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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.
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.
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
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 `<grooved terrain,,' likely formed by norm al faulting of ~ cold' iee-rich lithosphere above warmer' more ductile ice. Grooved terrain may be linked to satellite differentiation, during which high-density fee polymorphs were displaced from He deep interior resulting in volume expansion of He whole moon. The varied Estonia styles of Europa hint ~ ~ sub-iee ocem (Figure S.2~.~9 The satellites bright plains are crisscrossed by narrow troughs md enigmatic double ridges, with ~ morphological sequence from simple strue-
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
~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 <<hydrologic,, cycle assoei~ed win liquid hydrocarbons. The surface of Titm may be modified by me~me md edge rainfall md liquid hydrocarbon erosion, active ground-fluid processes, md li~ora1 processes Figure S.~. 17~ As does Early Tim has ~ atmosphere thy is primarily nitrogen md ~ surface pressure of 1.S bars. Titans thermal profile indie ate s that me~me (~10 percent abundance) md mmy minor organic constituents should exist in bow liquid md gas phase md should rain out of He Biospheres providing ~ liquid component to the surge.> Times liquid eyele, with clouds' rain, md perhaps seas, may resemble our ~rreshia1 eoun~rpart, win several key
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.
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
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.
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-
LAT~E SA=~S particle irradiation of ice-rich surfaces cart break molecular bonds' allowing recombination to form new com- pounds, as discussed below. Iapetus is of special ingress because the dark ma~ria1 on the leading hemisphere (al~do of 3 percent) is inferred to have ~ orgar~ic component. Id spectrum is consistent with ~ mixture of l~or~ory-s~thesi~d org~ics Termed Violins)' poly-H6N, md the hiurchison orgar~ic residue.~° The nature arid origin of ~e dark malaria is unclear. The among asymmetry with respect to ~e direction of orbital motion suggest some ex~rna1 control if not ex~rna1 origin. Sput~dmp~'on M&gnet~pheric Prowesses and Alterations The large sullies of ~e gaseous gimt planets spend all or most of their time in ~e coro~ting magneto- spheres of Base plumb. The interaction of sa~lli~ arid coro~ting plume modifies ~e sa~lli~' surfaces arid atmospheres arid leads to ~ net loss of vol~ile materials to ~e magnetospheres. At the present time' Io is known to low more Shari ~ ton per second of vol~ile Madrid (mostly S md 0) to Jupi~r~s m~n~osphere.~i Similarly' Europa is losing id icy surfed ~ the ram of ~2 cm per million years (Myr) to Jupi~r~s magnetosphere.52 Gar~ymede,~ magnetic field partially shields ~e equ~oria1 regions from plasma bombardment. However, it is estimated that the polar regions of C~ymede lose ~ average of ~ mrniMyr of ice from sput~ring.~3 Callisto, in ~ more benign radiation environment loses <0.4 mmMyr of ice to sputtering The plasma bombardment of icy surfaces result in He implosion of S derived from Iots torus into the crusts of icy satellites.54 The irradiation of icy satellite surfaces also results in the production of H2' O2, H2O~, md over stable oxides ~~ get embedded in the ices md also form tenuous atmospheres near He surf. The irradiation of other fee contaminate such as ~ md S produces CON SO2' md H2SO4. The radiolysis of the surface by magnetospherie particles continuously cycles ~ between SO2' H2SO4, md polymer S forms. At Europa' He fast recycling of the crust (believed to occur over ~ time se ale of 100~000 to 10 million years) may deliver oxidants from He surface to He subsurface ocem.57 These oxidants could fuel life in the absence of sunlight. s~ of ~~ Con The type md strength of sa~llitelmagnetospherie interaction depends on the shellings size, surface eomposi- hon' md electrical eonduetivi~, He presence or absence of ~ interns magnetic field in He satellite md He density, composition' md speed of He interacting plasma. Based on these factors' Tree distinct types of inter- aetions have been observed. In He nonconducting type of sa~lliteiplasma interaction' as in the ease of Callisto' the magnetospherie plasma slams into He satellite md is absorbed' but sputters some volatile myriad off He satellites surface. A second type of interaction, called He eondueting-satelli~ipl~ma interaction' is best illustrated by Io md Europa. Because of ~ well-developed ionosphere ~ Io md large plasma pickup near Europa' most of He magnetospherie plasma is diverted around the moons. Only ~ small fraction of the incoming plasma flux spikes the moons md sputters volatile materials off He surface. The strong Alf~en wing currents generated in He interaction are closed in the ionosphere of Jupiter where Hey generate visible footprint (see Figures 4.3 md 4.4~. The Bird type of interaction is epitomized by C~ymede' which generous its own internal magnetic fielded C~ymedets magnetic field is strong enough that it erects ~ minimagnetosphere of its own in Jupiter~s magneto- sphere, partially shielding the Nellie from plasma bombardment. The interaction between C~ymedets mag- netosphere md Jupi~r~s magnetosphere is similar to the interaction between Earths magnetosphere md He solar wind, in which magnetic recormeetion plays ~ key role. Curiously' He over Tree Calilem satellites were found not to have internal fields ~ present. However' it is likely that some or all of He over large moons of He solar system were endowed with ~ interns magnetic field ~ some time in Heir evolution.
Ado Ind~d Fang HEW FR0~ IN =E 50~R HIM Enmp~ G~ymed~ ~d Coo. Magnetic observations from ~e vicinities of Europa' Gar~yme~' arid Calli~o show ~t all Area moons generate electromagr~tic infusion folds in response to ~e romps fold of Jupi~r.~° The magnetic signatures are consistent with the presence of subsurface electrically conducting shells in ~~e bodies. Detailed ar~alyses for Europa arid Calli~o suggest ~~ liquid subsurface occurs with thicknesses exceeding ~ few kilometers could wcount for the enhanced subsurface conductivities.~i Geological arid geophysical lines of evidence are consistent win liquid subsurface oceans within Europa arid Gar~ymede. However, the prisms of electromagnetic induction from geologically inactive Calli~o was indeed ~ surprise. OFF. The only spacecraft to make in situ observation of ~e interaction of Titm with Saturn, s magnetosphere was Voyager 1' which flew through ~e plasma wake of Tim. No appreciable interns magnetic field was observed (surface field strength c30 nT).62 The main pickup ion is N+, md ~e in~gra~d surface pickup ram is ~10~ ions per second. The gnomes of the flyby was not suitable to infer the presence or absence of art electromagnetic induction signature, so magnetic measurements earmot yet speak to the question of art ocear~ within Titers. SPACE MISSIONS FOR LARGE SATELLITE EXPLORATION Spaceport exploration represent the cutting edge of research in addressing the key scientific questions (see the section `~Unifying Themes md Key Scientific Questions for Large Sa~lli~ Exploration'', below) ~~ are related to the theme of this shaper `<Aetive Worlds md Extreme Environments., The missions considered here rude from currently launched md flying (~assini) missions to Pose with extensive design already eomple~d (Europa Geophysical Explorer, win significant heritage from Europa Orbiter), to future mission concepts win varying degrees of design md study (e.g., Tim Explorer' Europa Landers' Neptune Orbiter). Input on Be characteristics md po~tia1 capabilities of these missions came from ~ variely of sources project briefings' studies by NASA Centers' industry' NASA advisory committee studies md reports md studies by Nations lleseareh Council panels (particularly COMPLEX). In evaluating He potential of these missions for addressing key scientific questions, He Large Satellites Panel had to reach ~ common understanding regarding mission md experiment capabilities. Naturally, Owing degrees of uncertainty occur in this process as one moves from well- understood missions md payloads to future e~dida~s for which multiple mission options md possible payloads are Hill being vigorously discussed. The following is ~ brief description of He key elements ~~ the panel considers to be related to each mission, largely on the basis of studies by the Jet Propulsion Laboratory. While details will edge as these And future) mission concept evolve, He panel believes them to be representative of the types of missions md measurement capabilities available for the period under study. Missions were considered within Free broad eon categories: large, medium, md small. At present, experi- enee indicates that He dollar cutoffs between these categories are about $750 million md $500 million, although cost estimates will edge as the missions become better defined md as new technologies become practical. The nature of the large satellites considered in this pmel~s study md the missions required for major advances over current knowledge (developed primarily from flyby recor~E~aissmee) dictate ~~ mmy of the high-priority missions would be ~ least in He medium md mostprobably in He large category. The costs of high-energy launch vehicles' radioisotope power systems, long flight times' md radi~ion-hard eleebonies all contribute to this situation. Of particular concern is the feet ~~ severe of the purely candidate missions are poorly studied to date. These missions are very eh~lenging by He standards of ironer planet missions md even past outer plme~ry recor~E~ais- smee. The panel urges more complex md competitive studies of these mission e~dida~s for understanding their true Gosh md capabilities.
LAT~E SA=~S Cause` Huyg=s Large Wisdom The Cassini mission with the Huyg~s Titm probe was launched in 1997. It will go into orbit around Saturn in July 2004 arid will deploy the Huygens prow into Titar~'s atmosphere in Jar~u~ 2005. For its evaluation of Saturn fillip scientific issues' ~e pme1 assumed ~ successful primp Cassini mission arid appropriate mission dam analysis. A principal large of the mission is Titar~. Huygens results combined with Cassini~s orbital remote sensing arid in situ sampling of the upper atmospheres should revolutionize our understanding of this sa~lli~s atmosphere, id structure arid composition' arid the complex chemical procesms occurring in it. Huygens descent dam arid mapping by several of Cassini~s instrument (radar, imaging, md near-infrared spectroscopy) should provide ~ first clom look ~ id h~-shrouded surface' identify lar~dforms md possible regions of liquid hydro- carbon lakes or seas, md give art indication of the age arid history of its surface. High-precision gravi~tiona1 measurement will place constraints on its interns structure arid history' arid may ~ able to determine if there exists ~ subsurface liquid-wa~r-rich layer in this fillip. Studies of ~e other sa~lli~s in the system will also be importers providing information on the history arid evolution of the sa~lli~ system' the interactions of ~e sa~lli~s with Satum~s magnetospheric environment' arid the origin of the dark, presumably orgar~ic-rich myriad on ~e enigmatic sa~lli~ Iapetus. Europa Copy Exposer The Europa Geophysical Explorer mission is designed to follow up md significantly expand upon He remark- able discoveries made by the Galileo mission, suggesting that Europa may have ~ global liquid-water ocem Renews ~ fee erupt that may be only ~ few kilometers to tens of kilometers thick. The primp objectives of He mission, as defiled by He Europa Orbiter Science Definition Team, em be split into two groups in terms of their privily. The highest-priority, or Group I' objectives are as follows: Determine the presence or absence of ~ ocem; Char aeterize the ~ree-dimensiona1 distribution of my subsurface liquid wear md its overlying fee layer; Understand the formation of surface features, including sips of recent or current activity; md Identify Redid lading sites for future lander missions. The lower-priority' or Group 2, objectives are the following: landers. Char aeterize the surface composition' especially compounds of interest to prebiotie ehemis~y; Map the distribution of importmt constituent on the surface; md ~ Char aeterize the radiation environment in order to reduce He uncertainly for future missions, especially Complements discussions of Europa objectives are confined in COh~LEX's 1999 report A Science strategy for t~ ~xpiora~on of ~~ropa.~3 For He present study, the panel has assumed He brie capabilities md <<s~awmm,' payload described in the 1999 Europa Orbiter Announcement of Opportuni~:64 ~ least ~ 30-day mission in orbit detailed gravi~ md altimetry measurement of He tides (with -m accuracy)' iee-peneb~ing radar, md ~ integrated eamer~remote-sensing package. The panel also assumes And recommends) some augmentation to this payload, including ~ magnetometer md some surface compositional experiments) capable of meeting He Group 2 objectives. ~ addition' He panel assumes that some signif~e~t dam will be returned during the Jupiter orbital tour phase of the mission from multiple Europa' C~ymede' md Callisto flybys md from more divot observations of Io.
Europa Fib ~~ HEW FR0~ IN =E 50~R HIM The pme1 considered To levels of po~ntia1 larded science ~ Europa. The Europa Pathfinder concept involves ~ small (~10- to 20-kg) payload delivered to the surface from ~ orbiting spacecraft using ~ retro- propulsion system md airbags to achieve landing. Total system mass is in the vicini~ of ~200 kg, including ~e re~o-propulsion md airbag lading systems. A key feature of the mission studied to dam is ~ compact larder body capable of operating from art arbitrary landed attitude. Proposed instrumentation could include ~ sophisticated geophysical Cation with seismiciacoustic sensors, magnetometer' arid possibly ~ tilt merry combined with surface elemental arid phase composition measurements of ~e immediate vicinity of ~e larder using some combination of optical, infrared' Ramm spectrometer' arid Laser Educed Breakdown Spectroscopy (LIBS) techniques. No subsurface sampling, sample har~dling' or prepa- ration systems are envisioned for He Europa Pathfinder. In addition to dam relayed from the lander to the orbiter' eomplemen~ry orbital science is assumed, win the devils to be determined by result of the Europa Geophysical Explorer mission. Technology needs include airbag arid larding systems for the Europa environment. Europa Astrobiology 1~r A more ambitious Europa mission concept involves ~ study of orgar~ie ehemis~y md possible biosignatures from ~ landed station. The science rationale md some of He experiment concepts for such ~ mission have developed recently in ~ series of workshops sponsored by the Europa Focus Group of He NASA Astrobiology Institute' md are eomplemen~ry to objectives developed by He 1999 NASA Campaign Science Working Group for Prebiotie Chemistry in the Outer Solar System, but no complete system mission studies of He concept have been performed. The key elements that distinguish this candidate mission (which could also carry some of the same payload as that on the Europa Pathfinder Lander) are He inclusion of subsurface sampling capability to obtain material that is less processed by radiation (~ depths greater thm approximately 10 em) md sample handling md sample prepa- ration for ~ sophi~iea~d chemical analysis suing including ~ gas ehrom~ographimass spectrometer md the coring instrument. This greatly extends the compositional eapabili~' md particularly He eharae~rization of organic materials' from ~~ envisioned for He Europa Pathfinder, but with ~ significant increase in eomplexily md cost (unquantified ~ presen0. As does He Pathfinder' this concept assumes either prior Europa Geophysical Explorer dam for global context mdior orbital science on id own supporting orbiter delivery spacecraft. ~ addition to radi~ion-hard electronics' this class of mission requires signif~e~t technology development in the experimental areas of highly compact md sophi~ie~ed chemical analysis systems. 17~ Explorer It is expend ~~ Cassini-Huygens results will set He agenda for the future exploration of Titan. However' ~ number of studies of mission concept that would form He basis for future exploration of Times atmosphere md surface have already been discussed. These are bred on anticipation of what Cassini-Huygens will accomplish md also on its known limitations. On He basis of these studies, He panel assumed ~ generic Titm Explorer that would be capable of addressing mmy key questions in the relevant areas. The mission assumes He use of aeroeapture ~ Titan to deliver ~ orbiter md ~ atmospheric <~aerobot.~, The key element of the proposed exploration are mobility within the Ionosphere so thy different levels' weather, md processes em be studied in detail win in situ experimentation, including aerosol collectors' mass spee~ome~rs, md over Ionospheric structure md composition ins~umen~tion. In addition' He system is assumed to be capable of making high-resolution remote observations of the surface from various altitudes md of descending to the surface multiple times during He mission to make elose-r~ge md possibly in situ measurement of surface composition md properties. Although landed packages delivered by the a~nospherie vehicle have also been discussed in various combinations win the atmospheric experimentation' He panel assumes He simpler
LAT~E SA=~S (single aerobof with surface lurching capability) for its evaluation ~ this time. The orbiter is assumed to have limited communications arid some science capability' perhaps focused on global context for ~e Tim Explorer dam md studies of the stratospheric regions not reached by ~e aerobot. Technology-development needs include ~ range of technologies for the aerobot (or over forms of Ionospheric mobiliW), ~ well as and radioisotope power sources for long-life operations. Ur~ 0761~r An orbiter mission to Urar~us is assumed to ~ able to address key sa~lli~ objectives Trough repeated flybys of the five major sullies in the system. Geological, geophysical' arid geochemica1 characterization of ~e sa~lli~s should ~ equivalent to thy achieved for the Galilear~ sa~lli~s by Galileo md ar~ticipa~d for the Satum system from Cassini-Huyg~s. A suite of remo~-sensing camer~spec~ome~r systems md space physics instru- men~tion for studying magn~osphere-sa~lli~ interactions is assumed. Neptune 0~r The Neptune Orbiter mission was of particular in~restto this parcel because of Be opportunity to study Triton' ~ world known to have intriguing volear~ie arid atmospheric aetivi~ despite its low surface temperatures. For in purposes, He panel assumes that such ~ mission would include repeated flybys of Triton with ~ instrument suite equivalent to ~~ of He Galileo or Cassini orbiter systems. As noted below' this mission is only feasible using advanced technology for solar electric propulsion combined win advanced aeroeapture or nuelear-elee~ie propul- sion to achieve orbit with ~ acceptable payloadiflight-time eombin~ion. Medium Missiom lo 065~r The mission concept for Io involves either ~ Jupiter orbiter dedicated to multiple close flybys of Io or ~ multirole mission, win part of the mission md payload being devoted to magnetospherie space physics goals md! or atmospheric md aurora1 observations. The assumption ~~ this mission could achieve the stand goals within this eon Amatory rests partially on assuming that heritage from He Europa Geophysical Explorer would plow significantly reduced posh. A suite of remote-sensing experiments is assumed' with emphasis on the monitoring of Iots active volemism md relend processes. Laymen 0~r The C~ymede Orbiter mission is similar in concept to the Europa Geophysical Explorer, but He impraeti- eable goal of measuring C~ymedets very small tides is replaced by ~ increased emphasis on C~ymedets internally generated magnetic field md id interaction win thy of Jupiter. No deviled studies are yet available' md the assumption that this mission could achieve the Hated goals within this eon Amatory rests partially on assuming thy the lesser radiation environment md heritage from the Europa Geophysical Explorer mission would allow signifiemily reduced eons. Neptune Flyby The Neptune Flyby mission eoneeptwould be similar to the Kuiper Belt-Pluto Explorer discussed in Chapter 1' but win ~ considerably expanded payload to achieve multiple objectives for Neptune, the ring system, He magnetosphere' md Triton. The panel assumes ~~ modern inshumen~tion designed for He study of Triton bred oneurrent knowledge from the Voyagerflybyin 1989eouldmake amajor advance over our current knowledge of Triton. The major limitation for observations of such ~ active world is that it obviously would provide only ~
~4 HEW FR0~ IN =E 50~R HIM brief snapshot of the Neptune system' arid clot definitely Carmine the presence or absence of ~ subsurface wear layer via derisions in the induced magnetic signature. Small Idiom For the reasons discussed earlier, depict missions to achieve major results in large sa~lli~ science rarely fit realistic ally in the small Gregory. Over Ins of science investment requiring resources in the rarer of Tom required by Discovery-cl~s missions or below Carl, however, make major contributions to large sa~lli~ science' although they are not, strictly speaking, new missions. Examples include the following: ~ ~~ memo for example, ~ it atom. Once ~e investment in ~ major mission is made' it is frequently possible to Bride very high science benefit from extending the lifetime ardor objectives where over resources permit. Past examples include the Voyager Urar~us arid Neptune missions, Galileo,~ extended exploration of Europa arid Io, arid the Edition of asteroid encounters to Calileo~s mission arid ~ Jupiter encounter for Cassini. A near-~rm opportunity is the likely extension of ~e Cassini orbital mission beyond ~e nominal 4-year prime mission. Detailed plarming for art expand mission has not yet bun undertaken' but sever al possible scenarios could result in major new Titar~ ardor icy sa~lli~ results for cost equivalent for or less Barb those for ~ single low-cost mission. ~ Orson- ~d pa- ~lescopm. The use of telescopic observations of all sorts has teem of ~emen- dous importmce to solar system science Id to sa~lli~ studies in particular. divestments in ~e continuing use Id upgrading of current facilities Id instrumentation' as well as the development of new systems' are vied park of balanced somatic program. (~e the more detailed discussion in the following subsection.) Key Enabling Technologies for Large Satellite Exploration New technologies cream opportunities for enhancing mdior enabling missions by ~ combination of increasing capabilities' decreasing resource use (mass' power, volume' Id so on), Id lower cost. Mmy of He key technologies are relend to all or mod of Be missions considered in this section. These include the following: ~ Telemetry. h~in~mee of essential Deep Space Network capabilities is crucial to all future missions. the period of time considered by this survey' signif~e~t improvements in systemwide telemetry capability are expend to be needed in order to handle Be dam requirement from inere~ingly sophisticated instrumentation Id the large number of potential deep-space missions. This is particularly Sue for missions relend to large satellite objectives, owing to their location in the outer solar system Id to the time-criticality of some mission phases. ~ Power systems. All past Id current missions targeting the large satellites of Be outer solar system have relied on radioisotope power systems because of the large heliocentric distances' long flight times, Id require- men~ for reliability Id radiation tolerance involved. Main~mee of this capability is critical for most if not all of the outer solar system missions considered in this study. Additions improvements in eff~eieney Id design of these systems are highly desirable for Be more ambitious missions involving landed packages md surface or atmospheric mobility. ~ For- Ed s~ rei`~ ~~ feud Do. All outer solar system missions involve' to some degree' long lifetimes' high reliability, md tolerance to reasonably large total radiation exposure from solar' g~aetie, Id planets magnetospherie sources. hey of the highest-privily large satellites (e.g.' Europa md Io) reside in extremely high radiation environments. Improvements in radiation-hard component Id design are essential to future exploration of these worlds. ~ M`croeLectron`~utonomy. More capability in smaller packages is ~ key component in achieving difficult science goals within mass' dollar, Id power eons~ain~. Hardware Id software Emcees in this area, coupled with He radiation tolerance Id reliability requirements noted above' are critical for making future missions capable of reaching their wienee goals.
LAT~E SA=~S ~5 ~ Propu~`on. Ou~r-plm~ missions in general, arid particularly missions to sa~lli~s residing pep in ~e gravity wells of large ply are severely limited by the physics of propulsion the rocket equation which dicers very small payload mass fractions compared with propulsion mass for current chemical systems. Tech- nology development in ~e area of electric propulsion is one importers component in improving this situation in the future.65 Unfortunately' current solar~lec~ic md future nuclear~lec~ic technologies do not offer large benefits for ~e mission Apes considered here' except for Neptune Orbiter. Nuclear-elw~ric systems could potentially yield huge improvements in payload mass arid capability for more dished' future missions win large energy requirements. ~ Acrocapture. The use of ~ planets or sa~lli~s atmosphere to slow art approaching spacecraft is another approach to solving the low-payload-mass problems noted Gove. The precursor Ethnology of aerobraking has already bun demonstrated ~ V=us arid Mars. Furler research into materials' structures, arid techniques required for full aerocapture are necessary in order for future missions to take at of this technique. Tim orbiters arid Ionospheric explorers are one highly promising use of this Ethnology. One of the po~ntia1 missions of brew interest for both large sa~lli~ md gimt pit research ~e Neptune Orbiter requires eider solar-electric propulsion combined with advanced aerocapture capability or nuclear-electric propulsion to achieve art acceptable payload md mission capability. ~ P~ proton. Marty of the satellites in this study are potentially interesting for the study of organic chemist' prebiotie ehemishy, md environments of biological interest. Examples include org~ie-rich Tim arid satellites that may have liquid subsurface ocems. Exploring these environments while maintaining ~ acceptably low risk of eon~minating Hem with terrestrial organisms poses new challenges. Improvements md research in techniques of planetary protection are needed to address these issues for future missions. Sinpporting lle~r~ for Large Satellite Exploration Mmy previous N1~C md NASA advisory report have stressed Be importance of bow adequate resources for mission dam analysis md ~ strong' ongoing research md analysis program in solar system science. It is partieu- larly important to emphasize these areas in ~ strategic study such as this' because their relationship to what is usually seen as the major science aetivily of NASA that is' flying missions is complex md frequently mis- understood. lleseareh relend to solar system exploration in He current era is unusual in that it is funded almost entirely by only one office within one federal agency (NASA). Other disciplines in physics, astronomy' md He geosciences typically are supported by programs in multiple agencies md offices' university programs, indus~ia1 research' md even s~te-sponsored research programs. The idealized' academic view of NASA,s relationship to He solar system research community is that NASA flies the missions ~~ the researchers say are most important md Den supplies He dam to the community' which proceeds to go about the business of "doing science,' with it. ~ reality, mission md research Utilities are so closely coupled within NASA ~~ He very research designed to utilize dam from past missions md develop He seientif~e basis md instrumentation for future missions is often in direct competition for scarce resources with He missions themselves. These areas must be given equal weighting with individual missions to arrive ~ ~ shone program of solar system exploration. The panel considers four closely fomented types of research: Mission dam analysis' lleseareh md analysis, Laboratory studies' md Earth-based astronomy. Meow Da~' Analysts Each mission has ~ e ore group of researchers involved directly with the mission md its experiments. In addition' Here is always ~ wider group of scientists win particular ingress in He missions objectives who
HEW FR0~ IN =E 50~R HIM independently participate in the analysis of mission dam ~ various levels. Typically mission d~-ar~alysis programs fund the acquisition arid initial analysis of mission dam during the missions active phase md for some years Forward, frequently with ~ broadened pool of research proposals. The split between what are regarded as direct project cons arid Gosh ~~ are part of ~e broader R&A budget has varied from mission to mission over time. h ~d Analysis As noted above' R&A is not always clemly separable from dam m~ysis' but $enera11y is the program area thy funds researchers to perform what is frequently referred to as ``basic research,, in the field. This includes theoretical' observational' md experiment studies arid the paralysis of dam from marry sources' not just one . . mlsslon. Laboratory Stymie Laboratory studies are, of course, one aspect of R&A, but historically they have been viewed separately' because support commonly requires subs~tia1 inveshnent in acquiring arid mainlining relatively lar$e-~ale arid costly equipment. Ec~-Bmed Astronomy Ground- md space-based telescopic studies have been important to the development of our understanding of large satellites dating back to He discovery of Jupiter~s moons by Galileo Calilei md Simon Marius in 1610 md continuing to Be present day with observations of Ions volcanoes' Tithes surface, md spee~oseopy of mmy satellites. These observations md mmy more provide the basis for formulating He plme~ry exploration missions' instrumentation' md experiment that have led to our current sate of knowledge. Telescopic observations also play ~ vital role in supporting md extending He results from missions, commonly ehm$in$ our way of analyzing these result or prompting further investigations' often while the mission is still active. Capabilities from He ground md from Earn orbit strongly complement Pose of missions by uniquely enabling He following: ~ Lon$-term studies of, for example, the se asona1 response of Titans md Triton~s Biospheres md the rapid evolution of Ions surface; on Tim; Investigations of rare events, such as major volemie eruptions on Io md large cloud systems' or Worms ~ Measurement win instruments ~~ are not yet feasible for spaceport observations' md He development of new techniques md instrumentation for future space applications; ~ Continual studies of satellites before md after space missions ~~ frame questions md provide ~mpora1 context; md ~ Technical support for He success of spacecraft missions' such as the ongoing determination of wind fields on Time needed to track the Huy$ens probe. At present planets astronomy is supported primarily Trough NASA's Infrared Telescope Facility' ~ 3-m telescope on Mauna Kea, for which half the time is Eloped to planetary investigations. ~ addition' limited observing opportunities exist on the Hubble Space Telescope md large ground-based systems (such as He Keek telescopes). The I1lTF plays ~ key role in plme~ry research, with st~e-of-the-art infrared instruments, quick response to time-eritiea1 events' md ~ scheduling facility that allows the investigation of lon$-term planets phenomena. Continued main~nmee md upgrading of these facilities are essential for future plmet~ satellite research. Mission development md scientific return md fundamental research also require ~ate-of-~e-art eap~ilities from He ground' such as the proposed Gist Segmented Mirror Telescope (GSMT)' md the James Webb Space Telescope (TWST) in Earn orbit. The advantage of ~ GSMT, win ~ aceomp~yin$ advance in adaptive optics'
LAT~E SA=~S ~7 is the increased sp~ia1 resolution arid sensitivity to faint sources. A GSMT cm address questions such as ~e weather on Titan, the vertical structure of Ions atmosphere arid its ~mpora1 evolution' volcar~ic activity arid surfwe charges on Io, arid the seasonal wind field on Titers. To address Base md other topics, perry astronomy must play art active role in the scientific syzygies for the proposed large-aperture systems. I>IFYING THEMES AM KEY SCIENTIFIC <'IJESTIONS FOR LARGE SATELLITE E~LO1lATION The Large Sullies Pme1 evaluated md orgar~i~d key scientific questions around four major themes ~~, in id opinion' best capture ~e most importmt scientific questions pertinent to large sa~lli~. They are as follows: ~ Origin ~~ ~o~`on of sami~ systems. Tidal hewing md orbital evolution have 1~ to complex histories for some large sullies. Sa~lli~ systems may form arid evolve in ways ar~alogous to ply systems but are much more accessible for detailed study Bars are extrasolar perry systems. ~ origin ~~ woiution of wamr-~4 Pro `n icy sami~. Evidence for wear within ~e icy Galilear~ satellites has led to ~ new paradigm for ~e potential habitability of plar~etsystems. Europa offers ~e greatest po~ntia1 for finding lifer because He subsurface wear may Ingram with He surface md the milieus miracle. ~ Exploring or- environment. Although orgar~ie materials are common in the solar systems only Earth md Titan allow He study of organic ehemisby in the presence of ~ Hick atmosphere' ~ solvent md ~ solid surface. Titm may enable study of He conditions leading to the origin of life. ~ Un~ng dynamic pantry pro. We em best understand physical processes by observing them in action' md satellites such as Io' Time md Triton offer ~ broad rude of current activity' from the interiors to the surfaces, atmospheres' md magnetospheres. Onion and Evolution of Satellite Systems The satellite systems around the gist plme~ were formed by processes reasonably analogous to Hose that formed He solar system. The proximity of these sa~lli~ systems (as opposed to ex~asolar planets systems) allows deviled study of the results of four different ~eretiona1 <<experiments.' The exhasolar plmet~ systems observed to day tend to contain Milt planets' md the apparent rarity of ~rrestria1 plme~ within ~ few as~onomiea1 unit of the eenLa1 par makes understanding the origin md evolution of satellite systems ~ sup toward underst~d- ing the origin md evolution of extrasolar planetary systems. Study ofthe Jovian system has revealed the importance of resonant orbital interactions in the evolution of satellite systems. Io demons the importune of tidal hewing in providing ~ energy source for internal dynamics' while Europa may provide ~ example of ~ habits that depends on this energy' ~ idea that has considerably broadened our concept of habitable worlds. Exploration of He ou~r-plmet satellites eon~ibu~s to our understanding of how He orbital md thermal evolution (coupled through tidy interactions) of satellites md satellite systems leads to He development of habitable environment. The following key questions emerge as He most important next sups toward underfunding He origin md evolution of satellite systems: ~ How do conditions in He protoplmet~ nebula influence the compositions' orbits' md sizes of He resulting satellites: ~ How do factors such as sing composition, orbital evolution, md tidal hewing influence the differentiation md ouches sing processes in large md midsized satellites: In particular' why is Tim the only large satellite win thick atmosphere: To what extent are the surfaces of icy satellites coupled to Heir interiors (chemically md physieally)9 ~ How has the impaetor population in the outer solar system evolved Trough time, md how is it different from He inner solar system: ~ What does the magnetic field of C~ymede tell us about its Herman evolution, md do over large s~elli~s have intrinsic magnetic fields:
Ads HEW FR0~ IN =E 50~R HIM Origin and Evolution of Wnter-llic~h Environments in Icy Satellites Perhaps the most significar~t question the humankind cart ask md effectively address about ~e universe around us is, Are we cloned In the coming decade arid continuing into the decade beyond' solar system exploration has the opportunity to make significant advances toward ar~swering the question of whether life does or cart exist beyond Earth in ~e solar system. Based on Galileo results ~ new paradigm has emerged in which martyr if not mock large icy sa~lli~s ~~ circle cold gas giant ply in the solar system arid other perry Xylems contain liquid-wa~r oceans. This paradigm shift implies ~~ the habitability zone around our star arid other stars is extended to include ciroumplar~ry belts surrounding Jupi~r-si~d ply. Four top-leve1 questions emerge: cart arid does life exist in the inferno occur of art icy sa~lli~9 What combination of sing energy sources, composition' arid history produce long-lived inferno occurs What is the distribution of inferno wa~r' in space arid timed ~ What is the chemical composition of ~e wa~r-rich phase' arid does surface chemistry reflect interior occur compositions Exploring Or~nnie-lli~h Environment Times weals of organic myriad md its possible seas uniquely resemble Pose of Earth. Titan illuminates the organic chemistry that proceeds in more reduced environments than Earths. It is ~ intact chemical laboratory where ultraviolet photolysis md electron bombardment initial the synthesis of carbon md nib open that ultimately forms complex organic solids in He s~ato sphere. Less well understood is Be long evolution of chemist Times surface' where both organic liquid md solid preeipi~tes are predicted. In addition, Tim is believed to support ~ liquid cycle involving atmospheric methane vapor md surface liquids. As such' clouds form over bodies of liquid, rain occurs, md the eireul~ion responds to Be release of latent head as on Earn. Yet' on Titan Be energeties driving these events differs from the terrestrial experience. Tim provides us with ~ new perspective on weather processes inherent to our home planet. Most imported it serves as ~ natural laboratory in which complex prebiotie chemist may have evolved. The following top-level questions emerge: What are the chemistry, distribution, md eyeling of organic materials on Titm: Is Titm internally active' producing water-rich environment with potential habi~hility: What are the current sate md the history of Times surface: What drives He meteorology of Titm: Has Here been climax edge on Titm: Could Tim support life forms ~~ do not require liquid wharf Understanding Dynamic Planetary Pro~ses The outer-plmet satellites are natural laboratories for ~ diverse rime of physical md chemical processes of grew interest to scientist md those who value science. These processes ergot be studied in small artificial laboratories, md some of ~em' such as active flood volemism' ergot be studied in nature on our own planet. Io is the most extreme example of ~ active world ~~ includes vigorous mmile convection, volemism' Platonism' atmospheric loss, md magnetospherie interactions. Cracking' faulting, md diapirism in Europa~s fee shell are probably still active. C~ymede has ~ active eve md magnetosphere. Titan has active meteorology' atmo- spherie chemist, md perhaps active effluvial,' md volemie processes. Eneeladus mud somehow have supplied the E ring of Saturn. Triton has active geysers md perhaps active glaciers md diapirism. Magnetospherie sputtering md implantation modify mmy satellite surfaces. Perhaps He best way to illustrate He rich science potential is to list the relevmt key questions in three categories: ~ Woo are t~ ~~e Boor pro ~~ t~r reunions ~ ti~i ~~ I ~~i patters of vo~sm ~d tec~sm~ Speeifieally: What is the nature md history of C~ymedets active cored Does Io have
LAT~E SA=~S ~ magma ocear~9 Are Bare active magmatic processes in European silicate cored Do Titar~' Triton' Enceladus, or other sa~lli~s have active interiors ~ Wit are t~ currently ~~ve endogen~c geology processes (vo~'sm, tecton~sm, ~d ~ap`~'sm) ~d wit c~ we ~~m Boat s~h processes 'n generalfrom t~e ~~e worm ~ Specifically: What cart Ions high hem flow, ulbam~c lavas, large-scale eruptions, arid tectonics ~11 us about ar~cient geologic processes on ~e rreshia1 ply How active is the fracturing' faulting' arid diapirism in Europe ice shell' arid how often does liquid wear reach ~e surfaced Are there active volcanic or tectonic procesms on Tiny What drives the geysers on Triton: solar or inferno energy ~ Woo ~~ '~ pompon processes cm~ On on die surfaces ~~ `~ Coo or geyse~e pats atmospheres Clomp ~~ mc~etosp~es ~ Specifically: How cart the dummies of plumes on Io arid Triton be explained~ Do my over sa~lli~s have active ventings What cart ~ learned about perry meteorology from Tiny How active is Tithes ``hydrologic', cycle' arid how does it modify the surfaced cart Io-like ma$n~ospheric interactions enable dim overy of large sa~lli~s around exhasolar Jovian ply How do sallies lose volatiles arid a~nospheres~ Key Measurement Ohje~ives for Exploring Large Satellites Table S.2 summaries this Barbells effort to quar~tify measurement objectives md rate the capabilities of current md future missions in meeting Hose objectives. For each key scientific question, He panel identifies several critical measurement objectives. Because ~ measurement objective may be met by using several different techniques' the suite of instruments that should be included in the payload of each of He missions is not explicitly identified. If ~ measurement objective is not applicable or is urmehievable by ~ mission this is designed by << ,,. However' if ~ significant advance in understanding of ~~ measurement objective would occur from ~ mission' He mission is assigned ~ single <<x.~, Major advances are signified by`<xx',, md my expend breakthroughs in understanding are indicted by "xxx.~, Through this approach' missions to Europa md Titm shod out as He highest priority. This analysis also illustrates that ~ flyby-lype mission such as the Neptune Flyby fairs poorly in the ruing matrix because mmy important measurement objectives em only be met from He global mdior Import coverage provided by ~ orbiting spaceport or from in situ surface measurements from ~ lander. It should be mood that the Uranus Orbiter also fares poorly compared win the Neptune Orbiter' because dynamic Triton is especially interesting. These results are incorporated into Table S.3 in He section below, together win ~ discussion of mission targets. 1tECOMMEN~ATIONS OF THE LARGE SATELLITES PANEL TO THE STEERING GllOl]P lintionale for lle~mmendntions As deviled above (see Table S.2), there are several key questions answers to which would lead to major seientif~e advances or breakthroughs in eharae~rizing the outer solar systems large satellites. However, space- er~ missions md other initiatives with costs approaching ~ billion dollars must do more ~m advance scientific disciplines. They must address He most basie questions of importune to all of humanity, such as He questions that motived this survey: Are we alone: Where did we come from: What is our destiny: The Large Sa~lli~s Panel has identified four relev=t high-priority questions ~~ em be addressed Trough the continued study of large satellites. They are as follows: I. Is there extmt life in the outer solar system: 2. How far toward life does organic ehemisby proceed in extreme environments: 3. How common are liquid-wa~r layers within icy satellites: 4. How does tidal hewing affect He evolution of worlds:
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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
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
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
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, <<Gravit~ior~1 Cor~rairds on the In~rr~1 Structure of G~yme~ >>' Nature 384 541-543' ~ 996. 2. J.~. Ar~rson' G. Schubert' R.A. Jacobsen E.L. Lau' Ed W.B . Moore' <<Europa>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<Shape' harm Radius' Gravi~ Field Ed tem:~l Structure of Calli~o>~' Icarus ~ 53: ~ 57- ~ ~ ~ ~ 200 ~ . 4. J.~. Ar~rsor~> R.A. Jacobson E.L. Lau' W.~. Moore' Ed G. Schubert' <<Io>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~> <<Subsurface 0~s ore Europa Ed Calli~o: ConskairJ~ from Galileo h4~etometer Ob~rvatior~> Icarus 1~: 329-3~> 2000. ~ . h] .G. Kivelsor~' K .K . Khur~' Ed h] . Volwerk' I<The Permuted Ed Elusive h] athletic h] omens of G~yme~> 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<P~yll Secondaries Ed Other Small Cr:~rs ore Europa>~' ?. 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' I<Tritorl>s 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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