod promising sa~lli~ for longhorn exploration. However, Cassini-Huygens will surely revolutionize our under~ar~ding of Time so it is premature to Flare ~ subsequent Titar~ mission in detail. Another consideration is thy my mission to ~e outer solar system requires ~ decade or more from ~e initial design to ~e end of the mission. Therefore, ~ logical approach is to continue to ~~ma~ between Europa arid Titar~ missions ~~ overlap in time. Cassini-Huygens followed Galileo, so ~e neximission should be to Europa' Hen ~ new mission to Titers. Any mission to Europa or Titar~ thy signifiear~tly advances our objectives is likely to be expensive. Alternations eollabor~ion is importmt seientifi- eally md may prove essential to adequately fund these endeavors. The other large satellites are also providing significant exploration opportunities. Whole satellite systems mud be studied in order to address theme A He origin md evolution of satellite systems. Theme ~ under- s~ding dynamic planetary processes leads us principally to Io md Triton in addition to Time as well as C~ymede, Europa' md Eneeladus. High-priorily questions ~ md 4 lead us to all of the six largest satellites md to Eneeladus md Miranda. Ground-~med Supporting Famlides The panel recommends continued support for the I1lTF along win He proposed adaptive optics upgrade in order to enhance the scientific results of He Cassini-Huygens exploration of Tim. While He I1lTF will continue to provide necessary support for plme~ry astronomy' it is ~ relatively small telescope, md mmy future inve~iga- tions require larger apertures' on He order of amp- to 30-meter~lass telescope. The advantage of such ~ telescope, for example' CANT' with ~ aceomp~ying advance in adaptive optics techniques, is He increased spatial resolution md sensitivity to dim sources. A GSMT would provide about I8~000 resolution elements across the disk of Io ~ opposition, allowing He study of the energeties of Io ~ s volcanoes by resolving mmy eomposition- ally md energetically distinct regions on the satellites surface (Figure S.~. It would resolve large Tim storms' providing information on Times weather. The C8MT would clarify the vertical structure of Ions Ionosphere through occultations. It would better eharwlerize the speeba of dark' likely organic, solids on satellite surfaces. ~ addition' the GSMT would enable critical mission support: for example, if it were available' it could better determine Times wind field md thus lead to better tracking of the Huygens probe. Summary of Panel R~ions Based on He summarized findings presented in Tables S.2 md S.3' He SSE Surveys Large Satellites Panel ranks id recommendations as follows. I. Cassini-Huygens, with preparation for enhanced science analysis md ~ extended mission 2. Continued support for Ear~-based telescopes' to include He acquisition of ~ appropriate amount of CHAT observing time

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146 HEW FR0~ IN =E SOLAR MUM FIGURE 5.d This Voyager ~ image of Io, the innermost of Jupi~r,~ Wilily Collins, has ~ spatial resolution approximately the Ems as 0t from ~ SO-mewr-aperture. E~h-~ ~lewopc equipped with active optics. Such ~ ~lewope would provi~ researchers with the abiliny to monitor the eruptions of Io.s numerous vol~ocs on ~ regular Axis for ~ period of years to decades. The pear-shaped plume of the volcano Pele is just visible on Io's upper-left-hand limb in the original image. Soured of NASA/JPL. Medium E I. New Orology developments to support future missions 2. Io Explorer 3. Rhymed Orbiter Wage Ed I. Europa Geophysics Explorer 2. Titan Explorer 3. Europa Larder Spender or Astrobiology) 4. Neptune Orbiter

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LAT~E SA=~S TOW Technology above: 747 Technology initiatives ~~ are needed are ranked below arid follow from the recommendations outlined I. Radi~ion-hard electronics for Europa Geophysical Explorer arid future Europa larders arid Io Observer' 2. Adverted Lemony arid power systems for all deep-space missions, 3. Atmospheric mobility for Tim Explorer, 4. Compact organic chemist l~or~ory for Tim Explorer arid Europa lar~ders, S. Plar~ry promotion for Europa lar~ders, 6. In situ age-dating for Europa landers arid Tim Explorer, arid 7. Solar~lec~ic propulsion arid aerocapture or nuclear-electric propulsion for Neptune Orbiter. Although the Ethnology recommendations above follow logically from the parcels science arid mission rankings, technologies may ~ developed for over reasons. For example' ~e administrations FY ZWS budget proposal includes funding for nuclear-electric propulsion. Once nuclear-electric propulsion is developed' this capability would Hen open up new mission possibilities, such as ~ spacecraft thy could sequentially orbit all three icy Galilear~ satellites. Why not postpone the Europa ~ophysica1 Explorer mission until nuclear-electric propul- sion is available: There are several good reasons for not postponing this impor~t mission. First, nuclear-elmtric propulsion is not expected to be ready for ~ actual mission for ~ least 10 years, md this panel considers Europa exploration too scientifically impor~t to postpone it for ~ d=ade. Second' ~ orbiter around Europa is far more important for the panel ~ s key objectives than are orbiters around Callisto or C~ymede' because Europa, s tides are much larger (i.e.' measurable via altimetry) md because id ice shell is significantly thirster (permitting radar sounding). Study of Callisto md C~ymede is importmt to undersold this class of icy satellite' but multiple flybys of these two moons expected from the Europa Geophysical Explorer will provide key information on Be surface morphology md composition' upper crusty structure' md magnetospheric indurations. The subsequent sup in Europa exploration should be ~ landed mission, which also requires ~ Europa orbiting spacecraft' md nuclear-electric propulsion md other new technologies may then enable ~ more capable mission. Finally, He panel emphasizes that strong support for adequate 1~&A is essential to all future initiatives. FIEF Elf EN C ES 1. J.~. Armor E.L. Lout W.L. Sjogrer~> G. Schubert' Ed W.~. Moore, <>' Nature 384 541-543' ~ 996. 2. J.~. Ar~rson' G. Schubert' R.A. Jacobsen E.L. Lau' Ed W.B . Moore' s Differer~ia~d retell Structure: Firers from Four Ga li le o Eric ouster s' ~ ~ Sc~e 28 1: 20 ~ ~ - 20 22> ~ ~ ~ ~ . 3. J.~. Arl~rsorl' R.A. Jacobsorl' T.P. hicElrath' G. Schubert' W.~. Moore' Ed P.~. Thomas' 1~' Icarus ~ 53: ~ 57- ~ ~ ~ ~ 200 ~ . 4. J.~. Ar~rsor~> R.A. Jacobson E.L. Lau' W.~. Moore' Ed G. Schubert' s Gr:~vi~ Field Ed Interior Skucture>~> Journal of Pro 106 (E14: 32963-32970' 2002. S. C. Zimmer' K.K. Khur~> Ed h4.G. Kivelsor~> < Icarus 1~: 329-3~> 2000. ~ . h] .G. Kivelsor~' K .K . Khur~' Ed h] . Volwerk' I Icarus ~ 57 In: 507-~) 2002. 7. C.R. Chapman Ed W.E . h] oKirmor~> ~lOr~erir~g of Pl=et~ S~ellites>~> ire J.A. Bums Ed h4.S. h4:~hews (eds.~> University of Arizona Press' Tumor 1986' pp. 293-341. S. E.~. Bierhaus' C.R. Chapman> W.J. h4erline' S.h] Barn 153: 264-~> 2001. . Brooked arid E. Asph:~ug, I~' ?. C.R. Chapman Ed W.E . h] oKirmorl' llOr~erirlg of Pl~et~ S~ellites>~' ir1 J.A. Bums Ed h4.S. Mathews (eds.~>SatelEt~' Urliversity of Arizona Press' Tucson 1986> pp. 293-341. 10. K. Zahrlle~ P. Scherlk~ S. Sobieszo~k~ L. Coolest ~ d H.F. Levisorl~ 11Differerrtia1 Cr~erirlg of S~6hrorlously Rotting Satellites by Ecliptic Comets>~' Icarus ~ 53: ~ ~ 1-~ ~~' 200 ~ . ~ ~ . S.A. Stem and W.B . h4cKi~on' Is Surfam AM Ed Impostor Popul~ior1 Revisited ir1 Light of Kuiper Be it Fluxes: Evi~rlm for Small Kuiper Belt Objects Ed Ream Geological A~ivity>~> A~tro~om`~l Journal ~ ~ ?: ?~-~> 2000 .

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