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Inner Solar System. Key to Habitable Worlds The inner plme~ provide ~ unique opportunity to study the processes ~~ lead to habitable worlds. Venus' Mercury, Mars' md the Moon (Figure 2.~) each hold glues to different aspects of the origin of the planets md habitable environment in the inner solar system. The Moon md Mercury preserve records of past events ~~ are largely erased on Earth md Venus. In mmy ways' Venus is Earths twin in Me solar system' md it provides ~ natural laboratory for understanding Me evolution of Parklike plme~ md their atmospheres' including how Earthts Exosphere might oh~ge in the future. Mars shows evidence for subs~ti~ climax oh~ge' which coup reflect processes ~~ ir~uenoed all of He irmer plmets. I>IFYING THEMES FOR STUDIES OF THE I~NE11 PLANETS At the mod fundamental level, Earn is unique. Through the study of over objects in the inner solar system' it is now understood that Each as ~ habitable plmet is the result of ~ series of stochastic event that occurred over id 4.~-billion-year history. The terrestrial or ``Ear~-like'' plme~ exhibit common geologic processes that bow reflect md determine Heir fate. Each of our neighbors is He result of plmet~-soale processes operating in He inner solar system with different boundary conditions. As He initial reco~aissmce of our solar system draws to ~ close' He scientific goals for exploration are Echoing. The initial exploratory sups were driven by the intense public md scientific interest in glimpsing new worlds for the first time. As articulated in this report, however' ~ new paradigm for solar system exploration is emerging, one that seeks to Ogress fundamental questions about our place in the universe. Thus, the unifying themes of He next decade of exploration of the ironer planets focus on He following: . lye paw Warm ~~ we come frame What led to He unique oharao~r of our home plme~ 1~ present: What `s going 0~2 ~ What common dynamic processes shape Ear~-like plme~9 1~: ~~ ~~ wego`ng~ Wh~fa~ swain Emus erwironme~ ~ Pose of Be over ~rres~ial plme~9 FIGURE 2. ~ (facing page) How do the ~npositions, interns makeup, and geologic history of the planed explain the formation and sustainment of habitable plane: enviromnents~ This image shows. from the left, Mercury, Venus, Each and the Moon, and Mars as they appear in slightly c~n~ natural color. Full images of Mercury do not Axis. The formic of the Moon is shown. Course of VesperiCioddard Spew Flight Or and Poor Ncivert.

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40 HEW FR0~ IN =E 50~R HIM Exploration of the inner solar system is vital to under~ar~ding how Earth-like ply form arid evolve arid how habitable ply may arise throughout the galaxy. Understar~ding processes on ~ ply - scale volcar~ism' Moronism' impact bombardment' evolution of ~e ahnosphere arid magnetosphere, arid development arid evolution of life requires comparative study of ~e Clarets closest to Earn in order to know the effects associated with sing diary from ~e Sun, composition, md the sale of dissipation of infamy energy over time. Comparative study of the irmer ply shows ~e imported of ~ large moon in making Each unique md perhaps uniquely suitable for life. Cue of the grew advar~es of geoscience has been to recognize thy present-day Earn represents just one sup in ~ progression of char~ges driven by ~ complex set of in~rrela~d perry factors. Coupled with this recognition is the revelation thy Earths atmosphere arid biosphere are fragile entities readily perjured by plar~ry-~ale processes. Much remains to ~ learned from the over ~rrestria1 planets' where similar processes have produced vastly different result. In this context' severe broad questions thy are fundamental to ~e hum art quest for understanding our place in the universe cart ~ addressed only by ~ detailed exploration of the irmer ply: Par~g On. What geologic arid atmospheric procesms s~bili~ climates P'~o~! On. How have large impacts affected the course of perry evolutions ~ Forenoon Ammo. How do the compositions' inferno makeup, arid geologic history of the ply explain ~e formation arid sustainment of habitable ply - environments: The past four decades of exploration, observation' Ad research have provided glimpses of Mercury' ~ first- order understanding of the Ear~-Moon system that laid the foundation for much of plme~ry science, Ad tm~lizing insights into the nature of the atmosphere' surface, Ad interior of Venus (Figure 2.~. Subst~tia1 advances have been made in the exploration of Mars from orbit Ad ~ three lading sins for in situ measurements. Clearly, mmy fundamental questions remain unresolved. Future progress will require detailed study of Ear~-like plme~ Ad of He eons~aints on how habitable worlds arise' evolve' Ad are sustained. The next decade holds immense promise for major advances in answering these questions. The next three major sections present ~ broad survey of He stay of knowledge of the irmer planets in He context of speeif~e seientif~e issues relying to He themes outlined above. The signifiemee of each issue is explained' Ad ~ summary is given of relevant scientific progress to date. Importmt questions are identified, Ad future directions for the Solar System Exploration program are Den outlined. By their very nature' several of He mod fundamental science investigations require commitment to ~ long-term integrated approach of observation' measurement Ad analysis. WHAT LED TO THE UNIQUE CHAlIACTEll OF OUR HOME PLANET' The factors leading to Earths unique eharae~ri~ies md' by extension' the unique characteristics of the other inner planets may be org~i~d as follows: The bulk compositions of the irmer plme~ Ad their derisions with disagree from the Sun; The internal structure Ad evolution of the core, crust' Ad mmile; The history Ad role of early impacts; Ad The history of water Ad other vol~iles Ad He evolution of inner planets' Ionospheres. lament progress in studies of each, likely future directions for research' Ad the to be addressed are outlined below. . ~mport~t questions that need Giver the scier~ific Ed programmatic importune of hears explor~ior~' detailed cor~si~r~ior~s Ed disoussior~s of the subject are deferred until Shaper 3.

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I~R boat ~~M Exploring Venus: Geologic and Atmospheric Processes atmospheric dynamics- snd superro~tion ~ lower atmospheric composition isotopes and trace gases ;~ surface meteorology 41 heat flop seismomeLy can. _ act- ~i. ail.,;, Fig cloud aerosol size : and composition nea r- l ~ descent and ascent imaging elemental abundances and m in eral ogy from core sample surface mineral microscopy FIGURE 2.2 A slice imp Earths sister planet Venus illustrates the urn own nature of the structure and ~~ of the interior; the composition and history of materials at We surface; md the composition' circulation' and evolution of Me atmosphere. Also indicated are some of Me mems by which Were ur~kr~wns may be investigated. Cour~sy of Jet Propulsion Laboratory' E. Sofas and M. Bullock. Bulk Compositions of the Inner Planed and Their Variation with [}issuance from the Sun A fundamental constraint on the formation of Earth-like planets is whether the inner planets are random accumulations of specific building blocks or whether ~ systematic cosmochemica1 trend related to distance from the Sun exists. The bulk compositions of the inner planets resulted from early nebular processes' planets accretion' and the removal or addition of material following accretion. The concentrations of volatile elements resulted from primp nebular processes (e.$.' condensation)' seconds processes (e.g' solar-wind erosion)' late addition from comets' or ~ combination of these processes. Determining the bulk compositions of the terrestrial planets is key to understanding the roles of these major formative processes. Especially important He the noble gases and their isotopes' which record early planets formation processes because they remain chemically inert.

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49 Recent Progress HEW FR0~ IN =E 50~R HIM Knowledge of bulk compositions has men gleaned from remote sensing, determination of orbital dynamics' arid samples of Earth the Moon, arid memories (including those from the Moon arid Mars). Good progress has been made in determining the surface compositions of ~e Moon, of some asteroids' Pride to some extent, of Mars arid Mercury. In situ measurements for Venus' made ~ mven locations by (short-lived) larded Sovi~ missions' suggest basaltic arid alkaline surfaces. However, the bulk composition remains poorly known. Isis of Base results suggests ~~ although some rock-forming elements occur in the inner solar system in chondritic relative proportions, ~e vol~ile elements are deple~d.i ~deed, compositions of perry basalt ruins possibly reflect gradient in ~e abundar~e of Jollily elements' increasing with dismay from the Sun. Isotopic dam on oxygen, hydrogen' arid other vol~iles provide clues to plenary compositions' their atmo- spheres' arid early solar system processes arid environment relevar~t to the origin of life. Isotopic dam from Earn arid Mars suggest ~~ primary ahnospheres were lost arid lair replied by volcar~ic outclassing or addition from come~.2 The (incomplete) measurements of ~e atmosphere of Venus are consis~t with solar values arid could reflect ~e influence of ~ primordial atmosphere' but the scam of chemical arid isotopic equilibrium of the surface arid atmosphere is urn own. Each of the inner pits has ~ complex history of postawretiona1 processes thy have contributed to Id modified id surface arid atmospheric compositions. Analysis of diverse surface materials' de~rmin~ion of their ages, arid assessment of the processes ~~ have affected them are needed in order to understand how volatile-element contents have evolved differently on each of ~e plmets. Future D`~`ons ~ Mercury. Basic information is needed on surface composition' internal structure, Id distribution of mass' each of which provides important eonshain~ on bulk major-element composition. ~ Beam. Compositional measurements of Be surface Id the atmosphere (especially the noble gases) are needed in order to understand the bulk composition Id Be origin of Venues abnosphere. Oxygen isotopic ratios would provide key geoehemiea1 constraints on the plmetts composition for underbidding differences among Be inner planets Id for testing models of formation. Measurement of the chemical sate of Be surface Id near- surface environment are needed to underst~d surface Id Exosphere interactions. ~ Moon. Seismic dam would resolve the interns structure, permitting ~ much-improved estimate of bulk composition. Samples of rocks from major unsampled terrains, primarily Be Soup Pole-AitkenDasin' are needed to determine ~ accurate deep erusta1 composition Id stratigraphy. I - ort~t cautions High-priorily investigations relating to bulk compositions of the inner planets Id Heir variation with disagree from He Sun are as follows: Determine elemental Id mineralogical surface compositions, Determine noble gas compositions of atmospheres' Determine oxygen isotopic compositions of He unaltered surface Id atmosphere, Id Determine interior (mmile) compositions. Internal Structure and Evolution of the Core' Crust, and Mantle Knowledge of the interns structure of He plme~ is fundamental to underfunding Heir history Her accretion. Key issues include dissipation of interns head eve formation Id assoeia~d issues concerning m~netie-field generation' distribution of heat-producing radioactive elements Id styles Id extent of volemism. Earthts crust is the product of differentiation Id several billion years of recycling through plan tectonics' with water being critical ingredient.

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INNET SoMT ~~M 43 Mercury is small' arid its ar~cient surface suggests ~ lack of crusta1 recycling or extensive resurfacing. Although models suggest thy the silicon portion of Mercury differentiated to form ~ crust arid mantle' little is known Bout id crust-mm~le structure or composition. Similarly' recent result suggest ~~ Mars probably has ~ marble arid core, but the interpretation is model-dependent. Based on Isis of lunar samples' ~e Moon Mar hot, win art occur of magma some 4~ km deep; its crud was extracted as the low-density component during solidification of the magma ovary' but insufficient hem remained to recycle Merrill. Venus, on the other hired, has been geologi- cally active within the past billion years' yet its surface is very different from Earths arid exhibits no similar plan phonics. Processes of crusta1 formation arid the dynamics of marble movements are poorly constrained. Recent Progress Knowledge of ~e interns structure of the Moon is constrained by samples, limited on-surface geophysical measurement' arid dam from orbit. Results indices ~ differentiated low-density' aluminosilica~ crust of about 40 km to 100 km, overlying ~ ferromagnesiar~ silicon upper mar~tle,3/ arid ~ small iron-rich cored Remo~-mnsing dam show thy the Moon has ~ strong hemispheric asymmetry. What caused the asymmetry is not known, but it is likely thy it influenced ~e distribution arid extent of subsequent volcanism. The topography of Mars also shows ~ dichotomy between Be northern lowlar~ds md the southern era~red highlar~ds. Although sever al hypotheses have been posed to explain He dichotomy' including those relend to both intem~ arid extema1 processes' these ideas remain untested. Venusts topography is known from Pioneer Venus md Moselle measurement md' although these dam reveal extensive Platonism md volemism' Be expressions of Parklike plan Ebonies are absent. Instead' topography md the relative your of the Venusim surface indigen ~ major, possibly global, resurfacing thy may have occurred episodically.~7 Although Venus appears to have ~ iron core' the absence of ~ magnetic field suggest ~~ it does nof have ~ magnetic dynamo, perhaps eonsis~nt with its slow rotation. Limited Mariner 10 dam ~ Mercury indie ate the presence of ~ magnetic field with ~ magnetosphere capable of standing off the solar wind most of the time. It is possible ~~ ~ dynamo or ~ strong remnant magnetic field is present. Either way' Mereury~s large mend eve plays ~ key role. Whether Mercury md Venus have solid inner cores md liquid outer cores is not known. Future D`~`ons Seismic dam for each of Be inner plme~ are ultimately needed to constrain Be structure' mineralogy' md composition of Be deep planetary interiors. Key investigations that address evolution of Be crust' mmile' md e ore include the following: ~ Determination of the horizontal Id vertical derisions in interns structures, Id Determination of the compositional derisions Id evolution of crush Id mmiles' Determination of the major heat-loss meehmisms Id resulting eh~ges in Estonia Id volemie styles' ~ Determination of He major eharae~risties of iron-rich metallic cores (size Id the nature of liquid Id solid components). The History Id Role of Early Impend An early paradigm shift in the understanding of the solar system was He realization ~~ impacts eonstitu~ ~ fundamental process, particularly in early planetary formation. For example, current understanding suggests that proto-Earth was shuck by ~ h~rs-size object' resulting in He formation of the Moon Id setting Earn on ~ distinctive evolutionary path. Impaet-genera~d heating likely caused p~ia1 to global melting of the terres~ia1 plme~, leading to He formation of Mama ocems Id differentiation of the interior.

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44 HEW FR0~ IN =E SOLAR MOM 1 ~ . ~ ~ ash AL 1 a - FIGURE 2.3 Cumulative cramr fr~uencics (i.~., the number of craters Dual to and larger On ~ km in diameter per Quark kilometer) with time, as ~rive<1 from lunar surfaces ~md from returne<1 Ampler. Cramr frequency ~m are then urn to climax the age of unknown planc~ surfaces. The recent cramr flux ~ ~ AU is partially conskaine<1 by mrreskia1 cramrs~ but the period prior to 4.0 billion y~rs is unknown. The apparent eluder of In-forming events near 3.9 billion y~rs is particularly puzzling and has =~era1 implications for early solar Doom history. Amps from O. Ncukum, B.A. Ivanov. and W.K. H~, `~Cramring llecords in the Nor Solar System in llelation to the Lunar Reference Sys~m,~~ ~~e She Reviews 96: SS-Bb, 2001. Large, early impacts played ~ key role in establishing Be structure of He early erupt which remains exposed on He Moon, Mars, md He observed part of Mercury. This structure influenced subsequent surface md near- surface evolution' such as the emplacement of lunar Ma flows. However' because d~a are incomplete' it is not known if global melting md differentiation occurred on all of He ~rrestria1 plme~' or if impact basins Domingo the erustal structure of Mercury ~ they do on the Moon md to some extent on Mars. The history of volatiles md plme~ry atmospheres were also affected by impacts' both through implication by comets md rem oval of gases through impact erosion. The lunar impact record, dated by samples' is used to exhapola~ surface ages throughout He solar system. However' there is considerable uneer~inly in the early flux of impacts' with two models proposed. ~ one' the flux decayed exponentially with time; in He other' the flux peaked ~ about 4 billion years (Figure 2.3~.8 Because He lunar era~ring record is used for dying events throughout He solar system' resolving He lunar era~ring record is key.

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INNET SoMT ~~M Recent Progress 45 Ply half of Mercury,~ surface has ~~n men grids while ~e Magellar~ mission provided global reco~ais- sar~e of Venus, it is not known if vestiges of early crust remain there. The paucily of impact Priers on Venus suggest ~ relatively youthful surface. However, estimates of surface ages are poorly constrained, raging from c250 million years to nearly ~ billion years old.9 Samples from nine locations on the Moon enable dying of key events in lunar evolution, but the chronology of most large, early impactors is poorly known. Future D`~`ons Understar~ding ~e role of early impacts requires age-d~ing key terrains on ~e Moon arid Mercury, mapping all of Mercury, md obtaining compositional' mineralogical' arid petrologic measurements of key terrains on ~e rreshia1 planets. The following specific investigations need to be conducted: Determination of large-impactor flux in the early solar system arid calibration of ~e lunar impact record' Determination of the global geology of He irmer plar~ets' arid Investigation of how major impacts early in ~ plar~et~s history earl alter its evolution arid orbital dummies. The History of Water Ed Other Volatiles and Evolution of the Inner Planets At~pher~ An accurate account of He history of water Ed other volatile compounds is essential for underfunding He origins of He environments of the irmer plme~. Plme~ry atmospheres have been severely affected by processes that occurred Per plme~ry formation, including interns processes (e.g.' volemism) Ed external processes (e.g.' impact Ed late-~ge accretion), but the dam are insufficient to determine the egret evolutionary paths. Aecura~ measurement of hydrogen Ed isotopic abundances of noble gases Ed oxygen in the Biospheres soil, Ed rocks are required. These dam are necessary to determine the fraction of pristine' nebular vol~iles versus lair eome~ry- impaet vol~iles Ed to underfed He loss rates of atmospheres in He early phases of plenary formation. Recent Progress The Moon Ed Mercury may have yet undetected significant species in their tenuous a~ospheres.~ At Mercury, coherent radar backwater indicates vol~iles ~ high latitudes.) ~ Clementine dam suggest buried wear fee ~ the lunar soup po le. while Lunar Prospector deleted significant quantities of hydrogen ~ both poles, but He form, extent, Ed origin of such deposits are not known. Although vol~iles are strongly depleted in the Moonts crust the volatile content of He deep interior is poorly constrained. Mars also exhibit significant amounts of hydrogen in the polar regions. On the over h~d, solar-wind implication deposits hydrogen Ed helium in He lunar regoli~. The prineip~ volatile eon~ituents of Venus Ed Mars are known. Much is known about the meehmisms of atmospheric lost Jems escape' hydrodynamic escape, exospheric sputtering' Ed solar-wind sweeping are He main processes. Application of these processes to Earth, Mars' Ed Venus is used to infer He pad Ed present scams of He atmospheres, particularly regarding He loss of water vapor.~4 For Mars Ed Venus, photoehemiea1 Ed other predictions of less abundant species exist, but no actual detections of them have been made. Future D`~`ons Wear Ed He volatile compounds ~~ make up the atmospheres of the ironer plme~ are also partitioned among the interiors, crusts' Ed hydrospheres of these plme~. The evolution of water Ed vol~iles since our solar systems formation is central to ~ understanding of terrestrial plme~, evolution to either support life or prevent id inception. An exploration program ~~ includes the following will achieve this new understanding:

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46 HEW FR0~ IN =E 50~R HIM High-precision measurements of noble gases arid light ~~61e isotopes' ~rmination of the composition of magmatic vol~iles, md ~rmination of the composition md source of the polar deposit on Mercury arid ~e Moon. WHAT COMMON DYNAMIC PROCESSES SHAPE EA1ITH-LIKE PLANETS' The d~amica1 influences shaping Earth-like ply include the following: Processes ~~ stabilize climate, Active interns processes ~~ shape the atmosphere arid surface environment arid Active ex~rna1 processes thy shape the atmosphere arid surfwe environment. Recent progress in studies of each' likely future directions for research' arid imports questions ~~ need to be addressed are outlined ~low. Proses That Stabilize Climate Venus, Earth, arid Mars have complex interactions between the surface, atmosphere' arid interior. Mars is mod likely geologically quiescent, although because of derisions in its motion around He Sun' it experiences echoes in solar energy input md thus in cycles of wear md carbon dioxide between polar caps, surface' md atmosphere. The eor~E~eetions between these environments on Mars md Venus are manifestly different from Hose on Earth. Looking md thinking beyond Earths climax system will enable us to more deeply underfed processes that affect climax md how they infer a~ in establishing planets environment. Underfunding the factors ir~ueneing plate Ebonies (initial planets composition, internal dynamics, md the role of wa~r) is of prime importance in understanding He stability of climate. The effects of clouds, volemism, md Platonism on climax stability are also impor~t but nof well understood. Studies of the Moon md id formation are impor~t to understanding how astronomical perturbations affect climate on ~rrestria1 planets. Recent Progress Comparative studies of He surfaces, atmospheres' md interiors of He inner planets show ~~, while common physical processes operate on these plme~' their interactions md eolle~ive effects are expressed differently. Earth' play Felonies recycles crush material md cools the interior. Carbon' oxygen, sulfur' md nitrogen cycle among the Ionospheres biosphere, ocems' crust, md interior. Ethos Moon helps stabilize obliquity md therefore seasonal vari~ions.~6 Although Mars md Venus have their own' unique versions of volatile eyeling' neither of them currently has plate tectonics or moons ~~ affect Heir orbital stability. Future D`~072s Critical interactions between He interior md Exosphere of Venus are nof understood. Science investigations central to understanding climax should do He following: Determine the general eireul~ion md dummies of the inner plmetts Ionospheres; Determine the composition of He atmospheres of He irmer plme~' especially trace gases md their isotopes; Determine how sunlight thermal radiation, md clouds drive greenhouse effects; md Determine processes md rates of surface/atmosphere interaction.

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INNET SoMT ~~M 47 Acme Intemal Proses That Shape the Atmosphere anal Surface Environment Processes taking plwe within ~ pit including volcar~ic outclassing, generation of magnetic fields' arid exchange or recycling between ~e surface' a~nosphere' md interior affect ~e current ~~ of the surface arid atmosphere. For example, prolonged volcanic eruptions cart affect climax md Ionospheric evolution. The magnetic field of ~ plar~trecycles ions back into the Ionosphere arid promos if from solar-wind erosion. Magnetic fields are generated by inferno processes' most likely from art inferno dynamo driven by differential rotation of solid inner arid liquid outer core. What would Earth currently ~ like without play tectonics or without id protective magnetic fields The lack of play tectonics on the other ~rrestria1 ply the lack of ~ magnetic field ~ Venus arid ~e weak field ~ Mercury, arid the remnar~t magnetization of Mars allow us to explore `~al~rn~ive scenarios,' for the current shy of processes active on Earn md to underbred the relative significance of ~e interplay between volcanic activity arid atmospheric composition in generating md sustaining habitable envi- ronments. Recent Progress The hot interiors of planets drive tectonic arid volcar~ic processes such as plate Felonies. Volcanic activity on the Moon arid Mercury occurred early' aetivi~ on Mars extended longer' while Earth arid proudly Venus remain geologically active. ~ all of Me irmer plme~, active internal processes contributed to Weir atmospheres Trough outclassing md' on mod bodies, interactions continue between the surface md Me atmosphere. Venus' the only other inner planet likely to still have ~ dynamic interior' lacks plate tectonics' md the evolution of id interior is ~ subject of much debate. i7 The rates of Estonia md volemie activity are not Justified for Venus' md the ages of major surface units thy would help eonshain rams of volemism e~of be determined from remotely sensed day. Earth has ~ strong, dipolar magnetic field thy sods off the solar wind. Mariner 10 day showed ~~ Mercury has ~ dipolar field, aligned in the same sense ~ thy of Each md with 0.001 of id surface field strength.~8 Although the Moon he remnant crush magnetism md momalies' Weir source is not clearly understood. lament results from Me Mars Global Surveyor spaceport indie ate ~~ Mars has remnant magnetism'i~ suggesting the possibility that ~ magnetic field once existed' enabling ~ Shrieker atmosphere.20 Venus has no measurable magnetic field' although it is not known if one existed in the past. ~ Earth' the lithosphere, hydrosphere, biosphere, md atmosphere participate in the cycling of vol~iles such as water md carbon dioxide. The current lack of ~ hydrosphere md biosphere on Venus provides ~ unique opportunity to Polyp Me links between processes in the interior' volemie aetivily, composition of the surface md atmosphere, generation md maintenance of Me global cloud layer' md chemical weltering of the surface. Key sups include measuring Me following: Me composition of the lower atmosphere' isotopic noble gas abundances in Me Ionospheres mineralogy md composition of surface rocks, md Me rates of active processes on Venus by accurately dying key surfaces. Future D`~`ons Knowledge of Me current sate of interns geologic aetivily as well as Me sate of evolution of the surface md of past or present magnetic fields is needed in order to eharae~rize active processes on the irmer planets. The highest-privily measurements are these: Characterize current volemie mdior tectonic activity md outclassing; Determine absolute ages of surfaces; md Char aeterize magnetic fields md relationships to surface, a~osphere, md Me interplmet~ medium.

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4s HEW FR0~ IN =E 50~R HIM Alive External Proses That Shape the Atmosphere and Subdue Environment The ironer ply share ~ common environment in our solar system in which active processes such as solar- wind bombardment aff=t how the atmospheres arid surfaces evolve. Because the inner ply are close to ~e Sun' ~ common loss process for their atmospheres is solar-wind sweeping, in which ionized species are removed from the top of exospheres by electric fields cormec~d to the in~rplar~tary medium. Magnetospheres cart help recycle ions into ~e neuba1 ahnosphere, but ~e efficiency of this process is urn own. Studies of ~e effects of ~e solar wind on ply win weak or no magnetic fields provide ~ basis for understar~ding how ex~rna1 processes affect atmospheric evolution. Microme~ori~ bombardment modifies the surfwes of Mercury arid ~e Moon md inject myriad into their exospheres. Bombardment by larger object is more infrequent, but it radically charges the surfaces arid atmo- spheres over time. Cosmic rays' memories' ion bombardments' arid implar~tion Mar the structure of ~e uppermost regoliths of Mercury arid ~e Moon. The same processes comminu~' vapor)=, arid mix the regoli~ while adding exogenous Merrill. Them ex~rna1 processes affect each of ~e inner ply in different ways arid ~ different scales' charming Weir surfaces arid atmospheres in ways ~~ determine how habitable environments are maintained. Recent Progress Pioneer Venus manured the radiation md particle environment for 14 years, resulting in our knowledge of Me effects of external processes on the plmetts upper Ionosphere. The extent to which Me lower atmosphere md surface are perturbed by external processes is not known but is thought to be minor because of Me dense atmosphere. Me exception could be deposition of volatiles by biometry impact. A better eharae~rization of Me escape ryes of various species from the atmosphere of Venus will aid in ~ understanding of how thy plmetts atmosphere has evolved. Mereury~s dipolar magnetic field is believed to shed off the solar wind much of Me time. The tenuous surface-bounded Biospheres of Mercury md the Moon are ~ result of meteoritic impwt volatilization of both He surface md the impostor (sodium' potassium, md calcium) md solar-wind-implm~d hydrogen md helium.2 i The origin of high-l~itude trapped lunar md mereurim vol~iles is currently ~ maker of intense discussion. The combined effects of small-male processes ~~ mobilize md alter the surface on airless bodies have recently been recognized Trough detailed analysis of lunar soils enabled by improved ins~umen~tion in Ear~- based laboratories. The nature md rate of such processes are Hill urn own. Future D`~`ons Sever al investigations are important for understanding external processes active in the inner solar system. They should do He following: Make precise compositional measurement of the surface-bounded atmospheres of Mercury md the Moon md determine He relationship between ionospheres md magnetospheres' Qumlify processes in the uppermost atmospheres of He terreshia1 planets' md Qumlify regolith processes on bodies win tenuous atmospheres. WHAT FATE AWAITS EAlITH,Si ENVI1tONMENT AND THOSE OF THE OTHER TERRESiTllIAL PLANETS' Discussion of the fad of Ens environment md those of He other terres~ia1 planets is organized under He following headings:

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INNET SoMT ~~M 55 arid of the surface on arrival ~ Emus. A core sample will ~ obtained ~ the surface arid lofted to altitude' where further geochemic~ arid mineralogical analyses will be maw. In situ measurements of winds arid radiomen will be obtained during ~scent' ascent' arid ~ ~e balloon station. Scientific dam obtained by this mission would help to constrain ~e history arid syphilis of the Venus greenhouse arid ~e recent geologic history' including resurfacing The Ethnology development achieved for this mission will pave ~e way for ~ potentially paradigm-al~ring sample-return mission in ~e following decade. 3. Bock Po~-A`~ke~ Bm`n Sit ~~m (SPARSE. The next highly ranked mission for inner solar system exploration is understanding basin-forming processes arid impact chronology by returning samples from the Soup Pole-Ai~en Basin on ~e far side of the Moon. The Moon provides ~ baseline for much of Claretscience, arid science questions associated with ~e Moon are ~ ~ high 1~1 of maturity. The Soup Pole-Ai~en Basin is ~e largest known basin in ~e solar system md ~e olden arid deepest impact structure well preserved on ~e Moon (Figure 2.4~. This gimt basin allows access to materials from ~e interior of ~ small, differentiated plum. The SPA-SR mission will obtain samples of materials produced during his enormous impact event' enabling ar~alysis FIGURE 2.4 The Moon South Pole-Ailken Basin ~ the momma of formation. A multi-ring twin formed as the initial mviby expand and rim structures slumped into the growing depression. The SPA imply is expert - to have ex=~ - through the crud and into the upper mantle. The current twin interior remains distinctly F~O-rich, as ~mrmine<1 from Clementine multispecka1~ Amy image). Lunar soils filly contain the diversify of repre~n~tive rock Hypes (rawer rigid: nod the millimeter ~1~. The ~~ of SPA formation is not known, but early Earth was proudly Floor to the Moon An it is now, and may have An rousing faker (producing such cloud Ends. Painting by W.K. H~r~, 2002. Figures mope from B.L. Jolliff, I.J. Gillis, L.A. Asking ILL. Korean, and M.A. Wiec~orek~ ``Major Lunar Musk Terrancs: Surfed Expression and Cru~-Mantle Origin ~Faumal of GO Ret 105: 4197-42165 2000, and J.A. Wood, J.S. Dickey, U.~. Marvin, and B.N. Powell. "Lunar Anorthosims and ~ Geophysi~1 Mode of the Moon,5, in A.A. Levinson (e~l.~. Praceedz~gs of the Apollo I] Lu~r Scheme Ca~fere~e Val ~ ~ Pergamon Press, New York 1970, pp. 965-~.

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5s HEW FR0~ IN =E 50~R HIM of the effects of early, large impacts on the structure arid evolution of the plows. Returned samples will include both soil arid diverse rock chips. Mediu~n-Cl~ ~siom bent ~ Sit Explorer The Irmer Ply Par~el~s highest-rar~ked new mission for the next decade is the Venus In Situ Explorer (VISE). It would provide key measurements of the ahnosphere arid surface, as well as ~st technologies needed for Venus surface md ahnospheric sample-return mission in the subsequent decade. Science measurement objectives of VISE are as follows: ~~rmine the composition of Venus~s atmosphere, including trwe gas species md light sable isotopes; Accurately measure noble gas isotopic abundar~e in the atmosphere; Provide descent surface, arid ascent me~orologica1 day; Measure Tonal Loud-11 winds over several Each days; Obtain near-infrared descent images of ~e surface from 10-km altitude to the surface; Accurately measure elemental abundar~ees arid mineralogy of ~ eve from He surface; arid Evaluate the texture of surface materials to eonshain weltering environment. A Venus atmospheric md surface sample-return mission has been identified as the sup in irmer-plme~ explor~ionth~is absolutelyessentialfor determining why~enus has evolved such a different environmentfrom that of Earth. This mission is eomplemen~ry to He Mars surface sample-return mission, utilizing some common elements, such as ~ ascent capability md orbits rendezvous. However, sever al elements are unique to returning ~ sample from Venus md need to be demonstrated in situ. Development of these technologies will enable future sample-return md po~tia1 Diseovery~lass missions to Venus. The key elements that will be tested by He Venus ~ Situ Explorer mission include the following: Aeroshell entry into Venues atmosphere; Passive insulation md survival in the extreme environment of Venus; Sample acquisition md handling ~ surface drill to obtain ~ sample quickly for example, in less ~m hour; md ~ An amens package ~~ would loft the sample by balloon to ~ altitude of 70 km md survive for sever al Earth days. Atmospheric Science Objectives. The composition of the lower atmosphere of Venus is urn own. Without this knowledge, comparisons of the factors ~~ affect climate on Earth md on Venus' including photoehemisby' clouds, volemism' surface-~osphere interactions' md He loss of light gases to space, are impossible. VISE will measure He abundance of trace gas species in the lower atmosphere of Venus to parts per million accuracy' enabling ~ underfunding of how these processes affect ~rrestria1 plme~ry climax. A fundamental quest is to undersold how md why Venus' roughly the same size' composition' md distance from the Sun as Earth has evolved to such ~ different sate. The record of planetary Biospheres is contained in the isotope ratios of the most inert gases xenon' krypton' argon' md neon. Are plme~ry Biospheres the remnant of gases that were originally solar in composition but Ben suffered massive hydrodynamic eseape,36 or did Hey acquire Biospheres from vol~iles ~~ had Trendy been differentiated~7 What was the role of impacts on Be ultimate compositions md evolution of Be ~rrestria1 plmets: Diserimin~ion between these events for each of the ironer plme~ is possible if noble gas isotopic ratios em be measured with ~ s~te-of-the-art neutral mass spectrometer. Previous spacecraft measurement have been inadequate to address these issues. VISE will determine He noble gas abundances md isotope ratios to sufficient accuracy to distinguish between hypotheses of He origin md evolution of Venues atmosphere. A meteorological package will measure atmospheric pressure md temperature profiles

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INNET SoMT ~~M 59 down to the surface' md pressure' temperature, md winds ~ ~e surface. Cloud-leve1 winds will be determined by tracking ~e ascent balloon during id 3.~-day lifetime, providing improved dam on ahnospheric dummies arid ~e origin of Venus~s mysterious atmospheric superrot~ion. St'rf=e Scathe Objectives. The former Soviet Unions Venera landers resumed basic elemental chemist md images of four sins on the surface,38~40 md Magellar~ dam provided evidence of possible evolved volcanic deposi~.4i However, we lack sufficient information on surface elemental abundar~es arid mineralogy to de~r- mine the degree of crusted evolution on Venus. The VISE mission would measure elemental compositions surface sin complementary to Hose of ~e Veneras. Mineralogy of ~ surface sample core will be obtained for ~e first time' allowing analysis of my weathered layer arid Asking for dupe of alteration arid occurrence of unfired Merrill. Textural ar~alysis of the sample using ~ microscope imaging system would provide information on ~e formation arid nature of surface rocks. These dam will ~ used to constrain questions outlined above. Aspic global radar coverage of Venus by Magellar~' little is known of ~e surface morphology ~ scales of ~ to 10 m. Without such information' it is difficult to determine how the plains formed arid to understand ~e nature of mobile materials on the surface. A descent camera on the larder will provide ~e firm broadscale visible images of ~e surface, with images returned from about 10 km altitude to ~e surface. These images will enhar~ee interpretation of He Magellar~ radar images by providing ground-tru~ dam on the surface texture of He 1~a flows ~~ make up Venus plains. The morphology arid texture of these flows earl be related to emplacement rate, volatile content md rheology, which are needed in order to underfed the role of volemism in shaping the atmosphere md surface of Venus. Images of Venues surface will also be returned from He lander' with filters chosen to provide compositional information. These images will help to determine the recent geologic history of Venus md will resolve differences in He in~rpre~tion of Venues resurfacing history.42 impleme'~'o'~. Science measurements will be made during Free VISE mission phases: (~) He descent phase' with atmospheric experiments md descent imaging; (~) the landed phase, win surface imaging md Ionospheric md surface chemistry; md (3) He ascent phase' win surface mineralogy md atmospheric eireul~ion analysis. The panel stresses that VISE needs to be kept simple' with limited but focused objectives. The panel assumes that the instrument portion of He mission will be competed either ~ ~ package or individually. Deep-atmosphere measurements should include these: A neutral mass speetrome~r win ~ enrichment cells ~ A meteorological package ~~ includes pressure md temperature sensors md wind-speed measurements the surface, md ~ Radio mienee investigations ~~ track the ascent balloon' measuring eloud-level bond winds. Surface science experiment should include these: Near-infrared descent md lander cameras' win filers chosen to maximize surface~omposition information; ~ An instrument to measure He elemental geochemist of ~ surface sample' likely ~ x-ray fluorescence malywr or ~ new instrument utilizing technologies ~~ are currently being developed. This measurement will be done inside the lander on He surface; An imaging microscope to analyze the e ore sample during ascent; ~ An instrument to measure surface mineralogy. As this measurement requires time md benign conditions' it will occur ~ high altitude inside He ascent package; md ~ Auxiliary experiments' such as ~ surface seismometer could be included, if mass margins md cost permit. Bomb Pi ole-A`~= Bare Sample ~~m The return to Earn of rock fragment from He largest impact structure on the Moon' the Soup Pole-Ai~en Basin, will address fundamental questions of ironer solar system impact processes md chronology. Because these

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do HEW FR0~ IN =E 50~R HIM materials sample the deep interior, Hey would greatly increase our knowledge of the differentiation of ply - bodies arid of ~e structure arid composition of ~e Moon. Key measurement to be made on returned samples include radiome~ic ages of impact-melt rocks from the South Pole-Ailken Basin-forming event arid chemical' isotopic, arid petrologic investigations of igneous arid volcanic rocks from the deep crud arid upper marble of ~e Moon. This mission is considered ~ medium~lass mission. The SPA-SR mission will address fundamental issues relevant to the impwt history of the inner planets arid the Ear~-Moon system md key remaining issues of lunar science. During Apollo, we sought to understand how ~ small Claret sorts itself out or differentia~af~r id formation. What the Apollo md Luna missions which inve~iga~d ~ limited region of the Moons nearside found was ~ considerably more complex ply body chart expected, arid we have not really ar~swered ~e question adequately. Remo~-sensing result have recently provided the global context to address this issue as well as the effects of gimt impact in the early solar system. Mostmodels for the early evolution of the Moon include initial lunar differentiation forming segregated layers thy are negatively buoymt md Gnome gravi~tion~ly unstable' sinking toward ~e interior of the ply. Marty aspects of ~e subsequent history, such ~ core formation md the generation of mare basalts' are linked to Best events. Similar models have teem proposed for Mars md Venus' arid thus the charac~riz~ion of the lower crust arid upper marble of ~e Moon will not only ~ ~ very significar~t Lop in distinguishing among severe models for early lunar evolution but it also will provide insight into procesms ~~ are likely to have occurred on our plumb. Bolt System Sc~e Objectives. The Soup Pole-Ai~en Basin is the largest known structure of its type in Be solar system md ~e olden well-preserved basin on the Moon. Its age provides ~ key constraint on understanding basin-forming impact chronology throughout ~e solar system. Samples of marries produced by this enormous impact event will help decipher the following: . Effect md timing of early' large impacts on plenary structure, differentiation' md orbital dummies; The dupe to which the impact penetrated (from sample composition md mineralogy); md The composition md origin of the impacting object (~rough trace-element md isotopic analyses). inner Sol~ System ~d E - maroon System Science Questions. lladiometric dating of samples of impact melt from ~e South Pole-Aitken Basin And possibly from nearby smaller but later basins) will provide key evidence regarding the irmer solar system md Earth-Moon system era~ring chronology. The age of Be South Pole-Ai~en Basin will constrain the period of 1~, heavy bombardment md will provide ~ critical Use of Be hypothesis that Be heavy bombardment was punetua~d by ~ e~aelysm, or spike' in Be flux of large impostors. Understanding Be bombardment flux is espeei~ly relevant for Be E~h-Moon system md Be evolution of earlyterrestria1 environments. Lun'Sczence P,rot~ ~d Fog f2,cestzo,~. The farside South Pole-Aitken Basin represent the prinei- pa1 major lunar terrain that remains unsampled.43 Despite numerous subsequent smaller impiety the enormous South Pole-Ai~en Basin repins id distinct regional geoehemica1 anomaly' observed rem only in FeO md thorium eoneentr~ions.44~45 lament remotely sensed information further suggests ~~ Be floor of Be basin may largely represent the mineralogy oftheMoonts lowereru~, although impaetbreeeias eouldeon~inm~tle rocks as clash And mmile rocks may be dishibu~d within the regolith).46~48 Analysis of materials from the basin Bus is expend to provide key information regarding fundamental problems of the present-day surface of Be Moon md id geologic history, including Be following: ~ Composition md mineralogy of the lower crust determined directly from samples, allowing teeing of models for Be differentiation of Be Moonts crust md mantle; ~ Composition md mineralogy of the mmile (potential rocks or clash in breeeia would be the firm direct samples of Be lunar motley; ~ Ancient materials from the lunar far side thy are not biased by the nearside impact basins' which Domingo current Apollo md Luna samples;

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INNET SoMT ~~M ~ Validation of compositional remote sensing over ~ major region of ~e Moon for which no represmt~ive samples are known to exist in Apollo, Lund or meteoric collections' arid where existing dam are ambiguous or otherwise nof well understood (enabling improved determination of bulk composition); ~ Sources of observed ar~omalous concentrations of thorium arid other he~-producing elements to under- shed lunar differentiation arid Herman evolution' including Solecism. This also addresses the origin of global perry asymmetry arid whether Th is enriched in the Moon relative to Earth which is imports for underst~d- ing the origin of the Moon from art early girt impact into ~e Earth; arid ~ Ages arid compositions of farside basalts to determine how mantle source regions on ~e farside of ~e Moon differ from regions sampled by Apollo arid Luna basalt. From experience with Apollo regolith samples md bum of ~e efficiency of loran arid vertical mixing, diversity of rock samples is expected in ~ representative sample from well-mlec~d sibs within SPA. implement~'on. The SPA-SR mission concept includes ~ robotic larder win automated scooping md sieving capability to enhance ~e return of rock fragment along with bulk regolith. A kilogram of returned mature lunar soil without sieving would ~ expected to include some S,000 rock frogmen in the I- to lO-mm rag. To maximize the likelihood of ob~ining ~ sample of original SPA basin impwt-melt rocks arid over desired materials' ~ roving capability or ~ multiple (three) lander concept could be employed. If multiple landers were incorporated' important lunar geophysical network science could ~ obtained to provide ~ structure context for ~e basin md He returned samples. The mission would include descent imaging to provide geologic context. A relay sa~lli~ is also required for command md control md to enable extended network science. This mission would serve as Hotbed for key Ethnology development associated win automated sampling, encapsulation' md return to Earn. Discovery and Sm~ll-CI=s Missions Summarized below are science objectives thy could likely be met within or below the Discovery cost caps for the ironer planets. These relate to objectives that em be achieved within He 1~&A program (Ear~-based facilities)' as missions of opportunity, or in eollabor~ion win foreign investigations' or as peer-reviewed Discovery missions. The panel provides ~ list of such prioritized objectives for Mercury, Venus' md the Moon. Mercury Scat The following investigations em be addressed by Ear~-orbiting mdior ground-based telescopes or by Discovery missions thy are eomplemen~ry to Messenger: ~ Investigate high-l~itude vol~iles to verify deposits (composition' extent depth)' to determine sources (solar wind' Biometry, meteoritic) md history (recent versus ancient), md to undersold the deposition process. ~ Acquire ~ complete mineralogical map of He surface to understand variations among the ~rrestria1 plme~ in crush formation md surface evolution. ~ Analyze the morphology md ~abili~ of the magnetosphere by measuring the intensity md distribution of ionic species emissions from beyond Mereury~s orbit. This provides ~ opportunity to observe active space weather before it reaches Earn. ~ Study the morphology of the neutral atmospheric species to determine if Hey are provided from surface materials' the interplmet~ medium, meteoritic flux' endogenie sources, or ~ combination of these. Significant technological advances or i~ov~ive approaches will be necessary in order for He following objectives to fit within ~ augmented Diseovery~lass mission: ~ lectern ~ sample from the surface to place Mercury in the context of solar system ehemis~y, determine volemie md thermal history, md calibrate the crater flux (age doings.

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HEW FR0~ IN =E 50~R HIM Emplwe ~ geophysical network (seismic, hem flow) to determine interns structure' distribution of he~- producing elements' loran arid vertical heterogeneity of crust md mantle' arid ~e Due density of the core. ~ophysica1 network science would address how small bodies differentiate arid how ~e bulk composition of Mercury is relend to the composition of ~e ~rres~i~ plus. V=~ Science Some of the following objectives cart ~ addressed, either alone or combined win others, by new Discovery missions; some will ~ addressed by missions of ~e European Space Agency (ESA) md Japm~s Inanity of Space arid Astronautical Science (ISAS); md others cart be addressed by ground- or space-bamd observing programs. ~ Lower-~tmosphere trace gases arid dummies. Information is needed on how trwe species vary over time arid space md how they participate in cloud-forming processes, ~ermochemica1 reactions, arid reactions win surface minerals. Such observations (~.g.' by art and infrared imaging spechome~r) cart also be used to look for direct evidence of exit volcar~ism. ~ Monitoring global geological processes' such as volcanism, ~tonics, arid mass wasting' by imagery arid topography win horizontal resolution in ~e few ~ns-of-me~rs rage. Techniques are available for detecting char~ges on perry surfaces (~.g.' ir~fl~ion of active volcar~oes before eruptions) on centimeter scales. ~ Exospheric mass loss md thermospheric dummies. A suite of instrument cm operate from Venus orbit or from HST md TWST to measure the loss of light species from Venus~s atmosphere (~is is key to understanding how Venus evolved to ~ sew so different from that of Earth). ~ ~otherma1 hem flow measured ~ multiple locations to determine rams of hem flow within the plmet md between the surfed md Ionosphere md to lead to better understanding of Solecism md tectonics of ~e crust md mmile. (This objective will likely require significant Ethnology development.) ~ Measurement of middle-~nosphere trace gases md dummies (by submillimeter heterod~e technology md direct Doppler wind measurements. The following objectives may be moved into the Discovery class win technologies developed for VISE: ~ Visual recor~E~aissmee of the surface below the clouds to provide impor~t ground-truth for Magellm radar images md far more refined geologies interpretation of the surface; ~ Global atmospheric dummies explored in detail win long-lived instrument (e.g., on ~ fleet of balloons)' including in situ pressure md temperature measurements, md possibly also direct measurements of solar md thermal radiation; ~ Noble-~as md trwe species measurements made with ~ simple Venus abno sphere sample-return mission. Such measurement are essential for understanding the origin md evolution of Venues atmosphere And for comparisons with Earth). Analyses on Earth would then be performed that would allow He measurement of noble md bate gas species to mmy orders higher precision ~m has been done for my planet over ~m Earn. L=~r Science New Diseovery-elass missions em address most of the following objectives wholly or in part. Sever al are plied to be addressed by Europem md Iapmese lunar missions that are scheduled for launch within the next years md will provide valuable opportunities for U.S. participation. ~ Geophysical network science (seismic, hem flow) to determine interns structure' distribution of he~- produeing elements, lateral md vertical heterogeneity of erupt md maples md He possible exis~ee of ~ iron- rich core. Geophysical network science would address how small planetary bodies differentiate' how the bulk composition of the Moon is relend to He composition of Earth md how planetary compositions are relend to

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INNET SoMT ~~M nebular condensation md ply - accretion processes (the Iapar~ese Lunar-A mission contains two such instru- men~d peneL~ors); Investigation of the extended history of basaltic volcar~ism arid calibration of ~e impact flux by returning ~ sample from the youngest lunar lavas (~.g., Lich~nberg-Rumker Hills). This would also address why balls formed where they did in space arid time (~.g., nearside-farside dichotomy arid originievolution of the Procellarum region), md how the Moon cooled generally; ~ Investigation of polar vol~iles to verify deposit (character' mineralogy' composition, extent' depths, to determine sources (solar wind' cometary' meteoric) arid history (recent versus mcimt)' arid to unarmed processes of vol~ile migration arid deposition on airless bodies; ~ ~~rmination of topography ~ high resolution (hundreds of Myers to kilometers), as done by the Mars Orbiter Lamr Altimeter (MOLA) on Mars Global Surveyor, in order to carry out deviled geologic investigations as well as to address the geophysical properties of the Moons crust arid marble md the Moons thermal evolution from hot arid weak to cold arid rigid; ~ ~~rmination arid mapping of the mineral composition of ~e surface (through h~rsp=tra1 imaging) ~ sufficient Spain resolution to advent underfunding of the petrologic relationships within arid the origins of principal geologic Unix; ~ Targeted area studies to understand impact chronology, especially the post-3 Cyr flux history md spikes or other periodici~ in the impact flux (accomplished by age-d~ing key s~igraphic Unix arid impact-cra~r melt rocks with returned samples); ~ Determining the major-element composition of the surface of He Moon (including magnesium' aluminum' md calcium md other measurable elements ~ improved resolution compared win existing daub in order to better characterize the distribution of materials on the lunar surface md to understand He formation, differentiation, md bulk composition of He Moon; ~ Stereo imaging eover~e for high-resolution (e.g., 10 m)' three-dimensiona1 definition of geology md surface morphology to address local md regional issues (geologic in~rpre~tion, resource evaluation' Moon-b~e planning); md ~ Geological site eharae~rization in order to derive He geological evolution of the surface ~ key locations md to deconvolve the interplay between Estonia' impact, md volemie processes (em., extendedilong-dur~ion rover traverse, impinge md in situ analysis). A Long-Term Explorabon Strategy for the Inner Planets ~ ~ . . ~ . ~ ~ ~ ~ r The inner solar system affords He opportunity to address broad objectives for understanding the history' current sates md po~ntia1 future of habitable planets. The Ever Plme~ Palely strategy is to focus on He n~gnest-pr~or`~ science o~eet~ves for mercury, venue, md He Moon in the decade 2003-2013. Exploration efforts in He subsequent decade should focus on He return of samples from Venus md Mercury md on essential network science. The later involves the establishment of multiple surface Anions operating concurrently on plmet md are referred to ~ `~Geophysiea1 Network Science'' in Table 2.~. Missions to implement these networks would involve individual projects for Mercury, Venus, md He Moon. Because of the challenges posed by network science md, in particular, sample-return missions, it is rarities ~~ key technologies be developed md proven in order to enable their implementation. Thus' ~ second-order aspect of the Venus ~ Situ Explorer mission includes developing technologies for obtaining samples md lifting them from the surface. This technology will draw on heritage currently being established for the Mars sample-return program. It should be noted ~~ ~ Venus sample- return mission' perhaps He highest-priori~ mission for ironer planet exploration in the subsequent decade' is He only way to accomplish He following: ~ Measure the isotopic composition of oxygen, to provide crucial information on this impor~t ehar~teristie of solar system formations processes; Obtain the isotopic composition of vermin elements (em.' Nd' W. Hf. Sr, Pb, Os, for which specific isotopes are products of radiogenie deeply), to address the timing md extent of metallic eve formation, the timing

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~4 HEW FR0~ IN =E 50~R HIM arid ex~nt of mantle differentiation' arid ~e depth' mineralogy, md chemical composition of source regions for them basalts; arid ~ ~~rmine the age of returned rocks, to constrain the geologic history of Venus arid allow comparison win the Earth. Not all of ~e fundamental science issues for the ironer ply cart ~ addressed by ~e priority missions proposed here. However' as discussed earlier' subs~tia1 advar~ces must be made in understanding how ply work, arid much cart ~ achieved through one or more focused Dimovery-class missions. In addition' art in~gra1 part of the exploration of the irmer pits is the inversion of day analysis' supporting research' Ethnology development' arid education arid public outreach programs with flight projects. For example, establishment of sample receiving facilities arid the laboratories for the ar~alysis of extr~rrestria1 materials must be in place well before ~e return of samples. Although such capabilities are likely to be implemented within ~e context of ~e Mars Exploration Program, Claris must accommodate non-martiar~ materials. Similarly, R&A supporting facilities arid studies of perry processes Ad., laboratory, field-ar~alog, computer modeling) mud ~ supported suffi- cim~ly to mable boy plarming arid scientific validation of results from flight projects. Mod importantly, robust day analysis programs are essential to harvest ~e investment made with flight programs. Ground-b~ed observa- tions are expected to continue to make major contributions to monitoring Ionospheric char~ges arid mapping. ~ eomlusion' the inner planets hold fundamental clues to Be development of E~h-like plar~e~ in our solar sham md elsewhere. They provide Equable insights into He paths toward md Philip of hale environments. The most fundame~ question about these plme~, such as Heir nature, Heir compositions' the interactions of Heir surfaces md atmospheres' md He role of impaetir~ objects, will be addressed by He missiom recommended here. FIEF Elf EN C ES 1. S.R. T~lor' So~r~m ~~ A New Pert Cambridge Ur~iversi~ Press' New York' 2001. 2. F. Robert' Scte~ce 293 1056-1058' 2001. 3. hl.A. Wiec~orek Id R.J. Phillips' ~' Journal of Moped Ternary ~ 03: 17 ~ 5-17~> ~ ?~$3 . 4. hl.A. Wiec~orek Id R.J. Phillips' < ~ add. 7. R.G. Aroma G.G. Schaber' Id D.~. Dawsorl' ~' Journey of hop ~~ar~ 99: ~0899- 10926) 1994 S. W.K. Herman> G. Ryder' L. Dories> D. Grir~spoor~> per~rd Alters Fombardme~ of the Primordial EarWhioor~ Systems> in R. Coup Id K. Righter ~ds.~' Origin of ~e Earth a~ Boors Urliversity of Arizorla Press' Tucsorl' 2000' pp. 493-5 ~ 2. ?. W.B . h4cKirmor~~ K.J. Zahr~le~ ~ .A Iv~ov~ Id H.J. file low <~Cr~erir~g ore Versus: he owls Id Ob~rv~ior~' ire S .W. ~ ought ~ .hl . Hurl~rl' Id R.J. Phillips ~ds.~> Ve~ ~ Urliversi~ of Arizorla Press' Tucsorl' 1997> pp. ?~-1014. 10. S.A.S~m' ~ews of Ocoph~cs37: 453-~' 1999. ~ ~ . ~ . Butler' D. hluhlem~> Id he . Slang ' Journey of hops ~~ar~ 98: ~ 5003-15023> ~ 993. 12. L.W. Esposito' R.G. Kr~olle~erg' FLY. h4arov' O.~. Tooth Id R.P. Turoo' IlThe Clouds arid Hams of Ver~us>~' ire D.hl. Hurry L. Colic T.hl. Donahue, arid V.I. Moron ~ds.~' Ve~' Ur~iversi~ of Arizona Press' Tucson ~ 983' pp. 484-~. 13. P.~. James' H.H. Kieffer' Id D.A. Paint ~' ire H.H. Kieffer' B.h4. Jakosky' C.W. Srly~r' Id hl.S. Mathews (eds.~' Mars' Urliversi~ of Arizorla Press' Tucsorl' 1992' pp. ?34-~. 14. See' for example' D.hl. Hurrah' T.h4. Donahue' J.~.G. Walker, Id J.F. Kayoing' < Or~g~oludo~ of P~rya~Atmo~phere~> Urliversi~ of Arizorla Press' Tucson ~ ?~' pp . 336422. S. D.J. Stever~sor~> <Tucsor~' 2000' pp. ~ ~ 3-~5 . 17. A.T. Basilevsky' J.W. Head' G.G. Schafer, G.G. Strom' Id R.G. Strom' ~' ir1 S.W. Brougher' D.hi. Hu~erl' Id R.J. Phillips ~ds.~> ~~ ~ Urliversi~ of Arizona Press' Tucson' 1997> pp. 10~-1085.

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INNET SoMT ~~M ~ a. N.F. Ness' < S.J. Bauer~ P. Cloutier~ Ad N.F. Ness~ 1>'Jour~l of Mop Rehears 106: 23403-23418> 2001. 20. R.J. Phillips' h1.T. Zuter'S.~. Solomon h4.P. Golombek' B.h4. Jakosky' W.~. Bakery' D.E. Smith' R.h4.E. Williams' B.h4. H~ek' ~5 lo. O. Aharor~sor~' Ad S.A. Hauck' Fanciers ~ od~amics Ad Global~ale Hydrology ore hi arson Scheme 291: 2587-~' 200 ~ . 21. See' for example' S.A. S~m' Tithe Lurker Atmosphere: History, Status, purrers Problems' arid CorJ~xt>~' Rw~ of Mop 37: 4534? ~ ~ ~ Ail. 22. C.h5. Pieters' J.W. Head m' L. Gaddis' B. Jolliff' Ad h4. Duke' 1lRook Types of South Pole-Aitker~ Basin Ad Extra of Bas:~ltic Volomism>>' Journal of Ocoph~al Regears ~ 06: 2800 ~ -28022' 200 ~ . 23. ~ . Hapke' IlSpam We~herir~g from Mercury to the Aneroid F3elt>~' Journal of Moped Reward ~ O 6: ~ 0039-10074' 200 ~ . M. P.F. Hoffman A.J. Kaufman G.P. Halversor~' Ad D.P. S6hrag' 11A Neoproterozoic Sr~owhall E~h>~>Scte~e 281: 1342-1346' 1998. 25. J.F. Ka~ir~g Ad O.~. Tooth 1lOlima~ Evolution ore the Terre~ri:~1 Pl=ets>~' in S.K. Atreya, J.~. Pollack' arid h4.S. Matthews ~ds.~> Origin a~ ~o~o~ of P~ry a~Sateiht;e Atmospheres' Ur~iversi~ of Arizona Press' Tucson ~ ?~' pp. 423-449. 26. J.~. Pollack'1lKuiper Prim Luxury: Prawn ar~dPa~ Climates ofthe Terre~ri:~l Pl=ets>~ar~ P1: 173-~> 1991. A. h4.A. Bullock Ad D.H. Grir~spoon' Tithe Rearm Evolution of Climax ore Ver~us>~' Icarus 150: 19-37' 2001. 28. H. Hiesin~r' R. J~um~' G. Neukum' Ad J.W. Head III' Glans of h4 are Basalts ore the Lurker Nearside' Journal of Copy Reward 105: 29239-~> 2001. 29. E.h4. Shoem:~ker~ 1lLor~g-Term V:~riatior~s ire the Imply Cra~rir~g ~ cord ore Earth>~' ire h] A. Grady R. Hutchisor~~ G.J.H. h4~all~ Ad D.A. Rothery ~ds.~' Meteor~. FI~ w`~ ~~e a~ Impact ~iiec~' Geological So ciety of London Special Publications 140: 7 -10' ~ ~ as. 30. W.~. Feldman S. h4aurim' D.J. Lawrer~' R.~. Li~le'S.L. Lawsor~> O. Gamault' R.~. Win B.L. Barraclough'R.~. Elphic'T.H. Pret~m~> J.T. Steir~erg' Ad A.E. Finer' 1lEvi~r~m for Wear Im Near the Lurker Poles>~' Journal of Mop Ternary ~06: 2323~- 23252' 2001. 31. G.L. Kuloir~ski' 1lUsir~g Lurker Helium-3 to Generate Nuclear Power Without the Production of Nuclear Wand 20th ~tematior~al Span >~elopmerd Cor~ferer~> Albuquerque' N. hoax.> hem ~-~> 2001. 32. G.W. Wetherill' 1>' i~teor~ a~ P~ry Science 34: 3 17-329> ~ Add. 34. L. Wilson Ed K. Keil' 11Volomic Eruptions Ed Ir~kusior~s ore the Aneroid ~ Vestal' Journal of Copy Ternary 101: ~ S9~- sP40) ~ 996. 35. h] J. Drake' Tithe EuoriteNe~a Story>~' AIeteor~ a~ P~ry Sow 36: 50 ~ -5 ~ 3> 200 ~ . 36. R.O. Pepir~> 1lEvolutior~ of E~h>s Noble Go:: Cor~quer~s of Assuming Hydrodynamic Loss Drivers by Gist Imp: Icarus 125: 148-~'1997. 37. K. Zahr~le~ J.~. Polls Ad J.F. Ka~ir~g~ 1lX6r~on Fra~ion~ior~ ire Porous Pl~simals>~' Geo~a et Cocoa Act 54: 2577-~> 1990. 38. Y.A. Surkov' V.L. Barsukov' L.P. hioskal~' V.P. Kh~ukova' Ad A.L. Kemurd~i~' Anew D~a or the Compositior~' Structure Ad Properties of Versus Rook Obtained by Vetoers 13 Ad Ver~era 14~' in Proceeder of ~e 14~ Lu~r a~P~rySc~e Co~fere~> Journal of Mop Te~ear~ Supp~t S?: F3393-F3402> 1984. 39. Y.A. Surkov' L.P. hioskal~' V.P. Kharyukova' A.~. Dudin' G.G. Smirr~ov' Ad S.~. Gaits 1lVenus Rook Composition ~ the Vega ~ Lading Sing' ire Procee~ of ~e 17~ Lu~r a~ PI She Co~fere~e Part 1> Jourml of Groped Reward 91: E21 5-E21 S' ~ 986. 40. V.L. Barsukov' Y.A. Surkov' Lid. Dimiki~' Ad I.L. Khodakovsky' Cryochemical Studies ore Versus with the Leers from the Vega ~ Ad Vega ~ Probes>~' GO ~~er~o~l 23: 53-~> 1986. 41. H.J. Moore' J.J. Plaut' Pod. Scher~k' ~dJ.W. He:~d' Earl Unusual Volomo or~er~us>~> Journal of Geoph~ar~ 97: 13~- 3494' ~ 992. 42. See' for example' A.T. B:~silevsky' J.W. Head, G.G. Schafer' G.G. Strom' Ad R.G. Strom' Tithe Resurfacir~ History of Ver~us`~' ire S .W. Prougher' ~ .h] . Hun~rl' Ad R .J. Phillips ~ds.~> T7~ ~ University of Arizorla Press' Tucsorl' 1 997. n~ . 1 O47- 1 OS:S or T Fit. It n~1 E.R. Scoffs < 2000. Ad ~ IT Lawrerlm) W.~. Feldman) B.L. Barraclough) A.~. Birlder' R.~. Elphic' S. h4aurim' Ad D.R. Thomserl) 1)) top Rehears tethers it: ~ 903) 1997. A. C.h5. Pieters) TOW Head m) L. Gaddis' ~ ~Tolliff) Ad h4. Duke) ~lRook Types of South Pole-Aitker~ Basin Ad Extra of Basaltic Volomism)~) Journal of top Rechart 106 (E) 1~: 28001-~8022) 2001. 48. P.G. Lucky' G.J. T:~vlor, ~ .R. Hawker Ad P.D. Srud.is~ 11 Fe O Ad. TiOo Cor~mr~r~ior~s ire the South Pole-Aitker~ Basin: Implic~ior~s ~ - . 1 for Marble Comp ositior1 Ad ~ asin Formation))) Jo arm! of top Research ~ 03 LEA: 370 ~ -3708' ~ ?~S .