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3 Mars. The Evolution of an Earth-like Planet The first high-resolution images acquired of Mars by Me Mariner 4 spacecraft ~ the dawn of the space age shadered popular notions of hears. Far from being ~ oasis' the surface of Mars appeared to be as battered md barren as the Moon. With id Din Exosphere md bitter cold temperatures' Mars was more parched than the dried places on Earth. The prosper ~~ life could have evoNed Here seemed dim. Each subsequent mission to Mars has eh~ged that impression in surprising ways. Mariner ~ revealed towering volcanoes' polar caps' md reheels apparently euthy water. Systematic observations of Be surface md atmosphere by Viking led to ~ huge increase in our knowledge of the breads of martim geologic history md the dummies of the current climax. We hi landed on Me surface for the firsttime. D~a from He recent Mars Globe Surveyor (MOS)have again revolutionized our underbuying ofthe evolution oftheplmet(Figure 3.~'revealing the importune of the very early development of Tharsis' discovering huge magnetic momalies from ~ early magnetic field' md showing evidence for recent or even ongoing climate Tahoe. Fundamental information also has been derived from the study of martim meteorites. Deviled analysis of these samples has invigorated the debate over whether life ever arose on Mars. Are we alone: is one of the most compelling questions in science. Is the development of life ~ common occurrence or ~ event that is exceedingly rare: ~ Earth wherever wear exists in ~ liquid staid viable organisms have been found. Mars is probably the most compelling pine to attempt to answer the question, Did life ever arise elsewhere in He solar systems' because we know now that water once existed And under some eireums~ees may exist today) in ~ liquid sate on the surface of Mars, md it likely exists in ~ liquid state ~ Kept in the crust. While the pre-space-~ge vision of eiviliz~ions on Mars has been replaced with ~ more informed understanding through exploration md discovery, Mars is still He most compelling md accessible target in He solar system on which to Duress He question of lifers existence beyond Earth. A synthesis of these discoveries md He results of scientific analyses show ~~' like Earth Mars is ~ planet of contrasts. Both planets have had complex geologic histories md climates that evolved md edged; in both eases FIC3UFtE 3. ~ (facing page; Da~ from the ~rs Orbiter Lear Altimeter MOLAR instrument on the ~rs Global Su - eyor spacecraft have enable the conclusion of highly assure images of Marsh topography. Them two images show the 11~1 Planers two dissimilar faces. The northem hemisphere (upper fight is flat and lightly cratered. In Unhasty the muthem hemisphere shows extremes of relief and is heavily cratered. Curlew of the MOLA ~m. ~7
Is HEW FR0~ IN =E 50~R HIM liquid wear played art importers role in ~e evolution of ~e surface arid Oration of art environment hospitable to life. Among ~e plar~ts' Mars is of particular ingress because of its similarity to Earth' yet the most importers lessons to ~ learned Hem from the differences between ~e two ply. Mars science might most usefully be thought of as ~ study of ~e evolution of art Earth-like ply. UNIFYING THEMES FOR STINGIER OF MARS The exploration of Mars has led us to the point of Ming able to understand the main elements or components of its systems. The sum total of information from spacecraft arid telescope observations md from Earth-b~ed research arid ar~alysis programs has led to ~ fairly complex first-order understar~ding of the ply: the composi- tion md first-order dummies of its atmosphere' ~ broad understanding of its wear md climax history' its crusty structure as inferred from global gravity arid topography' arid its surface arid crusty chemistry from remotely sensed measurements arid ~e study of martiar~ memories. While the indiv idua1 component systems of Mars have been illuminated' the relationships between Hem are less well underwood. Research addressing these crosmu~ing questions or themes has ~e po~ntia1 to significar~tly advar~ our understanding of Mars as ~ ply. The themes are as follows: . Mars as ~ po~nti~ abode of life; Wa~r' atmosphere' md climax on Mars; md Structure md evolution of Mars. The first theme recognizes ~~ Mars has had in ~e past on its surface' md may continue to have today in id subsurface' environments with all He ingredients needed to sustain life. Did life ever arise: md Does it exist today: are importmt first-order questions. To answer these questions' however, we need to know more Bout Mars md its evolution. If the answer to questions Bout life is yes, it will be impor~t to know where' how, md for how long life evolved' md id relationship to the plmetts evolution. If the answer is no' Hen it will be equally important to fry to understand why life did not arise. Clearly' He answer will be tied to He second theme' He history of volatiles md evolution of He climax md He atmosphere. One way of addressing He question of life will be by searching for ~ biological imprint on isotopic systems. But to use this type of approach will require more complete understanding of the atmosphere' the climate md its history, md, of course, water. Space exploration has taught us ~~ ~ strong coupling exists between the structure md evolution of planetary interiors md their atmospheres md climates: that is, between the second md third themes. For example' He discovery of localized, very strong remnant magnetism in its ancient crust suggest ~~ early Mars had ~ active dynamo md ~ strong magnetic field. If this was He ease, it would have shielded He plmet from biologically harmful solar And cosmic) radiation md inhibited the loss of vol~iles (w~er) to space. One of He distinctive characteristics of Earn relative to other bodies in the solar system is the presence of life. Over He past decade' we have begun to appreciate ~~ life on Earn has been more than ~ thin veneer of biology passively enjoying He ride; in feet life has strongly influenced the evolution of Earn. Clearly Mars is not as hiologieally active as Earn is, md it may even be inert. However' because Mars preserves part of id ancient geologic record ~~ is now lost on Earth, md because it has ~ atmosphere, evidence for liquid water ~ some time on its surface' md ~ ancient magnetic field, it provides ~ window into the early history of He evolution of Earth-like planet md perhaps the origins of life. MARS AS A POTENTIAL ABODE OF LIFE Present Life The surface of Mars today is cold' dry' chemically oxidizing, md exposed to ~ intense flux of solar ultraviolet radiation. These four factors are likely to limit or even to prohibit life ~ or near He surface of the martim regoli~.
Temperature is of ingress not only because of its controlling influence on microbial metabolic rams but also because of its influence on ~e stability of liquid wear. Although ~e peak denims surface temperature near ~e martiar~ equator cm rise above ~e freezing point of wear during much of the year, the average surfed ~mpera- ture is about 220 K' well Plow ~e freezing point of wear. Liquid wear is essmlia1 for life as we know it. Wear is abundar~t on Mars, but not in liquid form.) War vapor md ice crystals are premnt in the atmosphere, arid wear ice is almost certainly present within ~e martim regolith ~ high latitudes arid ~ the surface in polar regions. At increasing depth' where ~e rock is warmer as ~ result of the ply geothermal gradient liquid wear may be presmt in pore spaces.2 To dew ~ single mt of robotic studies he searched for exam life on Mars: ~e Viking life-de~tion experiments' which were desired to ~st for organisms ~~ used ~ Air carbon sours eider carbon dioxide or orchid molecules. Though ~e result obtained by the Area sew of experiments are regarded as having shown ~e materials Id to be devoid of both orgar~ic compounds md evidence of lift this in~rpre~tion has been subject to deba~.5 The lack of agreement highlights ~e difficulties inherent in ~e detection of viable microorganisms by robotic mems. Indeed, even were there unanimity thy the Viking experiments did not show the presence of life' ~e experiments could Fill be criticized as being overly ``geocentric,, in thy they showed ~ lack of evidence of metabolism only of Hose types particularly common among ~rres~ia1 microbes' not of ~1 conceivable metabolisms (nor even of various redox-reaction-~md microbial metabolisms well known on Early. The problem of distinguishing between biological Id nonbiologica1 orchid compounds is also complicated. The earbonweous ehondrites' interplme~ry dust particles' Id probably other bodies within the solar system contain abundant organic material ~~ is s~uetur~ly similar to biological products. Definitive resolution of He differences between biotic Id abiotie organic molecules requires highly sophisticated techniques well beyond my that could be mmaged robotieally. The accepted interpretation of result from He Viking landers is that He surface materials tested were devoid of organic molecules Id of my other evidence of life.6 However' even without consideration of alternative in~rpre~tions'7 the Viking results ergot be Eked as indicating thy life does not currently exist on Mars. Organisms ~ the Viking sites might have been missed because He experimental conditions (em.' the nutrient provided or processes followed) were not chosen correctly. Even more importantly' martim life might reside in aqueous oases' such as my recently Utile volemie vend or fumaroles disks from He Viking lading sites' or depths far beneath the surfeit regoli~ sampled by the Viking experiment. Put Life The surface environment of Mars may not always have been as hostile to life as it is today. Early in He plmet~s history, He average temperature may have been warmer Id He Exosphere more dense, Id liquid water may have existed ~ the surface. The geomorphologie evidence' especially valley networks, indicates that He martim climate was wetter, warmer, Id appreciably more hospitable to life prior to about 3.S billion years ago than it is ~ present. Fossil evidence of past martim life' if there was my' may be preserved in surface water-laid deposit such as lake- or Embed sediments in evaporitie mineral p~s,3 Id in hydro~erm~ly deposited mineral crush (Figures 3.2a Id by. An imports zone that seems likely to have been habitable throughout martim history is the eru~1 sub- surface, where wear may exist in ~ liquid ~ate. The geothermal gradient of Mars is probably such ~~ liquid water is present ~ depths as shallow as 2 km near He equ~or.9 The discovery of Crest microbes living deep within the Columbia [liver basely in He U.S. Pacific Northwest Id elsewhere on Earth' is ~ depths as grew as 3 km,~i is eonsis~nt win the possible presence of microbes living in similar settings on Mars. Samples from hypothetical subsurface settings of life would be very difficult to access, yet such materials may have been dislodged Id brought to He surface by meteoritic impacts. A study of the martim (SNC) meteoric ALH84001 produced evidence suggestive of biological Utility on Mars about 3.6 billion years Ho. This conclusion has not been widely accepted; the report has engendered much discussion' bow pro Id eon, regarding each of He several intriguing indicators of life proposed.
70 HEW FR0~ IN =E SOLAR MUM FIGURE 3.~ An acria1 view of the Greg Prismatic Hot Spring in Wyoming5s Ycllowstone Nation Park. The color variations are due to pigments in thermophyllic microns residing in the wears. Such ~~ms are Hug studied to unversed the limits of life on Each ~d as possible clogs for environments where life may have exisM on Mars. Age Source of louse Finley Island Park, Idaho. FIGURE 3.2b Travertine deposits ~ the hlincrva Terrace, M~noth Hot Sprinted Ycllowstone Nation Park. Such deposits are intimately asocial with microbial communities, aspects of which are commonly preserve in the kavertine ~posits. Hot springs ~d their deposits are Wing studied to un~rs~nd the limits of life on Each ~d as possible Clogs for environ- me~s where life may have exist on Mars. ~~e courtesy of lluss Finley, Island Parks Idaho.
71 Environmental Context for Life The question of life on Mars must trmmend ~ search for actual orgar~isms. It mud include ~e question of whether the martiar~ environment is or ever was hospitable to ~e beginning of life. This is ~ broad md complex questions arid ~e evidence may ~ so deeply buried in the past ~~ itcar~ be ar~swered only by gaining ~ extensive arid deep knowledge of Mars. For example' on Earth e~me-driven metabolic procesms cm create char~ristic biogenic isotopic signatures (affecting, in particular, ~e rages of compositions of ~e Cable isotopes of carbon' sulfur, nitrogm, hydrogen, arid possibly iron). However' in order to use such isotopic measurements to ~st for ~e past presence of life on Mars' we need to know ~e mope of abiotic fractionating processes there. The search for life should be based on the premise thy to understar~d the po~ntia1 habitability of Mars, we must fully understand the plar~et~s present arid past stales. We should be as prepared for ~ negative grower regarding Marcus po~tia1 habitability as for ~ positive one. The importar~ee of ~ positive mower is clear' but ~ neg~ive grower would prompt inquiries into what the implications are for the plar~et~ differences between Earth arid Mars. ELey Questions (questions win po~ntia1 for ~ paradigm-altering discovery relend to the question of life on Mars include He following: Does life currently exist on Mars: Did life ever exist Beret A question win potential for ~ pivotal seientif~e discovery is- How hospitable was md is Mars to life: Future Dictions The most important future activities with respect to the question of life on Mars are as follows: I. Sample-return missions will be required to permit definitive lens in ~rrestria1 laboratories for present md past life on hears (see section ``Priorities md llecommend~ions,, below); robotic missions preceding He sample- return missions will assist in locating the most fruitful sites to be sampled. 2. A broad program of study of He Mars environment' present md past, is needed to understand the context in which life did or did not arise on ~~ planet. WATER, ATMOSPHE1tE' AND CLIMATE ON MAIM Water The topics that comprise He theme of whorl atmosphere, md climax on Mars are closely linked. As on Earth water exists on Mars in mmy stabs md participates in ~ broad range of imports physical' chemical, md possible hiologiea1 processes. Water has played ~ key role in the evolution of the martim climate md in the shaping of Marks geological history. The question of where wear is on Mars today is difficult to answer fully. We have direct observations of four exposed martim wear reservoirs, which include water vapor in He a~osphere, water fee in the atmosphere' seasonal water fee deposits ~ the surface' md permanent water fee deposits ~ the polar caps. Of He four' He martim polar caps are by far the most massive. Recent MGS MOLA topographic profiles indie ate thy the mass of water fee contained within Marks norm md soup polar caps' assuming ~ high iee-to-du~ ratio, is He equivalent of ~ globe water layer 22 to 33 m thiek.~4
79 HEW FR0~ IN =E 50~R HIM Beyond the wear reservoirs the now cart be demand on Mars, bare is g God reason to suspect the presence of hidden wear reservoirs whose combined muses should be much grower bars bosh of the reservoirs the are currently exposed.~5 In Mares near-surface regolith, it is expected the wear is adsorbed on soil particles' arid there is fragmentary evidence from the Viking Cas Exchmge experiment ~~ the mass fraction of the wear could be on ~e order of ~ percent. Viking arid MOS observations have provided geomorphic evidence ~~ the layered deposit surrounding the norm arid south polar caps also contain wear ice' but its mass fraction is currently not well constrained. It is also expected ~~ near-surfa~ ground ice is to ~ found on Mars, as on Earth' arid numerous geomorphologica1 indictors support this idea.) ~ Models predict the it should be present within the top moors of the surface ~ latitudes as low as 20 degrees from ~e equator in favorable loc~ions.~7 Because of Marsha low surface temperatures, the partitioning of wear is heavily biamd toward its condensed phases' causing ~e martiar~ atmosphere to be extremely dry arid ineffective ~ Sporing large quantities of wear on seasonal time scales. Liquid wear on Mars is not expected to be sable on Mars today' because temperatures exceed 273 K only ~ low latitudes during ~e warmest periods of ~e day' arid my liquid generated would quickly evaporate arid be har~spor~d by the atmosphere to colder locations where it would then free=. Some of the most exciting questions concerning Mars deal with the past distribution md behavior of wear. Marty of them questions are motivated by geomorphic evidence such as runoff charmers' outflow charmers' arid other features ~~ have been in~rpre~d to merry ~~ liquid wear may have been present periodically on ~e surface of Mars in past epochs.~8 The recent MOS Mars Orbiter Camera arid Mars Orbital Lamr Altimeter observations have provided evidence for large ch~els ~~ once flowed from ~e southern highlands to Me nor~ern lowl~ds'i~ widespread mcient layering inferred by some to be of mdimm~ry origin,2° md small gullies on order walls that are considered to be evidence for recent erosion by fluids (Figure 3.3~.~i Atmosphere Our knowledge of the composition of ~e Mars Ionosphere is band on measurements of minor gases such as neon, krypton, md xenon md ratios of common isotopes in the ambient Exosphere 06Arp8Ar' i267~' i6Ofi7O' i6Ofi8O, i4Nii5N, thigh) by ~e Viking descent mass spechome~r, ground-based md airborne spectroscopy, md laboratory analysis of atmospheric gams captured in the vitreous components of martim memories. It is Nought that ~ combination of impact erosion md longhorn atmospheric loss from ~e top of ~e atmosphere by solar-wind sputtering md other processes' md possibly sequestration of COG md other gams in the crust of the plmet are responsible for the present low Ionospheric pressure ~ the surface of Mars (the yearly average is ~6 mbar). Marks present-day lower atmosphere is dominated by ~e behavior of CO2' wear vanor. md dub. as driven ~ O AL ~ ~ . ,~ , by the h~rs~un eor~f~guration md by the interactions of CO~, water vapor' md dust win the surface. A eombin~ion of the above. tweeter win issues of transport md cloud physics' eonstitu~s hears meteorology. ~easonai endues In me atmosphere mass of COG are up to 30 percent in He current epoch. Water vapor also inches with clouds md surface marries; id average ~ua1 column abundance is ~10 to 40 preeipitable microns of water ~ norm midlatitudes. Very little is known about the upper Exosphere of Mars. However, He interactions between Marks upper atmosphere md He impinging solar wind md solar ultraviolet light appear to have played ~ significant role in He evolution of the martim atmosphere md in the transition from ~ warmer md wetter environment to He present-day colder md drier environment. Only by understanding He processes that em occur in the upper atmosphere em we fully understand what drove the echoes in He volatile inventory md in He climate md thereby understand He evolution of habitability on Mars. The only in situ measurement of atmospheric composition came from the Viking descent neuba1 mass spee~ome~rs. These provided two midl~itude vertical profiles' in the altitude range of about 120 to 200 km, of CO2, CO' N2, O2' md Ar densities during low-solar-activity conditions. Using the male heights Bus measured' atmospheric temperature profiles were deduced. These temperatures showed quip large variations md averaged c200 K. Some indirect md limited information on composition md temperatures has been obtained using airflow md ionospheric information. The upper-atmospheric temperatures appear to vary by about IS0 K between solar cycle minimum md maximum conditions. The z-axis accelerometer carried by the MOS provided ~ grew deal of important information about total densities md temperatures during id extended aerobraking period.22
73 FIGURE 3.3 The Mars Orbiter Emery on Mars Global Surveyor imaged the' channels in ~ Wrier in the region End Gorgonum ~7.40 S. 168.00 W). Them features have En interpret by some researchers as Wing due to the ram flow of wamr across the surfed. The numerous channels ~d apron deposits indigo ~t may Ens to hundre~ls of individual cvems involving the flow of wear ~d Chris have occurred here. The channels ~d aprons have very crisps sharp reliefs ~d there are no small impel craters on them, subduing 0t shed features are extremely young relative to the 4.5-hillion-y~r history of Mars. The image is 2.3 km wide ~d illumination is from the upper left. Mars Global Surveyor, Mars Orbiter Camera. libeled No. MOC2-241~ toured of NASA!JPL~lin Sped Scien~ Systems.
74 HEW FR0~ IN =E 50~R DIM The only in situ measurement of the thermal plasma composition, density' arid temperature in ~e ionosphere of Mars were obtained by ~e retarding po~ntia1 ar~aly~rs carried Board ~e two Viking lar~ders' along win ~e mass spectrometers mentioned above. Electron density altitude profiles were also obtained by several U.S. arid Soviet spacecraft (~.g., Mariner 9~' using ~e radio occultation technique. Thus, we have some information on bow the dayside arid near-~rminator-nigh~ide electron density values, covering the altitude rar~ge of about 120 to 300 km. No clear presence of art ionopause was seen in this database. Cliche Climax encompasses ~ broad range of complex interacting systems with ~ wide rar~ge of time scales. The Mars climax system, which includes the surface' atmosphere' polar caps, arid accessible regions of the subsurface' has undergone significar~t charge during ~e plus history. Three time scales of climb variability cm be considered: in~r~ual' quasi-periodic' arid long term. Multidecade telescopic records of grew dust storms, multiyear surface pressure records acquired ~ the Viking larding sired multiyear orbiter observations of the appearar~e of ~e seasonal md residual polar caps, md large Derisions in ahnospheric wear make it clear thy the climax of Mars exhibit distinct Derisions from one year to the next (in~r~ua1 char~ges). Understanding the nature arid causes ofthese variations is importers for identifying interactions among Me cycles of carbon dioxide, dust' arid wear in Marsh present climate. Me of Me cornerstones of our understanding of the climax of Each is ~~ small, quasi-periodic variations in Earthts orbital md axial elements over time males of lens to hundreds of thousands of years result in large-scale chimes in Earths clim~e.~ Marks orbital md axial elements experience variability on time males that are comparable to those of Earth' but Be magnitudes of these Derisions for Mars are signif~tly grea~r.24 The consequent changes to the insolation ~ high latitudes undoubtedly have caused significant chimes in the seasonal cyc les of carbon dioxide, water, md dust. Awed on our preset understanding' hears is the plum in Be solar system that is likely to have experienced Be most signify quasi-periodic variations in its climate (Figure 3.4~. A wide rude of surface features on Mars cm be interpreted as evidence for warmer climatic conditions various times in Be plmetts history (longhorn climate change). There is general consensus that Mars possesses all Be volatile ingredient necessary to produce ~ warm md wet climate, but the problem is that ~ Marks disuse from Be Sun' the sable location for Marks volatiles is not in the atmosphere but in condensed phases, which makes it difficult to maintain ~ stable martim greenhouse.25 Although Be earliest martim atmosphere was probably lost by impact erosion md hydrodynamic escape during Be Early No~him era, ~ relatively robust atmosphere appears to have been reestablished during Be No~him by primitive volatiles released during Be erection of the Tharsis Plateau by volemie md igneous processes. The end of the No~ehim marked ~ huge eke in Be climax md probably in Be volatile inventory of Mars. Erosion rams declined, valley network formation largely ceased' md magm~ism declined. The intrinsic magnetic field appears to have declined or ceased ~ ~~ time; He loss of the protective magnetic field may have allowed subst~tia1 solar-wind erosion of Be Ionospheres with ~ consequent eke in elimate.26 Key Qu~tiom . . Questions with po~tia1 for ~ paradigm-altering discovery related to whorl Ionospheres md climax on Mars include He following: What are the sources' sinks' md reservoirs of volatiles on Mars: ~ How does the atmosphere evolve over long time periods: Questions with potential for ~ pivotal scientific discovery include the following: Is there ~ active water cycle on Mars: What are the dummies of the middle md upper Exosphere of the planets What are the rates of atmospheric escape:
75 FIGURE 3.4 The Mars Orbiter Emery on Mars Global Surveyor imaged the alternating layers of bright and dark mamria1 comprising the North Polar Cap. This image of one of the dark lanes crossing the cap reveals inferno layering. This layering is thought to Tonsil of mixtures of wamr in and bush with the alto variations indicating different dun conventions in an in matrix. The apparent regularity of the variations with depth may ~ indimlive of quasi-periodic variations in the martian climam. The image (~.4~:0 N. 279.540 W) shows ~ region I. - km wide and the vertical relief from the top of the image to the bonom is approximately 350 m. MOS MOLD M0002100~ Soured of NASA/JPL~lin Spew Scien~ Systems.
76 HEW FR0~ IN =E 50~R HIM A question whose crower would conbibu~ to building ~e foundation of knowledge of the solar system is- ~ ~ . - ~ ~ ~ ~ ~ ~ A ~ ~ A ~ ~ ~ A ~ ~ What is the thr=-dimensiona1 distribution of wear in the martiar~ crush Future Di~iom Importers directions for ~e future relating to Marsh whorl ahnosphere, md climax are the following: I. The ground-leve1 chemical arid isotopic composition of the ahnosphere, including humidity, should be tracked for ~ least ~ martim year ~ ~ network of trader stations. 2. The distribution of water (in both solid arid liquid form) in He crust' globally or ~ ~ wide varieW of sites' should be established (e.g., by sounding radar). 3. The composition arid dummies of the middle arid upper atmosphere arid the ram of escape of molecules from Be ahnosphere should be measured. STItI}CTI~E AND EVOLUTION OF hIAl~ Structure and Aridity of Be Crust and Interior Major advances in our understanding of the interior of hears have come recently in four impor~t areas: . . . ~ The bulk composition of Mars is better constrained owing to ~ "really improved estimate of the moment of 'Hertz mane possible by Pathfinder measurements. The moment of inertia depends on He distribution of density within ~ planet' md only ~ limited range of rock compositions have ~ given density. ~ Mars had ~ magnetic field in the past but Here is no present global field' ~ shown by high-amplitude magnetic mom alies detected in the southern highlands of Mars by He Mars Global Surveyor.28 ~ Crusta1 Slickness variations are fairly smooth across He dichotomy boundary between the northern md southern hemispheres of Mars; thus, ~ impact origin for He low-lying northern hemisphere is not favored.29 The crush thickness result are consistent with ~ plate tectonic hypothesis' but Hey do nof confirm that idea. ~ A key insight from the MOS topographic dam is ~~ the Tharsis Plateau predates He formation of apparently fluvia1 reheels. This suggests ~~ the outpouring of 1~a to make the plateau may have released enough carbon dioxide to form ~ insulating atmosphere md sufficient water to form the reheels md even ocem.~° Composition of the Crust and Interior Most of what we know about the composition of Mars comes from Free lypes of measurement: If} in situ analysis of He rocks md regolith by landers' At} orbital observations by emission md refiee~ee spectroscopy' md (3) studies of meteorites ~~ are inferred to have come from Mars. In situ Geyser by the Viking md Mars Pathfinder landers found rocks ~ the Pathfinder sin to be more siliceous than the basaltic rocks ~ the Viking sins. The soil is similar ~ both sites md less siliceous than rocks ~ either. Measurements from He Thermal Emission Speebome~r aboard MOS extended these compositions globally; mdesitie rock appears to dominate in He northern lowlands md basalt in the older southern highl~ds.32 Members of the SNG amatory of memories' comprising He shergodites, nakhli~s, md eh~signites' plus He unique memorize ALH84~' are thought to have come from Mars. Five different rock types are known in He SNG collection. They include basalts md lherzolites (shergo~ites), elinopyroxeni~s (nakhli~s)' ~ duni~ (Chassigny)' md ~ or~opyroxenite (ALH84~1). Most appear to be igneous eumul~es. None of these rocks mashes He composition of the basaltic mdesites found ~ He Mars Ponder lading sib. Similarly' none samples the surface-atmosphere interface, md Hey constitute ~ very inadequate sample of interior compositions.
77 Chronology and Strntigrnphy The geologic units of Mars are assigned to Area major time-str~igraphic Xylems. The oldest is the Noachiar~ system' which comprises ~e ar~cient southern highlar~ds. MOS dam indigen ~~ the Tharsis complex of volcanoes was initiated in the Upper Noachiar~ era. Rocks of the Hesperiar~ system overlie Noachiar~ unit; these include much of the northern lowlar~ds. The most recent system is ~e Am~oniar~' represented by the plains arid volcanic materials of Ammonia Plar~itia. The absolute ages of Marks geological events' arid Bus ~e time history ofthe ply evolution' will be fully understood only when the relative chronology derived from s~tigraphy is tied to art absolute chronology. The density of superposed armors provides ~ mourns of estimating absolute chronology' but this technique is dependent upon imperfect models of the cr~ring ram on Mars through time. The flux of ordering projectiles on Mars is underpin by about ~ factor of two.33 This uncertainty has relatively lisle effwt on interpretation of the absolute age of Noachiar~ terrains, expected to have been originally nearly spurned with craters' or of very young terrains' where ~ surface with ~ nominal age of ~10 million years is young in arty case. However, the factor-of-two uncer~in~ metros thy ages of terrains thy fall in middle maim history are very poorly constrained. Isotopic dating of Mars rocks from key str~igraphic levels will be required to establish the absolute chronol- ogy of ~e martiar~ geologic record. The most reliable dates will be obtained from samples returned to terres~ia1 laboratories; laboratory precision in ~es gotten by the K-Ar, 39Ar-40Ar' Rb-Sr' Sm-Nd' arid U-Th-Ph techniques will approach 10 million years. In order to extend the rape of sips dated beyond those thy em be reached by sample-return missions, it may be impor~t to develop ~ technique of in situ doings presumably by the K-Ar melody by robotic spacecraft. Using this method in the laboratory to dam martim meteoric samples of known radiogenie age, researchers find that K-Ar em be used to date samples in situ to ~ accuracy of ~20 pereent34 which for rocks of intermediate age would be ~ grew improvement over Me factor-of-two uncertainty in eratering chronology. Whether this technique em be effectively implemented on Mars has nof been demonstrated. Surface Pro~ses Wa~r' wind, volemism' md impact eratering have been fundamental drivers of large-scale surface modifiea- tion on Mars. ~ ~ smaller scale' surface materials are altered by reaction with the Exosphere in ways ~~ are poorly understood. Morphologic features creed by ruling water md, apparently, by Wading bodies of water em be seen on Mars. Fluvia1 features range in size from the Mint outflow reheels to valley networks to recently identified small' young eh~els.~5 Features indicative of siding bodies of wear range from putative shoreline features in the northern hemisphere' perhaps due to ~ ocem,36 to deltaie md in~aerater sediments' to finely layered bedding Sediments deposited in siding bodies of wear are high-privily sites for the preservation of fossils md hiosignatures.37 Mmy of the valley networks terminate in erasers, while Me outflow reheels primarily debouched to the northern plains. Wind has been ~ significant force in shaping He surface of Mars. Dunes are ubiquitous features' seen across Mars from orbiter to lander resolutions, while so much of the plmet exhibits ~ mmile of fine-grained material that true bedrock exposures are rare. A Hewer understanding of He importune of eolim processes through Marsh history will require thorough eharae~riz~ion of the current atmosphere md its dynamics, long-term surface observations of He surface md ahnosphere ~ ~ rude of sites' systematic imaging' md returned samples. The style of volemism varies in space md time across Mars, raging from large eons~ue~ in He Tharsis region win relatively young surface flows (Figure 3.~' to vast Hesparim ridged plains' to morphologies sugges- tive of old' explosive volemism in He central highlands. Our underfunding of magma chemistry md absolute chronology is' however' primitive' md it is nof yet clear whether the range in volemie styles represents eh~ges in source regions, eh~ges in near-surface environments' or a~nospherie evolution. Models for the physical md chemical alteration of He martim surface span ~ wide rude of possible meeha- nisms. The presence of ~ apparently deeply oxidized ancient crust coupled win apparently unoxidized later volemie l~dforms has led to the idea ~~ most of the weltering occurred early' during ~ warmer' wetter time'
Is HEW FR0~ IN =E SOLAR MUM FIGURE 3.5 This wi~-~gle view from the Mars Orbiter Emery on Mars Global Surveyor shows the martian volcanoes ~raunius Tholus (~awe: ~d Ur~ius Tholus ,(uppe0. The prawns of impel craters on the' vol~ocs. particularly on Ur~ius Tholus, indigens ~t they are quip ~cic~ ~d are not active by. The light-tone<] area on the south~em few jawaM sawer Aged of Ceraunius Tholus is ~ remet of ~ once-more-ex~nsive deposit of dub from the global dub storm cvems 1~t occurred in 2001. The cramr ~ the summit of ~raunius Tholus is about ~ km across. Sunlight illuminams the one from the lower left. ~~e Soured of NASA/JPL~lin Spew Science Systems.
79 arid thy alteration has ~~n sporadic since then. Estimated rams of weathering under current conditions are essentially negligible. ELey Question (questions with po~ntia1 for ~ paradigm-al~ring discovery with respect to the structure md evolution of Mars include ~e folowing: What rock Ins comprise ~e crust of Mars: ~ What are the nature md origin of Marks crusty magnetisms Questions with po~tia1 for ~ pivotal scientific discovery are ~ follows: What is the degree of internal activity in Marsh What is the size of the Martian cores arid is it partly or wholly liquids What was He origin arid fad of the Mars dynamo: What is the absolute chronology of the plar~et~ How does the oxidation sate of the Mars crust vary win depth Future Di~tiom Importmt directions for He future relating to Mars is structure md evolution include the following: A long-lived network of seismic stations is needed on Mars for determining the structure' properties, md aetivily of id interior. 2. Hem flow from hears ultimately should be measured ~ ~ series of surface stations. 3. The compositions md ages of ery~alline rocks from ~ distribution of martim sites should be measured. This will best be done by studying returned samples, but the database em be expanded win in situ measurement made by landers. 4. A high-resolution magnetic map of Marks southern highlands should be made. INTElICO - ECTIONS AND C~OSSC~l~llNG THEMES The fundamental questions for Mars exploration outlined in this chapter link strongly to the overall themes of this survey report as well ~ to the themes md directions of several of He over pmels. Belie to the overall themes of the survey (Where did we come from: Where are we going: Are we cloned, the scientific md exploration priorities for Mars are strongly linked to the Gird question. However, it is impossible to properly address this question md understand the Sue meming of ~ answer without ~ strategy for understanding He evolution of He interior md climate of He planet, which tie into He other survey themes. A key erosmu~ing theme for ``evolution of ~ Ear~-like planet,' is ~~ of coupled atmosphere-surface- interior processes. The evolution of the climate is intimately tied to the release of volatiles from the interior md the protection of ~ early ahnosphere by ~ magnetic field. As He climax evolves, id signature is recorded in minerals through surface-~nosphere interactions md preserved in weathering rinds mdior eoneentra~d in sedi- men~ry deposits. The isotopic signatures of current gaseous md solid phases' together with signatures preserved in He geologic record, document key element of this evolution. This clearly em across He various themes of reports from this panel md the Doer Planets Panel. The primary theme for the Wer Plme~ Panel is <<The Irmer Solar System: Key to Habitable Worlds.~, If we consider the solar system ~ ~ model for underfunding how Earth-like planets form md evolve, then Mars' like each of the inner planets, is ~ critical piece of the puzzle. Through the exploration of He inner planets md eompar~ive plmetology' we have developed ~ deeper understanding of He similarities md differences in planetary
so HEW FR0~ IN =E 50~R HIM evolution arid relationships to the basic physical md chemical properties of ~e ply. Yet we are still faced win fundamental questions' such as, What 1~ to the unique character of our home plumb How importers is relative position in ~e solar system' or even birth order: What is the real role of perry sing Why is plan tectonics observed only on Early How does ~ magnetic field affect clime md volatile evolution: How does ~e presence of ~ biosphere affect ply evolutions (We know Earths biosphere Mongols ~e composition of id atmo- sphere. Comparisons of Venus win Earn underline ~e difference ~~ this cart make.) The exploration of Mars will feed directly into ~e major questions defined by the Irmer PA Pared. However' ~ detailed hIars program is not ~ substitute for ~e exploration of the inner ply; Mars is but one `41~g of ~e stool., Just as we carmof duly understand the ramifications md implications of the trusser to the question' Did life ever evolve on Mars9, without ~ comprehensive knowledge of the planets evolution' so also we Honor address the fundamental questions of the inner solar system without comprehensive knowledge arid comparative study of all the plus. - WENT NASA AND INTERNATIONAL PLANS FOR MARS EXPLORATION The pace of Mars exploration for the next decade is breaking. A recently released report by the ARCH Committee on Planetary arid Lunar Exploration (COMPLEX), Assessment of Man Scat ~d Meow Pr~or`~' reviewed He current same of Mars mienee, identified critic questions for future investigation, md mapped He congruence between existing md proposed missions md these science priorities. The result are summarized in Table 3.~. Two impor~t points are evident from this summed. The first is than including spacecraft currently in orbit around Mars md excluding the Mars Sample lleturn mission' Here are nine missions plied or in operation' some involving multiple assets such as He two hears Exploration [lover (h~) missions md four landers for NetL~der' ~~ will fly before the end of 2009.~ In addition' ~ Mars Scout mission will be selected before He end of 2003 to fly in 2007. The range of Mars science that will be addressed by these missions is as broad md deep as the Mars science eommuni~' raging from the upper Exosphere to the deep interior. The second point however' is that even win this program, there are areas of science ~~ will not be addressed. For example, no plans have been made for conducting absolute dying of the surface by isotopic techniques; nor are there plans to inve~iga surface-~nosphere interactions in He polar regions. It has been argued that in order to properly address He highest-priority question for hears Did life ever evolve on the plme~ the planetary eons is crucial to underfunding the implications of ~ yes or no response. Given the range of investigations over He next decade, this foundation should be achieved. However' within fiscally constrained program it is still not possible to cover every topic to He level expected by every constituent. KEY MEASUREMENT OBJECTIVES On the basis of the current sate of Mars science reflected in this chapter as well as in other recent NO md NASA documents,39~40 the most importmt measurement objectives for hears have been identified md prioritized. The top priority is to obtain day to answer He question Did life ever arise on Hurst This panel concurs win He conclusions of earlier N1~C panels thy ~ definitive answer to this question em only be obtained via He study of samples returned to Ear~.4i Returned samples would also serve to support ~ number of other high-priority studies bearing on He climate md weathering history md geologic evolution of He planet. To undersold He overall evolution of hears md the intereormeetions among its systems (interior, surface' atmosphere), which are central to answering the question about life on the planet, key in situ measurements are required. The atmospheric md seismic measurements described above (see the sections <<Water, Atmosphere, md Climate on Mars,' md restructure md Evolution of hirers,,) require landers win long-dur~ion capabilities to establish the presence of interns aetivi~ md capture the full se asona1 dummies of atmospheric processes. The Editor room: Durir~g the period~her~this report w:~eir~g prepared for public~ior~' the Frer~ch-led NetL=~r minor candled.
i] TABLE 3.l Comparison of Recommer~tior~s of Seier~ee Priorities win Experiments On Projected Flight Missions Trlolu sion in Missions Skiers Priorities ~ ~ 0 0 0 cs~ 0 0 0 0 ~ O ~ 0 ~ ~ ~ ~ Parley Recommendir~g 00 0 ~ ~ ~ co ~ ~ ~ ~ ~ ~ ~ O O ~ ~ ~ ~ ~ ~ ~ ~ ~ ° ~ ~ v ~ ~ ~ ~ v O ~ O O ~ O ~ O O v v v v Z v ~ v v Z ~ : : NASA Other c~ O csx ~ O (N Z ~ ~ Z IrJerior Wh~ is the si~ md st~e of the core~ Is h4:~rs a~ive (irJerior :~ivity' tector~ics' voloar~ism)9 Wh~ is the thickr~e s~stru ~ure of the oru ~9 Wh~ i ~ the ~ otherm a1 gra die r~ Wh~ is the chara~eriorigir~Hvolutior~ of the ma~etic field~ Geochemisky ~d Petrology Wh~ v~ri~ior~s of ~ochemistry ar~d petrolo~ are prese~9 Wh~ h:~e beer~ mechmisms of ~ochemic:~1 differer~iation Is there evi~r~m for aqueous mir~eraliz~ior~9 Chrorlolo ~ ~d Str~igraphy Wh~ are the relative ages of geologica1 units ar~d ever~ts~ Wh~ are the absolu~ a~s of ~ologica1 ur~its ~d everJ~9 Wh~ are the absolu~ a~s of orystalline rooks Surfam Prooesses Wh~ are the preser~t rates of erosior~ ar~d ~positior~9 Wh~ were the past r~s ar~d prooesses: w~er ar~d coli~9 Wh~ h:~s the role of impa~ or~ering beer~9 Wh~ role has voloar~ism pl~ed ir~ surface evolutior~9 Surfami~mosphere ir~teractior~: wh~ vol~ile sourm~sir~ks W~er Pre~ cycle: souroes' sir~ks' mechar~isms' d~amics~ Wh~ is the 3-D orusta1 wa~r distributiorVorigir~ (liquidlice)9 How has the hydrologica1 cycle opera~d ir~ the pa~9 Life Does life exi~ or~ h4ars Can ~ y chemic:~1 products of life be ~tected~ Do isotopic pahems suggest life Wh~ o~ we leam from Artarotic meteorites Atmosphere Wh~ is the ourrer~ compositior~ of the atmosphere~ Wh~ are the ciroulatior~ d~amics o f the atmosphere (T, P)9 How has the atmosphere char~d over time Wh~ is the radi~ior1 environmerrt at the surfam of h5ars Wh~ is the r~:~ture of we:~ther or~ h4ars Clim~e Cordrol Wh~ is the ir~erarmua1 variatili~ of climate Wh~ has beer~ the long-term clima~ history of the plar~et~ Upper Atmosphere ~d Plasma Er~viror~mer~ Wh~ are the d~amics of the upper atmosphere Wh~ are the hot atom ~ur~d~ ~ s ~d e soape fluxes Wh~ are the ior~ esoape fluxes~ Wh~ are the ma~etic field cor~figur:~tior~9 Wh~ are the proms~s cor~trollir~g the ionospheric ener~tics o 00 o O O O O 00 0 00 0 00 0 00 0 O O O O O O O O O O O O 00 o o o o O O O o o o o o o O O O o o O O O ~ O ~ O O 00 00 O O NOTE: ~ the columr~ titled 1lP~1 Recommer~dir~g>~> solid ciroles i~r~ify the que~ions th~ eachpar~1 recommer~d for ~u~. The columr~ labeled 1lIr~olusion ir~ h~issions>> shows which missior~s will address these que~ions, solid ciroles si~ify missior~s th~ will cor~nka~ on e:~ch scien~ obje~ive' ~d oper~ ciroles si~ify ~ les~r 1~1 of ~er~ion to th~ obje~ive. h~issior~s ir~ NASA>s h4ars Explor~ior~ Program are li~ed sep~r:~ly from the missiorls proje~ed by other n~iorls. Dur~g the period wher1 this report was being prepared for public~iorl' the Frerlch-led NetLm~r missior1 was omm1e d.
so HEW FR0~ IN =E 50~R HIM dummies of ~e upper atmosphere of Mars arid rams of Ionospheric escape should be studied (among other reasons) to constrain the rams of wear loss from Mars, ~ key factor in the vol~ile history. In summary' ~e measurement objectives ~~ the Mars Parted has identified include ~e following: ~ Definitive measurements to ~st for the presence of extant or extinct life, or the geochemica1 arid organic chemical evidence for past biological activity. These measurements will require highly sophi~ic^d equipment procedures, arid sample proportion techniques not currently available' nor likely to be available in ~e foreseeable future' for in situ experimmts. Consequently' samples relend from well-documen~ sins of promising biological po~ntia1 mud ~ returned to Earth for detailed study. ~ Detailed charac~riz~ion of the geochemistry' mineralogy' trace elements' md chronology of samples selected from well-documen~d locations md returned to Earth to address questions relevar~t to the absolute chronology' climax arid wear history' igneous arid metamorphic evolution, arid weathering history of Mars. ~ ~~rmination of ~e sources, sinks, arid reservoirs of volatiles through inhered measurement of ~e composition of ~e atmosphere (including humidity)' isotopes of atmospheric gases, md volatile coning of arid processes in the subsurface' made over ~ least ~ martiar~ year, using long-lived gas Myers. Concurrent measurement of the composition of He middle md upper abnosphere is required to provide ~ systematic under- s~ding. ~ Determination of the sin of Marcus core, its current internal activity, arid id large-se~e plar~et~ structure using passive seismometry ~ ~ minimum of four sites' operating for ~ lent ~ maim year. ~ Determination of the absolute chronology of Mars. Required are the measurement of ages of erys~lline rocks from surfaces on ~ least four strategically chosen geologic units displaying conspicuously different crater densities. This measurement objective em be achieved Trough sample return if appropriate surfaces are sampled' mdior Trough in situ age de~rmin~ions made by landers if Be technology em be demoed to achieve sufficient precision md accuracy. ~ Measurement from orbit of the dummies of the middle md upper Exosphere of Mars md the rate of atmospheric escape. ~ Measurement of Me current neutral gas md ion escape fluxes; bow optical remote-sensing md in situ instruments carried on ~ orbiter are required to achieve these objectives. SiLGGESTED MISSIONS Mars Idle Return The Mars Panel attaches the greatest importance to Mars Sample lecture (MS1~' unquestionably ~ high-cost mission. While MS1l e~of replace vermin crucial in situ measurements (em., hem flow' seismicity, eleetro- magnetie sounding for whorl analyses of labile samples' md determination of atmospheric dynamics)' it is seientif~eally compelling in id own right, md the ground-tru~ acquired from returned samples will aid Me i~erpre~tion md greatly enhance the value of dam from orbital md robotic lander missions. Spaceport capabilities that would eon~ibute to effectiveness in sampling include mobility in situ reco~aissmee mal~iea1 instrumenta- tion' md ~ e ore drilling device. Cinder current conditions' it appears likely that living organisms' md more generally all organic material' would be destroyed by oxidizing conditions in Me surface layer of hears. They may be preserved only ~ depth in Me planet. Just what dep~entimeters, meters kilometers is urn own.) Necessary capabilities include the ability to mmipula~ md document samples eollee~d md to package them in ~ way eonsis~nt win requirement placed by Me plme~ry protection protocol imposed on the mission. A radio- isotope power system for He mission (see below) would expand the geographic range of sites that could be sampled md would extend He missions stay time' allowing He collection of ~ larger md more carefully selected suite of samples. Ample power undoubtedly will be important if drilling is eontempla~d. It is essential ~~ the site to be sampled be carefully chosen, win He choice drawing upon He large body of orbital md lander dam ~~ will be in place by He time He MS1l is flown. However, no single sample-return mission will completely satisfy the need for this form of exploration, no mater how carefully it is plied. Mars
AS is highly varied in its geology; prior to returning some martiar~ myriad to Earn it may be impossible for us to underfed which type of sin has the higher po~ntia1 for providing samples ~~ contain evidence of life arid other valuable scientific day; sample collection arid return represent ~ new endeavor, one thy may nof work perfectly the first time. It will ~ necessary to plan for ~ series of MSRs over whatever spars of time the budget permits. Mars Long-Lived Lander Network The Mars Pme1 also recommends the emplacement of ~ network of long-lived surface stations on thy ply ~ modera~<ost mission. The primp purpose of them stations should ~ to address two questions ~~ the pme1 believes are neglected by the Mars Exploration Program as currently constituted: (~) ~e infamy structure arid activity of the pit arid (2) the composition md activily of id ahnosphere. Such ~ mission' or series of missions' has not been designed by NASA' but ~e French space agency Venue Nations d'Etudes Sp~iales (CNES), in cooperation with in~m~iona1 partners' is planning ~ four-ion network science mission with goals thy are compatible win the parcels recommendations. Radioisotope power systems will ~ required to achieve ~e needed lifetimes arid global distribution of ~e stations. The Mars Long-Lived Lander Network - 3N) would use passive seismometers to explore the structure arid activity of Mars. Heat-flow probes also would contribute importar~tly to our knowledge ofthe martim interior' but these require the drilling of holes' arid they might more logically be emplaced by MSR if ~~ mission has drilling eapabilily; this would avoid placing ~ drilling requirement on He lander network. ML3N should also include me~orologiea1 serious ~~ measure pressure' temperature, relative humidify' atmospheric opacity' md wind velocity. Also included should be mass spectrometers ~~ permit high-precision' long-lived chemical md isotopic atmospheric analysis of the chemical dummies of C, H. md O ~ Marsh surface. Time variability of isotopic compositions em be in~rpre~d in terms of sources' sinks, md reservoirs of volatiles' md Ionospheric evo lution. Humidity sensors would track He flux of wear vapor into md out of the regolith win time of day md second providing impor~t insight into He water budget on Mars. The complement of instruments on the Freneh-led NetL~der mission' He four landers dis~ibu~d around He plmet md He expected lifetime of ~ martim year will be sufficient to eonshain the nature md size of the core' seismic activity' seismic velocities of the crust md motley md atmospheric properties of pressure' temperature' humidity' md wind speed. They will also have ~ magnetometer md electromagnetic sounding capabilities to sense crush structures md to search for subsurface water md ice. While this complement of instruments does not address all of He high-priority gods outlined for the ML3N, it represents ~ significant step forward. Mars lapper Atmosphere Orbiter The need for ~ orbits mission to study He upper atmosphere of hears is identified above (see the section <<Key Measurement Objectives,'). Areas to be addressed by this low-cost mission are the dummies of the upper abnosphere; hot atom abundances md escape fluxes; ion escape; minimagnetospheres md magnetic recormeetions; md energeties of He ionosphere. A hears Upper Atmosphere Orbiter (MAO) em explicitly explore these issues in the present-day environment md answer ~ number of impor~t scientific questions. Furthermore, such ~ mission could quantify present-day escape processes md allow certain backward extrapolations to earlier epochs in martim history. The instrument needed for ~ meaningful attack on these questions would require no new' basic instrument development md could be installed as ~ partial payload complement of ~ orbiting spacecraft. The neutral winds em be measured by either ~ <<baffled,, neutral mass spectrometer or ~ F~ry-Perot interferometer. The later instrument' along with ~ good ultraviolet spee~ome~r' could address in ~ meaningful way the hot atom md neutral escape flux questions. The neutral mass speebome~r would also provide neuron composition md temperature information. A plume instrument complement consisting of ~ magnetometer' low-energy ion mass spectrometer (capable of measuring flow velocities md temperatures), ~ electron speedometers ~ plasma wave detector' md ~ L~gmuir probe would go ~ long way toward resolving He questions of ion escape' minimagnetospheres md magnetic reco~eetions, md energeties of He ionosphere.
S4 HEW FR0~ IN =E 50~R HIM Mars Science Laboratory The Mars Exploration Program (h~P) projects development of ~ Mars Science Laboratory (MSL)' presum- ably ~ modera~-cost mission' for launch in 2009. Its instrument payload has teem stand only in ~e most general terms. The mission may ~ imp ornery indeed essential' as ~ ~chnology-demons~tion precursor mission to MSR. Mars Scout l\Ii~ions The Mars Scout program tonsils of competed, Dimovery-class' principal-investig~or-led missions win $300 million cost caps. The program was instituted by NASA to meet science goals arid opportunities not covered by other missions arid to provide ~ mechar~ism for ~e hop to ~ responsive to discoveries. As structured' ~e Scout program provides art excellent opportunity for NASA to accommodate science topics outside ~e principal objectives of ~e h~P, arid for the broad science community to respond to discoveries md ~chnologica1 advar~- meet. The Mars Parted strongly endorses NASA's desire to structure ~e Scout program after the successful Discovery program. In thy regard, it is essential thy the measurement gods for the Mars Scout program be direc~d toward ~e highest-priority science for Mars md be selected by peer review. As witnessed by the respond to ~e recent call for Scout propose ideas (more thm 40 submissions were received), tremendous enthusiasm has been stimulated by recent Mars discoveries for addressing scientific investigations not covered by ~e hop. Scout provides for He MAP ~ component ~~ is highly flexible md responsive to discovery, md the panel recommends that Scout missions be flown ~ every other Mars launch opportunity. Some of He mission priorities defined in this chapter (e.g.' the ML3N md MAO missions) could be aceommod~ed in He Se out program as ~md-alone missions or as target of opportunity on incarnations missions. The science priorities outlined in this chapter do nof encompass the full range of science topics of grew importune to Mars ~~ may fit within the Scout funding md mission profile. These are covered more completely in the - C report Assessmmt of Mars Sc~erzoe ~d Meow pr~or~42 as well as in the recent report of He Mars Exploration Payload Assessment Group (MEPAC).43 IMPACT OF SAMPLE llETl}RN ON THE MARS EXPLORATION PllOGllAM One of He major problems facing He MEP is choices. The abundance of new dam across all disciplines has led to extraordinary discoveries about Mars that are being reported in rapid succession, md with He plied program of NASA md intern~iona1 missions, this is likely to continue (see Table 3.~. The compelling nature of the planet md this vigorous exploration program has spawned ~ deep md broad scientific eommunily whose ingress md compelling questions span mmy orders of magnitude in space md time. Yet despite the apparent richness of this exploration program, the resources for NASA's REP are nevertheless finis. The scientific community md NASA are therefore faced with He critical question of prioritization. Central to this debug is the question of sample return, on which there are two points of view. The firm view is thy the costs of sample return will be high in terms of the spaceport resources md ir~frashueture needed to handle' house, md analyze the samples. This investment will undoubtedly defer in situ md orbital investigations of Mars during this effort. This view further advoea~s that because of this cost' sample return should be delayed until such time as the science questions to be addressed by sample return are so compelling md He technology so mature that success is assured. As the program moves forward then' the MEP resources should be directed toward continued in situ md orbital investigations. For example, He current best estimates of the cost of sample return rope between $~.S billion md $~.S billion' which would require NASA to combine the resources from two launch opportunities to fit within the MEP cost profile. It could be argued that for these same resources' four landed mienee packages win rovers could be sent to some of the mmy interesting places on Hurst to conduct in situ surface science md life-detection experiments md to establish well-ins~umen~d stations for interior' climate, md meteorology studies. This view that sample return should be delayed is motioned in part by ~ fear ~~ if sample return is approached too quickly, Hen all Mars Waldo kr~owr~ :~s the he ars Smart Leer or the Mobile Science Laboratory.
S5 science will be arrested to achieve this goal' md if ~e first samples are indistinguishable from SNG memories' further support for Mars exploration will be jeopardized. The contrary view is ~~ the mo ~ compelling question for Mars exploration' md one the is center to ~e SSE Survey, is Are we alone9, arid ~~ only Trough the analysis of samples returned to Earn cart this question be addressed to arty 1~1 of certainty. This view also holds thy the breads of Mars science to ~ addressed by ~e upcoming missions (see Table 3.1~ is enormous arid will do much to provide ~e esmnti~ context to address this question. However, the next leap in under~ar~ding Mars will only be achieved though the Physic of samples from the surface underwood in ~ perry context. This view also holds ~~ ~e first sample return will neither address all questions nor close the book on the life question. However' it will be critical for making the maximum use of the huge investment in dam sew made over the preceding decade (such as shown by the lunar example). Sub- sequent sample-return missions' interleaved win appropriate orbits md in situ exploration, will ultimately drive exploration to the sins thy will maximize our under~ar~ding of Mars arid answer the question Are we cloned This view is motivated in part by ~e sense ~~ sufficient information exists today to move toward the goal of sample return arid thy ~e ~chnologica1 challenges are sufficiently large thy ~e program needs to main now in order to achieve ~ launch early in the next decade (2013-2020), arid by ~ fear thy without ~ clear commitment to sample return He hap will never achieve this goal arid will lose support. The choice of which paw to take is not necessarily ~ either-or proposition. The true Gosh of sample return are not yet known md will be refined over the next few years. Even with ~ high cost' Here will be abundant other opportunities for Mars exploration. For example' following He flight of Mars Science Laboratory in 2009' He next opportunities to fly to Mars are in 201~ md 2013. If He costs of ~ simple sample-return mission come in ~ the low end of the cost estimates ($~.S billion) md it is flown in the 2013 opportunity, then' amording to recent reports of the hap budget to NEPAL, there should be sufficient resources to fly ~ competed Scout mission for He 2011 opportunity. If the Gosh for sample return are too high to bear for the 2013 opportunity this could be delayed till the 2016 opportunity, md MS1l together with commend Se out missions in 2011 md 2013 would easily fit within the current budget climax. 1lECOMMEN~ATIONS OF THE MARS PANEL TO THE STEERING GllOl}P Mission Priorities Mars Sample Deem The Mars Panel Hushes the higher privily to missions ~~ will collect samples on Mars md return them to Earth' bemiring ~ He 2011 opportunity if this is possible. Observations made by robotic orbiters md landers beyond 2005 e~of alone answer the most impor~t questions regarding Mars: whether life ever started on that plmet what the climate history of He planet was' md why hears evolved so differently from Earn. The definitive answers to these questions will come from the study of hears samples' in the eons of orbital md surface in situ measurement' of known provenance in laboratories on Earn. ~e Need for Sample Return ~e Search for Life. At our present sate of knowledge md ~ehnologiea1 expertise' md probably for the next sever al decades' it is unlikely ~~ robotic in situ exploration will prove capable of demonsh~ing to ~ acceptable level of eer~inly whether there once was or is now life on hears. llesul~ obtained from my life-detection experiment carried out by robotic mems are likely to be ambiguous for these reasons: ~ llesul~ interpreted as showing ~ absence of life will be challenged because He experiment that yielded them were too geocentric or otherwise inappropriately limited; ~ llesul~ eonsis~nt with but not definitive of, the exis~nee of life (em., He deletion of organic compounds of urn own, either biological or nonbiologie~, origin) will be regarded as incapable of providing ~ clear-cut answer; md
AS HEW FR0~ IN =E 50~R HIM Result in~rpre~d as showing ~e existence of life will be regarded as necessarily suspect since Hey might reflect the presence of earthly contaminants rawer Bars of art indigenous martiar~ biota. Similarly frustrating result cart ~ expected in attempt to march robotically for either of the two categories of fossil life thy might ~ preserved on hears: shomatoli~s arid microfossils. S~omatoli~s are accretions org~osedimen~ry structures' commonly thinly layered' produced on Earth by the activities of mat-building communities of mucilage-secreting microorgar~isms. Unfortunately, true s~omatoli~s on Earth cart be confused with nonbiologically deposited look-alikes (~.g., in thin, sometimes wavy layers of mineral precipitates commonly found in caves arid hot spring deposits on Earn; on hIars, such deposits may have ~m laid down, for ex ample' by repeated wading md drying or framing arid thawing of mineral-charged salt Paris or shallow lagoons). If shomatoli~-like structures were photographed on ~e surface of hIars, it seems vermin thy there would be widespread uncertainty as to whether the objects demand were in fact produced by life. Similarly, it seems unlikely thy robotic detection of objects resembling microfossils in or on the surfaces of rocks on hIars would prove sufficiently convincing to demons to art acceptable 1~1 of certainty thy pad life existed on thy ply. Me Ate ed for sample Ret'~r,~ C;eochem,~ry. In ~e area of geochemistry arid mineralogy, Win sections of returned samples cart be prepared in ~rres~ia1 laboratories arid studied by microbeam techniques as well as optically. Rocks contain ~ near-ir~'ni~ amount of information on ~ microscopic scale' some of it crucial to art understanding of ~e rocks origin md history. flocks cm ~ disaggrega~d' md their constituent minerals cm be studied chemically md isotopically. The dam obtained provide strong clues about md constraint on the nature of the differentiation events that led to He formation of the rock. They also make possible ~ variety of approaches to precisely dying igneous rocks in the sample collection. ~form~ion about the hIars climax will be found in the layer of weathering products ~~ are expected to be found on rock samples. These products will almost certainly be very complex minerals or amorphous reaction products that will tax the bed Ear~-based laboratory techniques to understand. It is very unlikely ~~ fling but ~ highly qualitative md ambiguous description of the weltering products could be made by robotic instruments operating on He martim surface. Me kneed for Sample Return CI'm~e ~d Coupled Atmosphere-Surface-~tenor Processes. Some surface- atmosphere md climax processes involving labile element or compounds must be studied in situ. Nevertheless' the key measurements for understanding the relative loss of portions of the atmosphere to space md to surface reservoirs are the compositions of surface minerals md Heir isotopic systemizes. Atmospherie-loss processes (em.' hydrodynamic escape, sputtering) leave eharwleristie isotopic signatures in vermin element. Loss to space versus to surface weltering (e.g., CO2 to carbonate minerals) is likely to produce isotopic fractionation in different directions. The ratio of i5N to i4N in He maim atmosphere is underwood to have evolved over He past 3.8 billion years (it is currently I.6 times the Crest value), md ~ de~rmin~ion of this ratio in near-surface materials may constrain the time of Heir formation. Compositional md isotopic analysis of surface minerals' weathering rinds' md sedimentary deposits will establish He role of liquid wear md processes such as weathering The corresponding measurement on volatiles released from near-surface materials are likely to be more heteroge- neous md may provide fossils of past atmospheric md chemical conditions ~~ allow the past climate to be better understood. MA Meteorites No' ~ Substitute for Sample Return. The SNG meteorites do not obviate the need for sample-return missions. SNG meteorites have provided ~ tantalizing view of ~ few maim rocks md ~ demonstra- tion of how much em be learned when samples em be examined in Earth-based laboratories; however' Hey represent ~ highly selected subset of martim materials, speeifie~ly' very coherent rocks of largely igneous origin from ~ small number of urn own locations. Thus SNG memories are unhelpful in answering one of our outstanding questions What is the absolute chronology of Hearst because although Hey em be accurately dated He geologic units from which they are derived are up own. While returned samples are also ~ selected subset of martim materials, Heir geolog ie context will be known' md they will be from sins selected because Hey em provide particularly valuable information.
S7 Regarding ~e climax history of Mars arid possible life there' the samples the will provide ~e most informa- tion are not igneous rocks' as the SNG memories are, but sediment arid soil samples. Taking Yosemite Valley as ~ ~rrestria1 ar~alog' ~e SNG memories represent ~e cliffs rather Bars ~e river muds md ~e sediment from ~e outwash sheathing into Californians Entrap Alley. It is the lair materials the cart provide information about chemical conditions, biological processes, arid timing; their martiar~ analogs, geologic features ~~ have ~e properties of river arid lake deposits, will help most in understanding wear md life on the plmet. Mars log- ~~r Network The Mars Pared considers the ~e h~3N should ~ ~e mooed-priority Mars mission. The principal experi- mend on Case landed stations should be passive seismometers arid maly~rs of ~e ground-leve1 atmosphere, bow of which must continue to record dam for ~ least ~ year to achieve their potential. Earlier NASA advisory parcels consistently recognized ~e imported of them experiments md recommended Heir implemen~tion.44 Seismic dam cart determine the sin of the core' which will constrain the bulk composition of ~e ply as will information on the seismic velocities in the marble. Knowing the bulk composition of Mars is importers for understanding the origin of the planets. Seismology cart tell us whiner the core is ~1 solid, all liquid' or part solid arid part liquid (~ is Earths cores' which has ~ direct arid profound bearing on our understanding of plar~etary dynamos arid Me present-day lack of ~ Mars global magnetic field. In the area of martim atmospheric science' Mere are open questions of meteorology' atmospheric origin md evolution, chemical stability, md atmospheric dummies. These questions are of particular interest for ~ broad community of scientist' because useful comparisons with Each em be made ~~ may prove important for understanding Me atmospheric evolution of both planets. The Mars Panel mashes high priority to ~ better understanding of the martim atmospheric composition' chemist' eireul~ion' md condensation of near-surf~e wear vapor as Me key components of climate systems md for comparative studies of atmospheric dummies md evolution. Mars tapper Atmosphere 0~r The third privily of The panel is given to the Mars Upper Atmosphere Orbiter mission. The upper atmosphere of hears drives the lower atmosphere in ~ variety of ways, md very little information is available on the martim upper atmo sphere. There are no existing plans in The current U.S. hears Exploration Program to address my of the seientif~e questions that are listed above concerning the upper atmosphere of hears (see the subsection <`Mars lower Atmosohere Orbiter''i. I:~n~s Naomi ~d Neurons MPars Express will :~Mress these Questions to some 1 1 extent but much more dam will be needed to mem~giully clued these open Issues. Cow the Nozom' md Mars Express will arrive ~ Mars during solar cycle minimum conditions' md dam from solar cycle maximum are required in order to answer some of The outriding questions (e.g., nontherma1 escape). l~nprionti~d Missions Mars Sc~e,~ Ikoratory The M8L mission may be important' indeed essential, as ~ teehnology-demonsh~ion precursor mission to MS1~' but the panel saw little science for M8L thy ergot be done as well or better by the missions discussed above. The deviled examination md analysis of rock samples em be done far more capably in terres~ia1 laboratories (though admittedly MSL could perform simpler analyses of ~ larger md more dispersed set of samples than those that ~ MS1l mission could return). The ML3N mission could conduct much more comprehensive atmospheric md seismic studies thm could M8L' which is ~ single mission' not ~ network. K-Ar ages remotely measured by MSL, if this technique em be made to work' will provide only one dam point toward ealibr~ing the martim geological column' with accuracy inferior to that obtained on MS1l samples in terrestrial laboratories.
as HEW FR0~ IN =E 50~R HIM Since the parcels task was to prioritize science missions arid since it sees M8L largely as ~ ~chnology- demonshation mission, it has not included MSL among the prioritized missions. Mars Scout The program of Mars Scout missions provides ~ excellent opportunity for NASA to accommodate science topics outside ~e principal objectives of the Mars Exploration Program arid for the broad science community to respond to discoveries arid ~chnologica1 advar~cement. If this activity is to be modeled after ~e successful Discovery program' it is essential thy ~e science goals for Mars Scout missions ~ direc~d toward the highest- priori~ science for Mars relend by peer review. There is concern in ~e Mars science community thy Scout missions may be vulnerable to Ming sacrificed in times of budget stringency. The parted urges thy ~e Mars Scout program ~ maintained win ~ high 1~1 of protection. Tedhnology Development Sample return will not be ~ simple task, md it has not been achieved by ~ robotic mission over chart ~e Russim sample return from the Moon 30 years ago. For the much more difficult sample return from Mars, marry technologies will have to be developed' tested, md validated. Them include heard avoidance in ladings sample selection' handling md delivery to the transfer chamber' Me Mars Ascent Vehicle' orbit rendezvous md capture' transfer to Earth md quarantine on Earth. It will be ess~tia1 for precursor missions to MS1l to incorporate Me teeing of essential technologies. Sample return md ~ long-lived surface network will require sophisticated ins~umm~tion for science md operations. While much Nought has been given to what sort of instruments might be required, Mere has been less direct investment in Me development of instrument md demonstration of Me technology required for flight- qualified systems. An extremely important consideration in establishing Me capabilities of landed packages on Mars' static or roving, is the power supply on which Hey rely Me options being solar panels md radioisotope power systems (l?P8s). The Viking landers lasted as long as 7 years because Hey had UPS power. The twin ME1l 2003 rovers' with solar panels, will operas for no longer than ~ estimated 90 days. This is because as He election of He Sun echoes, He available solar power decreases; for the same reason, He rovers get colder md need more power to keep warm. Meanwhile' dust is accumulating on the panels' furler reducing the power. The hi rovers are also restricted by He needs of Heir solar panels to led in He 100 N to IS~ ~ latitude belt ~ relatively low elections. The ML3N described above will not be able to operate within these constraints; m UPS will be essential. The power problem will seriously affect sample-return missions as well. llelimee on solar power would mem that samples will almost eerily have to be collected ~ low latitudes, which excludes those pans of Mars where ground fee is stable md where over volatiles are mod likely to be present. If He sample-return mission has ~ rover to collect samples' its lifetime will be short. The use of ~ drill to collect samples would require ~ generous supply of power. Data Any Ground-R=ed Oh~ervabons' and Lahoratory Studies The Mars Exploration Program, with its missions ~ 2-year intervals' presents ~ new problem in fully exploit- ing He amount md variety of dam ~~ will be collected. The volume md quality of dam returned by MOS alone have been extraordinary, md He analysis of these day is only begir~E~ing. Win the rapid pace of Mars missions plied for the next decade' the flood of dam em be expected to increase. While the Mars Exploration Program consists of flight missions, exploration md understanding of the planet as ~ system also depend on over modes of day acquisition. Some examples follow.
Te~cop`c studies By Continuing telescopic observation of Mars has played ~ key role in demonsh~ing ~~ the surfed of Mars charges on ~ relatively short time scale (~ with seasonal charges, dust storms, evolution of the polar caps.) Telescopic md spacecraft dam are highly synergistic, arid each plays ~ role in supporting the other. Support for future robotic arid possible m~ed missions to Mars will require ~ long clim~ologica1 baseline. The long baseline' partially obtained with ground-~md arid HST telescopic date will also con~ibu~ to art understanding of the wear cycles between the atmosphere, regolith' arid polar caps, as well as spatially resolved dam on Jollily cycles of whorl carbon dioxide' carbon monoxide, arid ozone. oret`~l Bomb Models are art essential component of arty scimlif`c endeavor. Examples of theoretical perry studies are those ~~ crew ~e geod~amics of Mars, its interior structure' ahnospheric loss arid fractionation, arid global climax arid general eireul~ion models. Maroon ME As already mentioned, the SNG category of martim meteorites plays ~ imports role in studies relating to martim life md He plmet~s structure md evolution. Studies of this small group of meteorites in terres~ia1 laboratories have provided invaluable' if fragmentary, information about He geochemist md chronology of Mars. NASA' the National Science Foundation' md He Smithsonian Institution have jointly supported ~ Antarctic meteorite program since 1976, in which teams of expert search areas known to contain ~ concentration of meteorites for ~ weeks every ausha1 summer; support of this program should continue. Astrob~olog`~l ~~h Studies of deep-sea hydrothermal environments hot springs, He deep subsurface' alkaline or acidic environ- men~' md sea fee have revealed amazing microbial diversity in the form of uncultured organisms from environ- men~1 extremes. Some of these habitats are po~tia1 malogues to past md present martim environment where life may have arisen or might continue to exist. Through expanded knowledge Tout the po~ntia1 diversity of He microbic world, we em explore how ancient microbial life might have impacted planets processes on Mars. P=parabons on Earlb for Sample Return A series of NASA md N1~C panels have considered the special problems assoeia~d with bringing samples from Mars to Earth,45-49 md NASA has acknowledged He need to prevent forward md back eon~mination every stage of the process of delivery. This includes He need to eons ~ quarantine facility to receive md contain He samples. A recent N1~C report drew at~tion to the long lead time required to prepare ~ hears Quarantine Facilily (M0F) for the reception of Mars samples once they are delivered to Earth.~° Cal He basis of prior experience win terrestrial biocontainment facilities md the Apollo Lunar lleeeiving Laboratory, He authoring eommi~ee estimated that 7 years would be required to design, eons~uet, md staff the M0F. To this must be added the time needed to clear ~ environmental impact statement md to carry out several N1~C recommendations for recormaissmee studies that are needed to inform the design md operation of He M0F.5 ~ The aggregate of time required will strain the schedule even of ~ 2011 launch (2014 return). It is impor~t ~~ scientific research md design studies that mud precede the design md eons~uetion of ~ Mars Quarantine Facility begin immediately, md design md eonshuetion of the facilily should begin ~ He earliest possible time.
Looo! To HEW FR0~ IN =E SOLAR MOM REFERENCES 1. See' for example, B.h4. Jakosky md R.h4. H~erle' tithe Seasonal Behavior of Wear on h4ars>~' ire H.H. Kieffer' B.h4. J:~kosky' C.W. Sr~y~r' Ad h1.S. Mathews (eds.~' Aura Ur~iversi~ of Arizona Press' Tucson 1992' pp. ?~-1016. 2. h4.H. Carr' Wear o~ Mars' Oxford University Press' New York' 1996. 3. K. Fiema~' J. Oro' P. Toulmir~ III' L.E. Oral, A.O. Nier, D.h4. Ar~rsor~> P.G. Simmor~ds' D. Flory' A.~. Bias D.R. Rushr~ck' J.E. Biller' Ad A.L. Lafleur' IlThe Search for Organic Suits Ad Ir~org~ic Vol~ile Compounds in the Surfam of hears>>' Journal of Sped Rewarm S: 4641-~> 1977 . 4. H.P. Klein' lathe Viking h~issior~ Ad the Search for Life ore h]ars>~' Rw~ of Opt aerospace Pit 17: ~ 655-1662> ~ 979. S. G.~. Levis Ad P.A. Strand 1lVikir~g Ladled Release ~ inflow Experimerd: Interim Results>~' Scow ~ Hi: ~ 322-1329' ~ ?76. 6. H.P. Klein' tithe Viking h~issior~ Ad the Search for Life ore h]ars>~> Rwtew~ of Is aerospace Ph~ 17: 1655-1662> 1?7?. 7. See' for example' G.V . Levitt Ad P .A. Skate Batik ing Ladled Rele a~ ~ igloo Experimer~: Interim Results>~' She ~ Id: ~ 322- 1329) 1976. S. J.L. Goodir~g' spoil hlir~eralo~ Ad Chemistry ore hears: Possible Clues from Salts Ad alms ire SNG h4eteori~> Icarus PP: 2841' . ?. hl.H. Carr' Wear on Mars' Oxford University Press' New York' 1996. ~ O. T .O. S~ver~s arid J.P . hloKinley' Blithe autotrophic h] icrobial Eco~ms ire Map ~ asalt Aqulfers>~> $~ce ~0: 450-~> ~ ?95 . 11. See' for example' S. Kos~lr~ikova arid K. Pe~rsor~' Recolor of h4ethar~o~r~ic Archea ire Gr~itic Grour~dw~er from Hard Rook L~oratory~ Swarm Croci of ~e Acre ~~ermt~o~ Spot of i M~cro~o~ogy~ September ~S-~1~ ~?~~ Devon Swit~rl~d, Swiss Soviet of hlicrobiolo~> Zurich' 1996. 1 2. D.S. h5cK~' E.K. Gibson Jr.' K. L. Thomas-Keprta' H. Vali' C.S . Rom~ck, S.J. Clemeh' X.~ .F. Chillier' C.R. h5~chliny' Ad R. hi. Zare' research for Pan Life on hears: Possible Relic Bio~r~ic A~ivity ire hi Meteoric ALH84001~)$~e ~3: 9~-930' 1296. 13. A. Treimar~' 11A Short' Critical Evaluation of Proposed Sims of Ancient hi Life ire Artar~ic Meteoric ALH84001~' ire G.A. Lemarch~d Ad K.J. haunch (eds.~> B`o=tro~o~ '99A New Art ~~ B~o~tro~o¢> ASP Cor~ferer~m Series' Vol. 213> Astronomical Soviet of the Pacific' Say Fr~cisco' 2000'pp. 303-314. 14. D.E. Smith, hl.T. Zuber, S.~. Solomon R.J. Phillips' J.W. Head, J.~. Garvir~' W.~. Bakery' D.O. hluhlem~' G.H. Pet~r~gill' G.A. Neumann> F.G. Lemoine' J.~. Ab~hire' O. Aharor~sor~> C.~. Brown S.A. Hauck' A.~. Ivar~ov' P.J. hioGovem' H.J. Zwally' Ad T.~. Duxtury' tithe Global Topography of Mars Ad Implicatior~s for Surface Evolutior~'Sc`~e 284: 1495-1503' ~ Aid. 15. hl.H. Carr, Water on Mars' Oxford University Press' New York' 1996. 16. h4.H. Carr' Wear on Mare> Oxford University Press' New York' 1996. 17. D.A. Paine tithe ThermalSt~ili~ofNear~urfam Grour~dIce ore hlars>~tare356:4345' 1992. 18. hl.H. Carr, Water on Mars' Oxford University Press' New York' 1996. ~ I. D.E. Smith' hi.T . Zuber' S A. Solomon R.J. Phillips' J.W. Head' J.E . Garvir~> W.E . ~ emery> D.O . hiuhlem~> G.H . Pet~r~gill' G.A. Neumann' F.G. Lemoine' J.~. Ab~hire' O. Aharor~sor~' C.~. Brokers S.A. Hauck' A.~. Ivar~ov' P.J. hloGovem' H.J. Zwally' Ad T.~. Dux~ury' tithe Global Topography of Mars Ad Implications for Surface Evolutior~'Sc`~e 284: 1495-1503' ~ add. 20. h4.~. h4alir~ arid K.S. Emmett' 1lSedime~ary Rooks of Early Be 290: 19~-~937> 2000. 21. hl.~. hlalir~ Ad K.S. Emmett' 1lEvi~r~e for Bars Grour~dwat.er Seepage Ad Surfam Turnoff ore hlars>~>Sc`~e 288: 2330-2335' 9.~.. G.h1. T<~nti~. S.W. Tar. TR.W. Derek. T;.TT. TV. G.T. Cn~.ro. S.hT. hTo11. Tip. Porker. T.T. Se.he11~her~. TOW. Shnook. TR.T.. a, ~ . . . . _ ,~ ~ . . . . _ ~ . . ~ . . ~ . .. . ~ . . ~ . . a, ~ Wilkersor~' IT R Murphy' IT L Hollir~gsworth' R.h4. H~erle' h4. ~Toshi' IT ~ Pearl' B.~. Cor~r~h' h4.~. Smith' R.T. Clamp' R.~. Pl~chard' R.G. Wilmoth' D.F. Rault' T.~. h4artin' D.T. Lyon P.B . Esposito' hi A. ~Tohr~or~> C.W. Whet~l' C.G. Tut Ad IT he Bahicke' tithe Structure 0 f the Upper Atmosphere of hiars>~> Sc~e Ad: ~ 672- ~ 676> ~ ?~S . 23 IT Imbrie' 11A~ronomica1 Theory of the Plei~omr~e Im Any: A Prief Historical Review>~'Icar~ 50: 408-~' 1932. At. W.R. Ward' 1lLor~g-Term Orbital md Spire Dynamics of hears>>' ire H.H. Kieffer' B.h4. ~Takosky' C.W. Sr~y~r' Ad h4.S. Matthews ds.~> Marty University of Arizona Press' Tumor 1992> pp. 298-320. 25. hl.H. Carr' Wear on Mars' Oxford Urliversity Press' Stew York' 1996. 26. B.h5. ~Takosky Ad R.`T. Phillips' Poplars' Vol~ile Ad Climate History>~'Na~re 412: 237-~44' 2001. A. W.h4. FolLr~er' C.F. Yo~r' ~ hi Yules> Em. Standish' arid R.A. Presort> 1lIr~t.erior Structure Ad Seasonal his Rediskibutior~ of hears from Radio Tracking of Mars Pathfin~r>~>Sc~e Ad: 1749-1752' 1997. 28. hi .H. Acura> IT E P Co~err~ey' hi F Mess' R.P. Lily D. Mitchell' C.W. Carlsor~' IT hi oFad~r~> K.A . Ar~rsor~> H . Reme, C . hi Gaulle' D. Views> P. Wasilewski' Ad P. Cloutier' 1lGloha1 Distribution of Frugal hia~eti~tior~ Discovered by the hears Global Surveyor hiA~ER Experime raid ~ Sale ~~e 284: 7 ~ O -7 ~ 3 ~ ~ ~ ~ ~ . 29. hl.T. Zuher' S.~. Solomorl' R.`T. Phillips' D.E. Smith' G.L. Tyler' O. Aharorlsorl' G. Balmirlo, W.B . Barler~> IT W Head' C.L. ~Tohrlsorl' F.G. Lemoir~> P IT h<oGoverr~> G.A. rheumy> D.~. Rowl~ds' Ad S.`T. Borg 11~tema1 Structure Ad Early Thermal Evolution of hears from Mars Global Surveyor Topography Ad Gravity>~>Sc~e 287: 1783-1793' 2000. 30. R.`T. Phillips, hl.T. ZuLer, S.~. Solomon hl.P. Golombek' B.h4. ~Takosky' W.~. Bakery> D.E. Smith' R.hl.E. Williams' B.h4. H~ek' O. Aharorlsorl' Ad S.A. Hauck' 11Ancierlt. ~ od~amics Ad Glohal~ale Hydrology or1 h] arson Sc~e 291: 2587-~> 200 ~ . 31. R. Rie~r' T. Ecor~omou' H. W~ke' A. Turkevich' IT Crisp' T Brilokr~er' G. Dreibus' Ad H.Y. hl~ween~Tr.' tithe Chemical Compo- sitior~ of hi Soil Ad Rooks Retumed by the mobile Alpha Proton X-r~ Spectrometer: Preliminary Adults from the X-r~ ho Sc~e Ad: 177 ~ -1774> ~ ?97 .
32. J.L. Backfield' V.E. Hamiltor~' Ad P.R. Chri~er~' <<A Glob al View of h4~im Surfed Compositions from hiGS-TES>~' Scow 287: ~ 626- ~ 630' 2000. 33. W.K. H:~rtm~=dG. Neukum' 14Cra~rir~gGhror~olo~dthe Evolution of hlars>~eScte~ew~6 (~ 165-~> 2001. 34. T.~. Swirl <~Could ~ Situ Desire Work or h4~rs93> 3~d Aquas Lurker Planetary Skiers Cor~feren~~ Air: No. 1492 (~- ROh1~' Lurker Ad Planetary Ir~itute' Hour Tex .' 200 ~ . 35. h4.~. heir d K.S. Ed~tt' <<Sedimentary Rooks of Early hiars>~>Scte~ce 290 19~-1937> 2000. 36. T.J. Parker, D.S. Gorcir~> R.S. Saur~ders' Dog. Pieri' arid D.h4. S~er~r' Coastal Geomorpholo~ of the Martin Northem Plains>~' JourmI of mop Rewarm As: ~ ~ 06 ~ -1 ~ 078' ~ ?93. 37. N.A. Cabrol Ad E.A. Grir~' 1lDi~ributior~> Classific~ior~> Ad Ads of Martin Impact Crater Lakes>~> Awry 142 160-172> 1999. 38. Spam Studies Poard' National Research Council, AD of Mars $~e a~ M=`o~ Prior National Academies Press' Washir~or~' D.~.' 2003. 39. See' for example' Up: Studies Board' National Re~:~rch Councils An of Mare Scte~e a~ Mt~`o~ Prtorttt~> National Academies Press' Washir~or~' D.~.> 2003. 40. See' for ~ xample' hears Explor~ior~ Payload Assessmerd Group (h] EPAG), 11h4ars Explor~ior~ Program: Scier~ific Goals' Oboe Fives' e ~ig~ior~> Ad Priorities>~> Chamber 2000> ire Scte~e PI for Explort~ Mary JPL 01-7> Jet Propulsion L~or~ory' Par Calif.' 2001. 41. See' for example' Span Studies Board' National Research Cour~cil' AD of Mare She FEZ M~`o~ Prior National Academies Press' Washir~or~> D.~.' 2003. 42. Span Studies Poard' National Research Council, AD of Mare Scow an Manor Prior N:~tior~al Academies Press' Washir~or~' D.~.' 2003. 43. hears Explor~ior~ Payload As~ssme~ Group (h4EPAG] shears Explor~ior~ Program Sciertific Goals' Objectives' ~~estig~ior~' Ad Priorities>~' Demmber 2000> irk Poor Explore Mary JPL 0 ~ -7> Jet Propulsion L~or~ory' Par Calif.> 2001. Art. Span Studies Poard' National Research Council' AD of Mars Scow a~ Manor Priorly National Academies Press' Washir~or~' D.~.' 2003. 45. N~ior~:~l Aeronautics Ad Span Admini~r~iorg Mare Sample Awry Protocol Workshop' NASA/CP-~-208772> Washin~or~> D.~.) 1999. 46. N~ior~:~l Aeronautics Ad Span Admir~i~r~ior~' Mare Program Art Recomme~o~ of ~e NASA A~trob~olo~ I~> Ames Research Cerder' Coffee Field' Calif.> 2000. ~ . hears Sample H~dlir~g Ad Requiremer~s Pan l (UNSHARP) ~ F`~l Report' NASAfTh] -1?~-209145' Jet Propu lotion Laboratory' Pasader~' Calif.' 1999. 48. Span Studies Foard' National Research Councils Mare Sample Tears: I~ a~ Tecomme~ho~ N~ior~:~l Academy Press' Washir~or~' D.~.' 1997. 49. Span Studies ~ card' National Research Councils Awry a~ Cerh4~ho~ of Marha~ Samples, N~ior~:~l Academy Press' Wash- ir~or~> He.> 2002. 50. Span Studies ~ card' National Research Councils Brat a~ C:erEficado~ of Marha~ Sampler' National Academy Press' Wash- ir~or~' D.~.> 2002. ~ ~ . Span Studies ~ card' National Research Councils Awry a~ Cerhbcado~ of Marha~ Samples' N~ior~:~l Academy Press' Wash- ir~or~' D.~.' 20 02.