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7 Priority Questions for Solar System Exploration' 2003-20 13 * The Basis for an Integrated Exploration Strategy The IS themes md more than 100 scientific questions spring six categories of targets Vised in Part One give ~ clear view of Me scope' complexity, md diversity of con~mpor~ solar system studies. They provide evidence for the richness md breads of We knowledge that has been gained from four decades of solar system exploration. These questions also ~~l of how much more Were is to learn win regard to vital fundamental issues about the solar system. To Egress these questions' the SSE Surveys panels proposed ~ broad range of future flight-mission dads (see Part One), which are summarized in Table 7.~. The purpose of this chapter is to indurate We mmy scientific questions posed by the individual panels into ~ small At of key questions of We highest scientific privily' from which it is possible to Drive ~ pra~ica1 program of exploration for We next decade md ~ glimpse of the future ~~ it herpes. SiEl1ING PllIOllITIES ll~iona1 judgments as to scientific priorities must take into account con~mpor~ motivations for solar system exploration' which tend to be reflections of We mod profound questions md the most signif~mt of recent discoveries. The most basic motivating questions for solar system exploration, which also reflect the ingress of the public' must play ~ role in sewing priorities for the future: Are we alone: Where did we come from: What is our destiny: The discussion in the previous chapter documents the intimate associations of these questions with robust planetary exploration program. Assessment of priorities for Me next Deere mud take into account the discoveries md successes of the recent past md Me po~ntia1 for resolution of high-level questions. ~ its analysis of Me inputs from id panels md from the so lar system exploration community (see Appendix B) the SSE Survey arrived ~ ~ list of what it asserts to be the mod signifiem~diseoveries of the pest decade (see Box 6.~. Moreover' Me mmy questions raised in Part One illustrate some of the more profound mysteries thy still confront us (see Box 6.2~. Lastly, Me Survey nods thy it is intrinsic to Me nature of science ~~ priorities must be continually adjusted to take account of new findings, md that such adjus~nen~ are sometimes unexpee~d md su~en. 175

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176 TABLE 7.l hlission Concepts Proposed by the SSE Survey~s Par~els HEW FR0~ IN =E SOLAT ~M Par~1 h~issior~ Concept Name Cost C1~s I~er Plar~ets V~us In Situ Explor er h4edium South Pole-Aitkm B~in Sample Return h4edium Geophysica1 Network Scier~e h4edium Venus Sample Returr~ Large h4eroury Sample Returr~ Large Discovery missions Small Pr im itiv ~ ~ o die ~ Kuip er ~ elt -P luto Exp 1 or er h] ~ d iu m C:om et Surface Sample Return h4edium Troj a~er~taur Reco~aissance Fl~y h4edium As~roid Rover~ amp 1e Returr~ h4edium Comet Cryoger~ic S ample Retum Large Discovery missions Small Gi~t Pl~ets C:~smi Extended Small Jupiter Polar Orhiter with Probes h4edium Neptune Orhiter with Pr oh ~ Large Saturr~ Rir~g Ob~rver Large Ur~us Orbiter with Probes Large Discovery missions Small L arge S ~11 ite ~ Eur op ~ ~ ~ ophy sic a1 Expl or er L arge Eur op ~ Lander Large Titan Explorer Large Neptur~e OrbiteriTritor~ Exp lorer Large Io Ob ~rver hiedium G~yme ~ Orb i~r h4edium Discovery missions Small h] ars hl ars S ample Return Large I`l ars S cience L~ or atory h4edium I`lar Long-Lived Lander Network hiedium hlars Upper Atmosphere Orhiter Small I`lars Scout missions Small NOTE: h~issior~s ir~ boldf:~ are ~ short li~ ~eloped in this ch:~p~r ir~ re spon~ to the ~ ~ key scierdific que~ior~. Judging wientifie priority requires careful eonsider~ion md ehoiee of the eri~ria ~ are used to make ~e judgment. The SSE Survey~s criteria are these: Seientifie merit < OCR for page 175
priory ~oUE~oNS FoT SoMP ~~M EXpLomHoN ``OpportuniW,, has to do with ~e practical mater of achieving ~ resolution to the question under considera- t~on. A positive measure of opportunity could be ~ favorable budgetary situation, or ~e favorable orbital con- figur~ion of ~ ply. Other possibilities exist for example' successes in relend research in Brother scientific field' or ~e concurrent development of ~ mission or ~ Ethnology win relend objectives. Assessment of ~chnologica1 readiness is ~ powerful tool for making judgment' as is seen more clearly in ~e following shaper on mission priorities. It cm also ~ of use in judging the relative priorities of fundamental scientific questions. For example, if answering such ~ question demar~ds deep drilling into ~e subsurface of some diary solar system body md the subsequent return of ~ sample to laboratories on Earth his will surely affect thy questions priority with respect to ~ question thy perhaps has Bum scientific merit but requires little more Earl' 177 . - 1 . . ~ ~ 1 . - . . ~ say, the easier task of collecting remo~-sensing day for its resolution. TWELVE KEY SCIENTIFIC <'UESTIONS THAT I}N~ElIPIN THE OVERALL EXPLORATION STRATEGY The SSE Survey defines four broad' erosseuding themes thy integrate the various goals identified by He parcels in Part One: The First Billion Years of Solar System History; Vol~iles md Organdies: The Stuff of Life; The Origin md Evolution of Habitable Worlds: md Processes: How Planets Systems Work. Next, the SSE Survey identifies 12 top-level questions that represent the disillusion of more than 100 individual questions identified by Be Survey pmels; the 12 questions are glamorized within the four erosmu~ing themes. ~ 1~ first Brow Yeas of Sour System History. The processes ~~ occurred during this epoch propelled the evolution of Each md He over planets. Plmetary-seale dramas were played out during those formative years' including He emergence of life on Earn. Yet this epoch in the solar sys~m~s history is poorly known. Three top- level questions emerge: I. What processes marked Be initial stages of planet md sa~lli~ formation: 2. How long did it take He gas gist Jupiter to form' md how was He formation of the fee gists (Urmus md Neptune) different from that of Jupiter md its gas gist sibling, Saturn: 3. How did the impaetor flux deeply during the solar systems youth' md in what way~s) did this decline ir~uenee the timing of lifers emergence on Earth: ~ Vocables ~d Organist: 1~ Stu~ofL`fe. We are truly made of Restuff. Life requires organic materials md vol~iles' Doubly liquid whorl originally condensed from or acquired by the protoplme~ry nebula md later delivered in some degree to He plme~ by org~ie-rich biometry md as~roida1 debris. The distribution md transport of volatiles md org~ies are intimately linked to He evolution of our plme~ry system md the sate in which we find it today. These Tree top-level questions emerge: 4. What is the history of volatile compounds, espeei~ly whorl across the solar system: S. What is the nature md history of organic material in the solar system: 6. What global meehmisms affect the evolution of vol~iles on planetary bodies: ~ ~ origin ~~voiution of ~~ worm. Our concept of the <`h~itable zone,' is being expanded by recent discoveries on Earth md elsewhere in He solar system. Whether or not life has Ken hold in He solar system beyond Earth' the implications are equally profound. Understanding our plme~ry neighborhood will help

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778 HEW FR0~ IN =E 50~R HIM to trace the evolutionary paths of ~e other ply md ~e few of our own. The four top-leve1 questions thy emerge are Apse: 7. Where are ~e habitable zones for life in ~e solar system, arid what are ~e perry processes respon- sible for producing arid sustaining habitable worlds: 8. Does (or did) life exist beyond Early 9. Why did the ~rres~ia1 plar~ets differ so drily in Weir evolutions 10. What hoards do solar system objects present to Ethos biosphere: ~ Processes: How Cry Stems Work. Understanding the operation of fundamental processes is the firm foundation of perry science. Studies of perry interiors, surfaces, atmospheres' rings, md magneto- spheres are windows into the evolution of worlds. Studying procesms in our perry system allows extrapolation to extr~olar ply arid of bosh ply - systems to ours. Two top-leve1 questions emerge: ~ I. How do the processes thy shape the contemporary character of plar~e~ry bodies operate arid Ingram 12. What does He solar system tell us about the development arid evolution of ex~asolar planetary systems arid vice versa: All of these questions are of high scientific merit. Most have He potential to lead to major paradigm shifts in our general understanding. All have He po~ntia1 of being pivotal md could lead to new pathways in solar system research. All will lead to ~ inere~e in the factual base of our knowledge. All' as indicted below, em be subs~ti~ly addressed win reasonable levels of ~ehniea1 development' md most em be addressed within He envelope of opportunities that are implied in He proposed New Frontiers program md NASA,s ongoing Mars md Discovery flight programs, along win less frequent Flagship-elass missions. Mmy of these questions build directly on recent discoveries. They will also help elucidate the outriding mysteries Tout He nature of the solar system md make significant progress toward answering the most basic motioning questions. The missions win the greatest potential for answering these high-priori questions are specified below md listed in Table 7.2. llECOh~lENDED MISSIONS TO ANSWER ELEY QIJESiTIONS The First Billion Years of Solar system History Wit Processes Aid the Jilt Stages of P~t ~d I Formf~~on ~ The plmet~ system secreted from ~ spirming disk of gas md dust (the solar nebula) surrounding the proto- Sun about 4.6 billion years ago. Beyond Neptune' He solid material never acereted into the major plmets but remains as ~ vast collection of objects known as the Kuiper Belt. These icy bodies hold clues not only to the origin of He outer plied but also to the origin of Earths inventory of volatiles md possibly to the origin of prebiologiea1 organic myriad on Each. Because of He cold temperatures ~ trms-neptunim distances md because smaller objects are less likely to have undergone internal differentiation' the smaller Kuiper Belt objects (KBOs) are thought to be relatively unmodified since Heir formation. It is therefore expend that studies of the chemical composition of KBOs will provide knowledge of the pathways of volatile md organic molecular materials from their in~rs~llar origins to their disposition in Ens hydrosphere' atmosphere' md biosphere. The Kuiper Belt- Pluto Explorer (KBP) mission constrains the bulk properties of sever al KBOs' including the be~-studied of these icy objects, Pluto md Charon, by determining their densities. Radii are precisely measured by high-resolution imaging md solar occultations, md masses by measurement of He gravitations deflection of the spacecraft. Moreover' the mission observes He surfaces md atmospheric constituent of Pluto, Charon, md over KBOs high spatial md spectral resolution in order to determine He composition md distribution of vol~iles. These measurement e~of be performed win neatest precision from Earth-based telescopes but em be achieved with KBP. Thus' the KBP mission is Gentry to addressing He nature md composition of the plme~simals that are

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priorly ~oUE~oNS FoT SoMP ~~M EXpLomHoN TABLE 7.? Most Relevant Missions to Address Fund~nenta1 Scientific Questions 179 Fur~damer~ta1 Scientific Question Most Relevart Missions The Few E'[aio~ Yews of so'= System Eat 1. What protests marked the initial ~~s of plant arid satellite formations 2. How lord did it take the gas diary Jupiter to form' arid how was the formation of the ice gists (Ur~us Id Capture differed from that of Jupiter Id its gas-gi~t sibling' Spume 3. How did the imp actor flux deem during the solar sy~em>s youth' Id ire what warm did this Flirt ir~fluer~e the timing of lifers emergence ore Earths vo~s =4 of: ~~e stuff of oft 4. What is the history of volatile compounds' especially where across the solar systems 5. What is the recurs of the organic m~eria1 ire the solar ~~em~ Its history 6. What glob al me char~isms affect the evo lutists of vo lazily ore planetary ~ odie s The 0~ a~d Erg of Eat Worlds 7. Where are the habitable zones for life ire the solar ~em' arid what are the planetary pro ~ sse ~ re sp onsible for pro ducir~g Id sustair~ir~g he itable worldly S. Does (or did) life exist beyond Earths P. Why did the ~rre~ria1 plants differ so dramatically ire their evolutions 10. What hawrds do solar system objects present to Earth>s biospheres Processes: Eow P~ - Systems Wow 11. How do the processes that shape the contemporary character of planetary bodies operate arid i~era~9 12. What does the solar system tell us gout the development Id evolution of xtrasolar p l=etary ~ems' Id ~ ice verse Comet Surface Sample Return Kuiper Belt-Pluto Explorer South Pole-Aitker1 ~ asir1 Sample Retum Jupiter Polar Orbiter with Probes Neptune Orbiter with Probes Kuiper Belt-Pluto Explorer South Pole -Aitker~ ~ shirt Sample Retum Comet Surface Sample Return Jupiter Polar Orbiter with Probes Kuiper Belt-Pluto Explorer Cassini Extruded Comet Surfed Sample Return Titan Explorer Mars Explor~ior1 Program Mars Upper Atmosphere Orbiter Venus Ir1 S itu Explorer Europa Geophysical Explorer Mars Lor~g-Lived Larder Network hi ars Samp le Retum hi ars Scierloe L ~ ornery Europa Leer hi ars Samp le Retum Mars Lor~g-Lived Larder Network al ars Samp le Retum al ars Scierloe L ~ ornery Venus Ir1 S itu Explorer Large Skeptic Survey Telescope Cassini Exter~d' Comet Surfam Sample Returrl' Europa Geophysical Explorer' Jupiter Polar Orbiter with Probes' Kuiper Be it-Pluto Explorer' Mars Lorlg-Live d Labeler' Mars Sample Returrl' Mars Science Laboratory' hears Upper Atmosphere Orbiter' South Pole-Aitker1 ~ asir1 Sample Retum' Venus Ir1 S itu Explorer Cassini Extruded Jupiter Polar Orbiter with Probes Kuiper Belt-Pluto Explorer Large Skeptic Survey Telescope Nepturle Orbiter with Probes Ethic Fume lids alphabetically drily what the SSE Survey oor~si~rs to be the mo~t relevant mission c~did~es to address the scientific Junior. However' the Survey recommits that some of the mission Redid could address specific aspects of ~ scientific question exert if riot liked. h~issior~s within the Discovery program could fall ire any of the m 11s ire this column.

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So HEW FR0~ IN =E 50~R HIM best preserved from ~e initial Cages of plmet arid sa~lli~ formation. The value of this mission increases as it observes more KBOs arid investigates ~e diversity of Heir properties. KBP will make the first survey of this most poorly known but very significar~t portion of the solar Xylem. The Kuiper Belt is the birthplace of short-period comets; therefore, sampling one of them comets is ~ mems of examining the ma~ria1 from which ~e ply were built. The Comet Surface Sample Return (CSSR) mission will gently collect Madrid from one or more sins on the surface or in the near-surface layer of ~ short-period comet md return it to Earth. This will permit ~ full suite of sophisticated elemental, isotopic, orgar~ic' arid mineralogical measurement to ~ performed in ~rres~i~ laboratories' studies thy will yield unprecedented information on the materials arid chemical processes thy dominated the initial sages of planet arid Ill formation. Although it is ultimately desirable to return ~ nucleus sample ~ ~ temperature sufficiently low to preserve the full suite of ices, ~e highest priority is given to ~ mission returning ~e full suite of organic materials arid non-ice minerals from the surface of art accessible short-period comet. The Moon records some of ~e most ar~cient history of ~rrestria1 ply evolution. The Apollo arid Luna missions investigated ~ limited region of the Moons nearside arid did not resolve fundamental questions of how the Moons interior differentiated into layers Mar its formation. The recently recognized South Pole-Ai~en Basin is Be largest known impact structure in the solar system arid the oldest arid deepest well-preserved impact structure on Be Moon. This gimt basin allows access to materials from Be interior of Me Moon. The Soup Pole-Ai~en Basin Sample-Return (SPA-SK) mission will obtain samples of materials produced during this enormous impact event Id return Rem to Earth. Analyses of these samples in terreshia1 laboratories will permit deviled charw~r- iz~ion of the mineralogy, elemental composition, Id isotopic makeup of Be lower crust Id upper mantle of Be Moon. This will allow Be m~era1 models for early lunar evolution to be tested Id distinguished' providing insight into processes that are likely to have occurred on Earn Id the over terreshia1 plme~ during Be initial stages of their formation. 0~r Wit Period D'd ]up`~r Form, c'nd How D`d Jim Bash Differ from ~t of t~ JIce O`~ ~ During the formation of the gist planets' timing is critical' with dramatically different consequences for Be inner solar system depending on whether Mint planet formation was slow or fast. A commonly cited model is that the gas gist Jupiter formed relatively slowly' in about 10 million to 100 million years, by condensation of gas around ~ accumulated roek-iee eve of Bout 10 Earn masses. If this occurred, then Jupi~rts composition should reflect that of ~ evolving solar nebula while the solar wind was blowing the nebula away' rawer ~m ~ pristine nebula. Beyond Jupiter' where the density of the solar nebula was very low, Be over gist plme~ formed even more slowly. Consistent with this, Be bulk properties of Uranus Id Neptune suggest ~~ these fee gist plme~ formed too lye to capture Be gas of the solar nebula. Without Jupiter~s enormous gravi~tiona1 effect to disturb them, over solar system bodies would have formed in ~ relatively benign environment. On Be other h~d' observations of gas disks around nearby stars show that the gas is depleted on time scales of just ~ few million years, suggesting that the formation of Be gist plme~ may have been relatively rapid. If Jupiter formed relatively quickly, in about 100~000 years by the process of hydrodynamic collapse, Den the planet should have ~ negligible core. In this ease Be planets composition mud represent ~ relatively pristine sample of the solar nebula. This model is attractive because it might partly explain Be apparent abundance of gas give around other stars' which are commonly in close-in orbits where roek-iee cores seem unlikely to have formed. A quickly formed Jupiter would have prevented objects in what is now the asteroid belt from ever forming into ~ single planet' Id inside Be asteroid belt Earn Id over terrestrial plme~ would have been subjected to ~ rain of impacting objects matured by the forming gimt planet. Whether Jupiter formed rapidly or slowly em be deduced by whether the planet has ~ roek-iee core. Thus' Be existence of ~ eve is key to whether Jupi~rts composition reflects the pristine or evolved solar nebula Id whether the plmetts formation dramatically affected the burgeoning inner solar system. The Jupiter Polar Orbiter win Probes (JPOP) mission concept reveals whether Jupiter has ~ roek-iee e ore Id determines its mass' by carefully

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priorly ~oUE~oNS FoT SoMP ~~M EXpLomHoN Ask manuring ~e plus gravily field. Moreover, remo~-~ensing instruments md three deeply penetrating probes search for wear arid over volatiles md measure their abundar~es to provide compositional clues into the nature of the solar sys~m~s largest Claret arid the timing of its formation. The Neptune Orbiter win Proms (NOP) mission concept samples ~ distinctly different class of diary Blarney art ice gimt. Neptune~s chemistry will ~ measured by remo~-mnsing instruments md in situ with army probes. This enables direct comparison between art ice gimt (Neptune) arid ~ gas girt (Jupiter) in terms of their chemis- tries, apparent time scales of formation, md effects on ~e evolution of ~e irmer solar system. How D`d t~ Impostor Fax Decay During the Soar Systems Youth ~d ~n Wit Way(;J D~d As Decl'ne Inf ~~e t~ Timing of L`feN Emergence on Eanh~ The formation of the massive gas arid ice gimt ply probably had ~ profound influence on the early armoring ram throughout the solar system as they cleared Heir formation zones of unused debris. A sustained' solar sys~m-wide rain of projectiles could have resulted' arid this may have delayed life from gaining ~ foothold on plar~et Earth. The lunar impact record, dyed by eollee~d rock samples, is used to extrapolate surface ages throughout He solar system. However, there is considerable uneer~inty in He early flux of impacts' with two models proposed. In one, He flux decayed exponentially with time. In He other' He flux peaked ~ about 4 billion years in ~ period of enhanced bombardment ~~ would have profoundly ir~ueneed ~1 of the ~rrestria1 planets. These two scenarios have wetly different implications for the conditions under which life might have emerged on early Earth. The record of most early events on Earn is long gone, but impor~t information is preserved in the face of our neighboring Moon. To undersold the conditions on early Earth, it is impor~t to establish the age of the Moonts olden surface units. Dating of samples returned from He interior of He recently identified Soup Pole-Ai~en Basin, ~ major impact structure on the Moons farside' would establish ~ benchmark date for this earliest chronology. From this benchmark' He ancient impaetor flux would be more firmly established' with important implications for early impact processes reassessed for Earth md the other terres~ia1 planets. The SPA-S1l mission collects samples of the Moons oldest well-preserved impact basin md returns Hem to Earth where precise age- d~ing techniques em be applied. From the derived age of the Soup Pole-Ai~en Basin imply event ~ vital point of reference is established for He eratering ram during He earliest history of the Moon md infant Each. This is well-grounded md straightforward experiment ~~ builds on ~ subst~tia1 base of knowledge about our satellite eonsolid~ing id position as ~ cornerstone for underfunding the history of Earth md He over ~rrestria1 plme~. If ~ solar system-wide rain of projectiles did indeed arise from He formation of the Mint plmets notably Uranus md Neptune Men the formation of He later may also have dynamically excited the Kuiper BelL leading to id present eollisionally sculpted structure md triggering ~ influx of KBOs into the inner solar system. An investigation of Pluto, Charon' md Kuiper Belt objects will yield ~ valuable record of He size distribution md flux of impostors within this yet-unexplored region. The atmospheric escape rate on Pluto md inferred kilometers' worth of volatile sublimation erosion of its surface over He age of the solar system suggest ~~ Plums surface may be young md hence may record the present-day impaetor rate md impaetor size distribution in He Kuiper Belt. ~ contrast, the record on Charon md over KBOs is expected to be eumul~ive md to reflect He size distribution md flux of impaetors in He ancient Kuiper Belt before clearing occurred. The comparison of Pluto to Charon md other KBOs thus has impor~t implications for whether the late heavy bombardment was ~ solar sys~m-wide phenomenon' indeed whether or not it occurred ~ all. The KBP mission will image the sunlit hemispheres of Pluto md Charon ~ resolutions sufficient to determine the populations of large craters on their surfaces md to lead to ~ understanding of the modifying geological processes that have affected each surface. Analysis of these images will coonskin the role that He clearing of He Kuiper Belt played in the bombardment of the irmer solar system md in the transport of vol~iles md organdies from the deep outer solar system to early Earn.

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Ash Volntiles and Or~nni=' The Stuff of Life Wit ~ t~ History of Vo~ Mamr`~4 Espec'al~ Wait; Across t~ Sour Systems HEW FR0~ IN =E 50~R HIM Earth formed too hot to contain the large proportions of volatile materials now present, giving rise to the idea the its volatiles' including whorl were delivered to the ~rrestria1 ply after Weir accretion. Even Jupiter may have received much of id complement of vol~iles from farther out in the solar Xylem. The observed comets are volatile-rich' arid mmy move in orbits thy cross those of the plar~ts, resulting in collisions. Comets are leading dads as deliverers of volatiles to the plumed including art uncertain fraction of ~e wear now found in Earths oceans. Asteroids from the outer regions of ~e main belt may also contain vol~iles in sufficient abun- dar~e to con~ibu~ significantly to the ~rrestria1 ply. For Base reasons, strong scientific motivation exists for exploring ~e reservoirs arid trar~sport mechar~isms of vol~iles in ~e solar system. The CSSR mission will approach the surface of ~ short-period comet md gently collect myriad from one or more Dies on the surface or in ~e near-surface layer' returning orgar~ics arid non-ice minerals together with wear maintained in ~ frozen Bare. While this mission does not address the full range of scientific issues thy could be accomplished by collection of vol~ile-rich material from depth md returned ~ deep cryogenic temperatures' laboratory analyses of the cometary volatile minerals will firmly establish Be chemical standard for Be elemental arid isotopic Boundaries in short-period domed. Such comets are thought to come from the Kuiper BelL which contains some of He most primitive, unprocessed myriad in the solar system. Although repeated trips through Be inner solar system will have altered the surface regions of ~ comet considerably, mmy impor~t ehemie~ ratios will be preserved' providing impor~t insight into the history md hmsport meehmisms of water md other volatiles in the solar system. The Galileo probe returned day within Jupiter where the pressure reached some 22 bars, but it entered the ~ ~ gist planet in ~ unusually dry downdraft region md so did nof sample Be deep-water abundance that is believed to be characteristic of the planet as ~ whole. As ~ consequence, Be wear abundance in Jupiter remains uncertain by ~ least ~ order of magnitude. The JPOP mission sends three probes deep into Jupiterts clouds ~ different latitudes to measure Be abundances of Jovian wear md other elements. In addition to determining composition with depth the probes also measure winds, temperatures' clouds' md sunlight to ~ depth where Be pressure reaches 100 bars. Understanding Be abundance of Jovian water is very important to underfunding the volatile history of Earth md other plme~' because fee is Be medium by which opera less-abundmt volatiles would have been incorporated into Jupiter by plme~simals' md' similarly' could have been Spored to Be irmer solar system' including Earn. Wit ~ t~ Namre of t~ Organic All `n t~ Sour System, ~d Wit ~ I~ H`~tory~ Stardu~' ~ Discovery mission to return minute samples of cometary dust, is under way md will provide information about Be chemical composition of dust grains captured during flight Trough the coma of ~ active comet ~ high veloeily. More comprehensive investigations demand access to ~ larger sample of biometry matter' preferably one collected directly md gently from the nucleus in order to preserve the composition md structure of the sample. The CSS1l mission will collect ~ full suite of cometary organic materials md non-iee minerals' md return Rem to Earth' where deviled elemental' isotopic, organic, md mineralogical measurements em be per- formed. The mission provides ~ vital stepping-stone by sampling Be organic md nonvolatile mineralogy of comet. It would provide fundamental new day about the chemical md structural properties of prebiotie organic maher, addressing vital questions such as these: What is the handedness of cometary molecules' md what bearing does this have on the handedness of life on Eared ~ What is the ratio of carbon chain molecules to carbon rings in the domed' md how does this compare win the corresponding quantities in Be in~rs~llar dusk

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priorly ~oUE~oNS FoT SoMP ~~M EXpLomHoN Asp ~ Were the materials in domed incorporated ~ low temperatures win lisle modification, as suggested by ~e abundar~e of ~e vol~iles carbon monoxide arid carbon dioxides Or was the constituent ma~ria1 first cycled through ~ wide rar~ge of solar dietaries arid temperatures by turbulent motions in ~ heavily mixed solar nebula, as suggested by ~e presence of high-~mperature silicas in comets The atmosphere arid surface of Tim are inferred to be rich in organic materials' providing ~ natural arena for the study of orgar~ic chemistry over immoral arid specie scales unattainable in ~rrestria1 laboratories. Under- s~ding ~e pathways of orgar~ic synthesis on Titar~ may hold answers to the evolution of prebiotic chemistry on ar~cimt Earth. Cassini will enter orbit around Saturn in July 2004 arid will release ~e Huygens prom into Titmice atmosphere in 2005. Huygens will sample Titans atmosphere in situ' identifying arid quantifying its constituents. Huygens descent dam arid mapping by several orbiter instruments will provide ~ first close look ~ Titans h~- shrouded surface arid identify possible regions of liquid hydrocarbon lakes or seas. Result from Cassini arid Huygens will elucidate the fillips surfed sand atmospheric composition, md complex chemical procesms. However' War the nominal Cassini mission ends, coverage of Titar~'s surface will be incomplete. Cassini Extended (~asX) provides art opportunity to follow up on major dim overies of the nominal Cassini-Huygens mission with focused orbiter remote-sensing observations arid scientific paralyses. Because the pathways arid products of long-term orgar~io evolution on Tim may have implications for He origin of life on Early it is imports to thoroughly investigate He neural orgar~io chemist in He atmosphere arid on the surface of Tim. A future Titm Explorer ([EX) mission might consist of ~ orbiter md ~ `~aerobot,' that is able to move within He atmosphere to obtain samples md conduct experiments ~ multiple locations. The graft would include aerosol collectors' mass spectrometers, md other a~ospherio-shuoture md -composition instru- men~tion. In addition' the system would make high-resolution remote observations of the surface from various altitudes md would descend to the surface multiple times to make olose-r~ge md possibly in situ measurement of surface composition md properties. Through analyses of the products of organic synthesis on Time we will Deter understand the prebiotio processes that led to the origin of life on Earth. Wit Global MeeF~ AT t~ E~olut~ors of Cow on Pantry Bow Once delivered to the planets' volatiles may be sequestered in surface md interior reservoirs, partitioned into the atmosphere, or lost to space. For example' on Earth, CO2 dissolves in ooem water' preoipita~s as oarbona~ rook' md reemerges in subduction zone volomio eruptions. On Venus' He lack of liquid wear md plan ~otonios precludes this mechanism' md CO2 remains in ~ gaseous same md contributes to He atmospheric greenhouse. Both He CO2 md He nitrogen abundance are similar on Earth md Venus' so ~ real mystery is what happened to the wear thy should once have been present on Venus: The atmosphere md surface of Venus preserves records of ~~ plmetts evolutionary history' including He interaction of the Exosphere md surface rooks. Therefore, compositional md isotopic measurements of He atmosphere md of the surface rooks would reveal He planets interns md atmospheric evolution. The proposed Venus In Situ Explorer (VISE) mission would measure the composition md isotope ratios of the Exosphere on descent md of surface rooks on lading Moreover' He mission would retrieve ~ core sample md then undertake sophi~io~ed geochemioa1 md mineralogical measurement from ~ more benign environment ~ high altitude. These VISE result would oonshain the original complement of water md other volatiles on Venus, mechanisms of volatile origin md loss, md the interns evolution of He planet. The mission is central to understanding terreshia1 planet volatile evolution, which om proceed toward either supporting life or preventing id inception. At the other extreme, Mars vol~iles are largely happed in the polar caps md in vast buried reservoirs of frozen perm~fro~ probably overlying liquid groundwater. Mars missions currently under way, including Mars Odyssey, are revealing the pad md present reservoirs of water on Mars, as well as He processes that control He distribution. Mars Exploration Program (MEP) missions offer important po~ntia1 to continue the theme < OCR for page 175
4 HEW FR0~ IN =E 50~R HIM variability of isotopic compositions will indices sources, sinks, arid reservoirs of volatiles' arid the ply atmospheric evolution. The hears Upper Atmosphere Orbiter LAOS mission will study the upper ahnosphere of the planet to Carmine its dynamics, hot-~om abundar~es arid escape fluxes' ion escape' minimagnetospheres arid magnetic recollections, arid the energetics of the ionosphere. Them results will for the firm time reveal the coupling between the lower md upper atmosphere of Mars arid thus are key to understanding the evolution of Ionospheric volatiles. The Mars Science Laboratory (MSL - art approved mission, currently scheduled for launch in 2009 will conduct deviled in situ investigations of ~ sip ~~ orbital day identify as ~ wa~r-modified environment, provid- ing critical ground-truth for orbital remo~-~ensing day arid Aping hypotheses for the formation arid composition of wa~r-modified environments. The Ems of in situ measurements possible on ~e howl are directly relevant to martiar~ vol~ile evolution, including atmospheric sampling, surface mineralogy' arid chemical composition. The Mars Scout program also provides opportunities for missions thy investigate ~e evolution of the plus volatiles. Thus the stage will ~ set for Mars Sample Return (MSR), in which samples from carefully chosen sips will be returned to Earn arid subjected to ~ full array of ar~ica1 techniques' merging new understanding of ~e geological evolution of Mars win deviled knowledge of the chemistry, mineralogy, arid chronology of the crust the role of vol~iles' arid elucidation of the conditions ~~ could potentially have led to the emergence of life on Mars. The Origin and Evolution of Hahitahle Worlds Warm Are t~ Ale 7~s for Lift `n t~ Soar System, ~d Wit Are t~ P~ Processes ~spons~le for PI Doing ~d Sag H~ Ate Wor4 2 The boundary conditions for habitable zones in the solar system are principally constrained by ~e occurrence of liquid wear md ~ source of energy for biologist activity. On Earth, life exists wherever water occurs. Microbes thrive in both extremely hot md subfreezing temperatures' under acidic or alkaline conditions, md in He presmce of high concentrations of salts or heavy meals. Life forms capable of surviving similar conditions may have existed, md might persist today, in the subsurface of Mars md within large icy satellites, no~bly Europa. Study md comparison of planets md satellites ~~ have ~ wear history allow ~ understanding of how habitable worlds evolve. Mars is ~ Be outer edge of the traditionally defined habitability zone' md today id near-surface wear resides largely as ice. MEP missions will improve our understanding of He lied Planets potential current md past habitability by investigating He distribution md history of its vol~iles (see above), md Trough remo~-sensing md in situ investigations of He geological md geoehemiea1 processes that have operated Here. Debate will continue as to whether Mars support or ever supported life ~ least until samples are returned from carefully chosen sins on the planet. The Hall missions will collect samples from carefully selected locations md return Hem to Earth, where Hey Cube subjected to deviled mineralogical' chemical, md isotopic analyses. When correlated with remo~-sensing md in situ dam md inferred geological processes' the result of sample analyses will clarify whether He plmetts environmental conditions have ever been conducive to life. Thus, MS1l missions are ultimately critical to understanding He limits of habitability in He solar system. The putative sub-iee ocem of Europa might provide ~ different type of habitable world, one that does not rely upon solar energy. Tidal hewing provides ~ source of energy to maintain liquid water behead Europe icy earap~e. Morphological features there surged surface motions broadly analogous to the jostling of floating fee plays in Earths polar ocems. Geological processes would allow for eommunie~ion between He ocem md surface, md therefore the transport of nutrients md perhaps organisms between the surface md the subsurface ocem. Inferences about ocems within Europa md the other icy Calilem satellites have received dramatic support from induced magnetic-field measurement from Galileo' md the existence of subsurface liquid water is now widely accepted. However' mmy uneer~inties remain regarding the level of current activity, He nature of He

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priorly ~oUE~oNS FoT SoMP ~~M EXpLomHoN ~S5 sullies geological processes' the thickness of the ice shell' ~e chemist of the surface arid ocear~' arid po~tia1 energy sources for life. The Europa Geophysics Explorer (ECE) mission will address the po~ntia1 habitability of Europa. This mission orbits Europa arid employs geophysical methods specifically' gravity arid altimetry measurements of Europe tidal fluctuations to confirm ~e presence of art interior ocem arid charw~ri~ the sullies ice shell. Additions remo~-~sing observations will examine the three-dimensiona1 distribution of subsurface liquid wear; elucidate the formation of surface features' including sins of current or recent activity; arid identify arid map surface composition' including compounds of astrobiological ingress. ECE is ~e vied next sup in understanding the po~ntia1 habitability of Europa arid ~e procesms the might produce arid sustain habitable environments within icy sullies. Bow for ~~> fife ~~t Beyond ~2 Whether life exists in ~e solar system beyond Earth is among the most profound questions we cart ask. Even more profound is the fact the we cart make sub~tia1 progress toward ar~swering it during the next decade arid ~e decade beyond. Today, Mars appears hostile to life because of id thin atmosphere arid harsh radiation environment; yet life may have exited in ~e plus distant past or may still exist in subsurface reservoirs. The SNG memories are of martim origin but' because of Heir origin by random impact ejection' have urn own provenance md are unlikely to be typical of the surface rocks on hears. Already it has been suggested ~~ the SNG memories contain evidence for ex~rres~i~ life. The ambiguous md controversial nature of the evidence, however' suggest that ~ def~ni- tive answer to the question of whether or not Mars fossils exist mud await Be return of carefully retrieved samples' as proposed for Be MS1l missions. The MS1l missions will carefully collect md return martim samples for comprehensive examination on Earth, employing sophisticated analyses that could not be done in situ ~ Mars. Ply close analysis using the full range of mal~iea1 facilities available in ~ terrestrial laboratory em provide Be detail md experiment confidence to address the subst~tia1 issue of past md current life on Mars. The popular md scientific interest in Europa lies win Be possibility ~~ id subsurface ocem might eonstitu~ ~ habitable zone for past or present life. Following Be ECE mission' if ~ ocem is indeed confirmed, ~ subsequent Europa Lander (ELAN) mission should be aimed ~ in situ investigation of Be surface md its ehemis~y. Such ~ mission em search for md eharae~ri~ near-surface organic materials md perform deviled geophysical inve~iga- tions pertinent to the potential for Europa to harbor life. The potential for life in protected environments beneath the surfaces of otherwise inhospitable worlds is ~ fasein~ing possibility' undreamed of just ~ few decades ago. Why Du t~ Terres~! P~ D`~r So Dr~N ]n ~~r Evolution ~ The terrestrial planetary bodies share mmy similarities, but solar system exploration has revealed ~~ they are also fundamentally different in mmy other ways. The Moon' Mercury, md Mars stabilized their crush md lithospheres early in plenary evolution md became < OCR for page 175
USA HEW FR0~ IN =E 50~R HIM upper crust. It is critical to understar~d whether climax charade has Duly occurred ~ Mars, Prods if so' what id causes arid effects are. The hop missions will explicitly address Marcus clime charge md atmospheric evolution. To understand the plumps current sources, sinks, arid reservoirs of vol~iles' ~e h~3N mission will determine the ground-1~1 chemical arid isotopic composition of the atmosphere' including humidity, ~ ~ network of surface stations for 1~t ~ martiar~ year. To bear underbred the longer-~rm evolution of ~e atmosphere' the MAO determines ~e composition arid dummies of ~e middle arid upper ahnosphere arid measures the escape ram of Ionospheric molecules. The MSL is scheduled to conduct deviled in situ investigations of ~ sip thy orbital dam identify as wa~r-modified environment, Asking hypes for the formation md composition of wa~r-modified regions' arid providing critical ground-tru~ for orbits remo~-sensing dam sex thy are used to infer past wear. Mars Scout missions provide ~e po~ntia1 for focused studies of Mars climax charge arid Ionospheric evolution not otherwise addressed in ~e hap. MSR missions will establish ~e role of liquid wear arid weathering processes by enabling detailed laboratory study of the chemical md isotopic signatures of mineral samples md weathered materials. Corresponding measurements on vol~iles within returned samples may provide definitive evidence of past atmospheric arid chemical conditions' allowing past climax conditions to ~ underwood. Understar~ding the causes md offers of climax charge also requires in situ investigations of Venus where surface temperatures hour chart ~ oven are produced by ~ C0~ greenhouse. Cloba1 monitoring of Venus,~ atmosphere arid climate, in situ elemental' mineralogical, arid geochemica1 measurements of ~e surface' arid detailed dam on ~e noble gas isotopes md trace gas abundances of ~e atmosphere are necessary to understand Venues climate, md po~nti~ly ~e fax of Earths climax. Them are goals of the VISE mission' which will also prepare ~e groundwork for ~ future Venus sample-return mission. Wit Hazard Do Sour System Objects Present ~ Ear~s B`osp~2 Cosmic impact has the po~tia1 to eliminate humankind ~ we know it. Therefore' it is critical for us to systematically assess the magnitude of them thread. The atmospheric, geological' md biological effects of cosmic impact have become apparent only since the early I98Os, when the likely cause of the Cretaceous-Tertiary extinction was first linked to Me impact of ~ lO-km asteroid. Even much smaller impaetors still possess enormous energies md may cause local to regional devas~tion. At Congresses direction' NASA has supported ~ ground- based program to identify Me NEOs larger than ~ km in diameter. This task is about 50 percent complete' win estimates for the dam of completion rowing from 2010 to 2020 md beyond. The kilome~r-sized impaetors would be globally devastating' but much smaller projectiles would wreak unimaginable local havoc md are much more frequent. The high-~titude explosion of ~ SO-m-diame~r body above Tunguska' Siberia, in 1908 flattened trees over ~ broad area. A differently aimed impwt of this scale could flatten ~ modern eight' with deaths in the millions. Bodies larger than about 300 m in size cause ground-level explosions in the gigaton range. Such impacts would devastate whole counties. There is about ~ ~ percent ehmee thy such ~ imply will occur in the next eentury.i Assessment of the NEO population down to 300-m males' as pax of ~ organized inventory of Me small bodies of Me solar system, is recognized as ~ high priority for NASA's Solar System Exploration program. ~ . ~ .. ~ . .. . .~ . . ~ ~ ~ ~ ~ ~ ~ .~ ~ ~~ . . ~ ~ ~ xtrapol~'ons from ex~snug surveys suggest m~ me number of reams larger mm low m IS on me order of 10~000 to 20~000. These bodies are too faint to have been detected by the current surveys' md almost all remain undetected. For each object we need to determine Me orbital element with accuracy sufficient to predict Me probability of ~rrestria1 impact within Me next 100 years. This time seine gives sufficiently early warning for Me development of mitigation species as needed md is compatible win Me intrinsic time scale for d=amiea1 chaos among the NEOs. For Dose objects with ~ non-negligible impact probability, we also need physical observations to determine the size' which, when combined with ~ < OCR for page 175
priorly ~oUE~oNS FoT SoMP ~~M EXpLomHoN Processes+ How Plnne~ry Systems Work How Do t~ Processes Act S~pe t~ contemporary it of By Cods Operate ~~ ~2 ~S7 An understanding of plar~tary formation, evolution' arid po~ntia1 habitability is possible only win ~ detailed knowledge of the individual processes ~~ shape perry interiors' surfaces, atmospheres' rings, arid magneto- spheres. Physical processes define ~e mechar~isms by which plenary interiors, surfaces' atmospheres' arid magnetospheres evolve arid interact. Relevar~t interior processes include chemical differentiation arid core forma- tion arid the mechanisms of he~trm~fer throughout planetary history. Impact cra~ring, ~ctonism' arid volcar~ism represent geological procesms thy have shaped perry surfers throughout history. Ply - ahnospheres hold the record of the volatile evolution of the planet arid interactions win surface materials' weather' arid climax. The nature of the procesms ~~ are responsible for ~e remarkable diversity of perry ring systems must be bower understood. The nature arid evolution of the magnetosphere are critical to ~ wide rar~ge of phenomena' from perry interior processes (~.~., core dynamos) to loss of surface arid atmospheric species with time. Together' art improved md in~gra~d understanding of perry processes is necessary to determine fully how ply work. Virtually all of the missions suggested in PA One contribute to ~ Afar understanding of perry processes' rar~ging from our deepening knowledge of the Saturn system (~asX), to ~e inferior structures arid gaseous arid magnetospheric environments of ~e gist plmets (JPOP md NOP)' the surfaces md interiors of icy sa~lli~s (ECE, ELAN' md TEX)' the surface md atmosphere of Venus (VISE), impact basin formation md the interior of the Moon (SPA-S1~' the history md environment of Mars (MEP), md the hod of more specific aspects addressed by Discovery missions. Collectively these results will substantially enhance our understanding of planetary processes. Wit Don t~ Sole System Ted Us About tFw Development ~d Evolution of E;xtrmo~r Cry Systems ~d V`~e Versa~ Extrasolar planets are increasingly becoming ~ focus of both scientific md popular attention. Mmy more extrasolar planets will be detected in the next decade. Some will be imaged, md their spectra will be partially resolved. To provide critical ground-truth for these exciting discoveries, NASA should pursue ~ parallel program of close-up exploration md analysis of our own Mint planets, their ring systems' md the Kuiper Belt. The solar system provides ground-truth to He study of gist plme~ around other stars. The answer to He question of whether or not Jupiter has ~ roek-iee eve is critical to understanding how He plmet formed md' by extension' how exhasolar plme~ form. Two formation meehmisms are believed to be possible. The first or slow process' invokes the initial aggregation of ~ roek-iee eve of approximately 10 Earn masses. This embryo Men Streets gas, but He rate ~ which it does so is limited by how fad the growing object em radiate energy. The second, or fast process' invokes hydrodynamic instabilities that cause ~ subcondens~ion of the solar nebula to collapse as ~ result of its own self gravily. Stars form this way when the density of mater in Milt molecular clouds reaches ~ critical value. Brown stars failed stars' thy is' substellar obie~s insufficiently massive to sustain nuclear reactions in their cores also form in this mower. The rate md mems of formation of Jupiter em be understood by determining whether it has ~ roek-iee core. Measuring the mass of Jupiterts eve is ~ major objective of the JPOP mission. The NOP mission samples He chemist of fee gist Neptune' enabling direct comparison to Jupiter in terms of ehemisby md inferred time scale of formation. Observations of extrasolar plme~ mass, radius, temperature, md composition will be difficult to interpret unless we draw on our knowledge of gist plme~ in our own solar system. We need to understand how differences in bulk density translate into differences in composition md origin. The fee aims md gas gibe of our solar system will help provide ~~ knowledge. We also need to know how atmospheric circulation md over me~orologiea1 phenomena affect the ~mpera- tures md compositions of extrasolar gist plme~. These objects have clouds in their atmospheres. Clouds lead to preeipi~tion md release of latent hem. The Mint planets found close to their parent Ears have large day-night

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Kiss HEW FR0~ IN =E 50~R HIM temperature gradient. The temperature gradient lead to winds, which affect both temperature arid composition. Clouds, precipitation, ~mper~ure gradients, arid winds are me~orologica1 phenomena. We know about Best things from studying the atmospheres in our own solar system. ~~rmining ~e atmospheric composition, properties' arid dynamics of Jupiter is ~ major objective of ~e JPOP mission. Three proms are deployed deep into ~e gas gimt ply ~ three different latitudes' measuring composition, winds' temperatures' clouds, md sunlight as functions of pressure to ~ depth of 100 bars. The NOP mission samples the chemistry arid Ionospheric dynamics of ice girt Neptune' enabling dirwt comparisons to Jupiter. The study of protoplar~ary disks cart ~ influenced by studies of perry rings in our own solar system. The concept of migration of bodies due to circular momentum exchange with surrounding myriad was first advar~ced in the ring context arid is now ~ mainstay of perry formation models. Moreover, defiled understanding of ring processes would yield signif`~ar~t scientific benefit to ~ broad rar~ge of ashophysica1 investigations' including studies of accretion disks' spiral disk galaxies' arid the disks surrounding interacting binary stars' arid inve~iga- tions of active galactic nuclei. Here the Gemini mission to Saturn md the proposed NOP mission are fundamental to ~ understanding of ring procesms arid to ~ baker under~ar~ding of the accretion of plar~e~ in the solar system arid over plar~et~ systems. We have only jusidiseovered Be vast' unexplored region of the solar system known as He Kuiper Belt. At He same time, we have now begun to image He dust arid plar~etesima1 debris disks around other stars in our search for plme~ around other stars. We have discovered close Dialogues to our own Kuiper Belt around some of these stars for example, around He Car Epsilon Eridmi. These observations show ares md local voids th~may be due to the gravit~iona1 effects of embedded large planets. If we were to look ~ He solar system from afar' as we look out ~ other planets disks today, we would see ~ similar void eked by the gravitations sectoring of Neptune. In order to undersold md interpret imaging md speetromopy of plme~ry bodies around other stars, we need to undersold the structure md composition of our own Kuiper Belt. ~ the coming decade, studies of ex~asolar plme~ry systems will continue from new large Lopes on He ground md in Each orbit. At the same time' He LSST will be able to determine He distribution of objects in He Kuiper Belt in grew devils which will enable comparison win the structure of extrasolar plme~ry disks. Moreover' the KBP mission will explore Kuiper Belt objects, including Pluto md Charon, first in order to understand their nature' composition, md evolution. These missions will provide local truth for understanding dam from other seller equivalent. llEFEllENCE . C.R. Chaps Id D. hiorrisor~> ~ure367: 3340> 1994.