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Mars Evolution

by: Mr. Bo Greenfelder

Mars Evolution EPS 465

Mr. Bo Greenfelder
GPA 3.67


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This 12 page Class Notes was uploaded by Mr. Bo Greenfelder on Wednesday September 23, 2015. The Class Notes belongs to EPS 465 at University of New Mexico taught by Staff in Fall. Since its upload, it has received 22 views. For similar materials see /class/212184/eps-465-university-of-new-mexico in Earth And Space Sciences at University of New Mexico.


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Date Created: 09/23/15
insight review articles Mars core and magnetism David J Stevenson California Institute of Technology 15072 Pasadena California 91125 USA ermaiI djsgpscaltechedu The detection of strongly magnetized ancient crust on Mars is one of the most surprising outcomes of recent Mars exploration and provides important insight about the history and nature of the martian core The ironrich core probably formed during the hot accretion of Mars 45 billion years ago and subsequently cooled at a rate dictated by the overlying mantle A core dynamo operated much like Earth39s current dynamo but was probably limited in duration to several hundred million years The early demise of the dynamo could have arisen through a change in the cooling rate of the mantle or even a switch in convective style that led to mantle heating Presently Mars probably has a liquid conductive outer core and might have a solid inner core like Earth lthough the existence of the martian core has been accepted for many decades is interesting for several reasons First its size and composition tell us about Mars as a whole 7 its constituents and provenance Second its antiquity tells us about early conditions on Mars we believe that the core formed early and this requires that Mars had a hot beginning Third this core is the likely source of a magnetic field for some part of Mars history probably the earliest part just as Earth s core is the source of the current geomagnetic field Fourth the field may have in uenced the early climate through its in uence on atmospheric escape It could also have affected the environment for early life on Mars Fifth the heat ow from the core may have fed mantle plumes and in uenced volcanic activity much as hot spots such as Hawaii are thought to be fed by core heat ow on Earth Sixth a core if partly or entirely liquid in uences rotational dynamics just as for example changes in length of day are in uenced by Earth s liquid core Mars is built from roughly the same ingredients as Earth silicates and oxides of magnesium and iron as well as metallic iron alloyed with various constituents The mantle and crustal components are discussed in the accompanying article by Zuber pages 2207227 When we refer to a core for the terrestrial planets Mercury Venus Earth and Mars we mean a central region that is rich in metallic iron Because this material is about twice as dense as the silicates and oxides making up the crust and mantle its presence as a core is revealed through its in uence on the mean density of the planet and through its effect on the moment of inertia Old measurements of gravity and more recent geodetic data from Pathfinder1 reveal that the mean moment of inertia for Mars is 0365MRZ where Mis the mass of Mars and Ris its mean radius Together with the martian mean density of 393 g cm g this suggests a model of Mars that is not too different from a scale model of Earth that is similar ingredients distributed similarly The martian core is proportionately a little smaller than Earth39s core and proportionately more iron is found in the mantle in oxides or silicates As we do not know the composition of the core we cannot be certain about core size but a core radius of around 130071500 km depth to core of 190072100 km is indicated Figure 1 shows a simple interior structure of Mars Bertka and Fei2 suggest a come position for Mars that is different from partly devolatilized primitive meteorites 214 2001 Macmillan Magazines Ltd Far less is known about the martian core than Earth s core because we lack seismological evidence or geodetic data of sufficient precision In particular we do not know whether the core is entirely liquid partly liquid like Earth or entirely solid although there are indirect arguments against an entirely solid core Fortunately because Mars is at lower pressures than Earth and so is more accessible to highepressure experiment it is possible to assess the likely phase composition for the core I begin with a discussion of the timing of core formation and the new evidence on the nature and origin of martian magnetism An assessment of the dynamo process follows and is applied to the possible thermal histories of Mars I conclude with other implications of martian magnetism and core structure and some comments on future exploration Core formation It is widely accepted that terrestrial planetary cores owe their existence to a process of gravitational separation of mostly liquid immiscible iron from the partly solid silicates The supporting arguments are partly physicals but increasingly geochemical Although we have no samples of either the core of Earth or the core of Mars we do have rocks that are probably indicative of mantle composition For Mars these are the very limited yet highly important SNC meteorites for shergottites nakhilites and chassignites As on Earth these igneous rocks show a striking depletion of ironrloving elements called siderophiles whose extracr tion testifies to the conditions of core formation4 lsotopic data5 6 also suggest that this corerforming event was early in Mars39 history These data together with physical modelling suggest a scenario similar to the following Mars accumulated from smaller bodies over a period of perhaps as long as 100 million years Myr but possibly much shorter around 4546 billion years Gyr ago Isotopic evidence is come patible with a very short accretion time suggesting that Mars might even have been a runaway isolated embryo rather than a slowly accumulated body like Earth In this accretional process the impacting bodies may have already had iron cores but the energetics of the impact events would have caused extensive melting and mixing of the immiscible metallic iron and silicateoxide components allowing chemical rerequilibration on a small scale centimetres to metres 3 A substantial mass of the impacting bodies may have been in the form of giant impacts bodies of the order of the mass of Earth s moon NATURE VOL 412 t 12 JULY 2001 wwwnaturecom Crust otvartable thickness l 00 km in south 30 km in north Convecttng silicate mantle may be layered FerSESW liquid outer core Possible solid Fe inner core Possible transition discontinuity tn Earth Figure1 Cutawayvlew ofthe martian interior ht review articles nr example following dike injection Most of these cooling events took place at a time when a large global eld was present Mos 39 rather than all39 is appropriate here because the surface magnetizas magnetized through cooling in the presence of other crustal fields rather than a global field Other origins ofthe magnetization for example due to impact as suggested for the Moon are conceivable in principle but seem insufficient given the magnitude of the r quiremenrs There is great interest in the meaning of the spatial pattern of Emmi minquot 39 39 39 39 39 to plate tectonically derived lineations of magnetization on Earth39s crustal magnetization suggest that the martian veundergone reversals 39 Mars requires at least one and preferably several ofthe following high abundance ofappro iate rragnetic materials for example magnetite a particularly favourable magnetic mineralization for example single domains large vols crust that are coherently magnetized andor an unusually large field in which the magnetization was ac uire per unit mass where G is the gravitational constant If all this were converted into heat it would be sufficient to heat the Marss forming material to several thousand degrees above the melting nnim 39 39 39 39 likely This ocean might be transient surviving for a briefperiod any L m A did have a global magnetic field for one or core dynamo the process responsible for Earth39s current eld The strongest magnetizations are observed in the ancient southern highlands of Mars which predate 4 Gyr The antiquity of these e review in this issue by Zuber pages 220227 However not all 39 Vviarsm rlrr 39 39 atmosphere 39 39 ofthecore formation Althou not as hot or at such high pressure as the likely conditions that formed Earth39s core the lower gravity on Mars M u r l viscous and highspressure deep mantle In this scenario the core might initially be either a few hundred degrees hotter than the mantle if the energy of core formation is retained substantially L L rra altitude 39 39 39 39 This prevents firm conclusions being made about the timing of acquisition ofmagnetization and hence the timing ofa postulated martian core dynamo Schubert et a1 have suggested a later post 40 Gyr period of rmcmmi minquot 39 39 39 39 39 ity 39 39 42 Gyr or earlier acquisition First as noted by Acuna et a1 the ancient impact signature and seems to be surrounded by a region with very little L particularly that preserved in the south may have formed in this p andLhicknessarediscussedbyZuber pageszz0227 Martian magnetism Mars unlike Earth has no global dipole magnetic field The Mars Global Surveyor spacecraft confirmed this but also found strong m c am i a 39 39 TH not a plausible outcome if the southern crust were subsequently reheated and then cooled to acquire magnetization during a later epoch in which a global field was active Second it is difficult to ace e u rim an 1 r t r a much of the power in their spatial variability is at length scales comparable to the distance fromthe surface Inversion39 ofthese data nonsunique b L f L f39 H must L r observed magnetization rather than some thin layer of ossibly remagnetized material In particular revived igneous activity of the large amounts required Third evidence from the ancient martian meteorite ALH84001 suggests that its magnetization was A A M outermost several tens of kilometres of the crust and possibly confined to an even thinner layer A deeper layer or source of currents could not provide the observed spatial structure except r netizations are up to 1030 A mquot an order of magnitude higher the strongest magnetizations typically encountered in Earth thinner magnetized layers on 39 39 39 39 Given the large amounts of magnetized crust required it seen very NATURE VOLAlz lZJULYZOOI wwwnsturetorn 9 2001 Macmillan Magazines Ltd A regionthat lacks large magnetic fields at the spacecraftaltitude mi t still consist of crust Lhal f magnetic field or examp e incoherent the cooling history may have voured multidomain magnetite or less hvourable mineralization or the field may Maren er r 39 h that seemto be 39 in a 100 Myr and thus cooled in a different magnetic field even though their surface 215 insight review articles Figure 2 Radral magnetretreld at 200m altttude based on data collected by the Mars Global Surveyor spacecraft Thesouthem hernrsphere exhlblls the largestfleld anomalles Notttethatthe oppostte hemtsphere has held anomalresthataretypreallyan order of magnltudesmallerand exhlblt erertspatral structure These mole drtterent prqettlon tn ref 39 Centred orr 0 E 15 X15 grld Centred orr 180 E Lambert Azrrnuthal EquzleArez prqectron 7397 7250 400 eat 725 40 lo 25 so 100 250 500 666 HT appearance and crater density seem to be identical last but not lea t 39 uuu a Uull er trust39 localities thereby allowing preservation ofa relatively weak and tch magnetization even when the surface age postdates any Thermal m mmrnslllunal convection globalfield If one accepts that core convection is needed then a probable A martian dyllauu The dynamo mechanism see Box 1 is much studied but still neoesary condition for a dynamo is the presence of convection In The reason lul imperfectly understoodn39 numerical simulation In particular we do not know the conditions sufficient for the existence ofa planetary dynamo Because we can only speculate about early conditions on Mars the problem of interim Ul 39 39 39 martian 39 39 formidable Earth39s dynamo is also imperfectly understood A large byinnencoregrovlLhm xtsts then it is likely that the expected field magnitude B inside the region of field generation is given b the lsasser number oforder unity This implies B Zp U 2 where p is the fluid density is th 2 is the planetary rotation rate an r thi ielrl 10 3tesla it unusually large heat flows or the development of an inner core In ei 39 a ther case the core mist be cooling To appreciate this rgument conduction along an adiabat Fm gt law 2 kotTgCP cgtLhermalconveclion Where It is Lhe Lhermal conductivity 0 is Lhe coef cient of Lhermal eman inn 39 39 39 39 39 but because this is the same prediction as for present Earth the the level ofthis crude argument from the present field at Earth39s urfare but it does not admit fields substantially larger than B ZpQU 2 n 22 at present for the idea that the field scales in some direct way with the ener y ource so that it might undergo slow decline over geological time or large changes arising from innerscore I n nuclealio Mars is that the field was generated in a magma ocean Plausible numbers are a characteristic fluid velocity vof10 msquot because 105 m and a magnetic diffusivity A of 10 m2 squot possibly riate to higlrlenlperalure and higlrpressure silicate me tsquot which together give a magnetic Reynolds number o 10 This is marginal at best but would be attractive because large L The ezoot CP is the speci cheal at constant pressure parameters are all uted except for g which is approxirrately linear in radius r the is the lanet centre If the core is simply cooling and releasing the stored sensible heat provided by gravity during planetary accretion then the total heat flux is also linear in r W r e pCPd1dt3 where 2 is the mean core temperature and H firm 39 39 39 39 39 39 39 to unknown very high pressure effects Consequently if thermal 39 39 lth it ill also cease to operate at about thesametime elsewhere in thecore If the core is cooling and the central tenperaurne drops below the 39 39 39 39 Tn Fanh e irnn ms relevant to Mars the dominant light element may be sulphurz For 39 39 39 2 N1 rm N Tuppl l l l chemical arguments suggest the inner core will be nearly pure iron with some nickel and the sulphur will be entirely in the outer core The introduction of this light element into the uid of the lowermost core willtend r t 39 J 39 39 a all or most of the outer core provided the cooling is sufficiently fast Latent heat release at the innerecoreouterecore boundary will also contribute to the likelihood of convection However innerecore rowth permits outerecore convection even when the heat ow 0 39 y l manure heat carried by conduction along an adiabat In this regime possibly carried by conduction is large This state is possible because the buoye ancy release associated with the compositional change exceeds the work done against the unfavourable thermal strati cation Unlike thermal convection compositional convection may not cease every 0 39 I 39 i i ierl in detail but not in general outline should the core include a light elementthatrlne nnte hi i 39 39 JUIE ample silicon as well as the universally expected complement of sulphur Required cooling rates For plausible choices of parameters the cooling rate ofthe core must estimate is uncertain by perhaps as much as a factor o required cooling rate in the presence ofa growing inner core is much smaller by a factor ofseveralzw but has not been studied in detail for Mars As a consequence models with an inner core will tend to suse tain a dynamo for along time perhaps even to the present day unless there is something unusual about the thermal history as suggested below The overlying mantle determines the cooling rate Indeed it is the mantle that determines whether a terrestrial planet has core convection andwhether it can have a dynamo convectionzs Simple scaling laws for convection compatible with a i cists as mixing length theory suggest that V 01 Emp 3 where p 39 m quot 39 F n39 39 im in it composii tional equivalent when the convection is driven by compositional density differences I define 5 EonJEondad Eotal EondadFondad r L uiauui themartiancoreL106mtlmzs lp104kgm 3Rmmaybelarge even if slt 1 That is the heat ow has to only slightly exceedthatfor any convection in order to reach that for convection of sufficient vigour to sustain a dynamo This claim must be tested er numerical work It is conceivable but diffith energetically for a dynamo to function for s lt 0 for example because of baroclinic instabilities and thermal winds arising from horizontal temperature gradients that are caused by lateral differences in heat ow throug the coreemantle boundaryEveninrhi m p I hm 39 Slltl a me erLlLariuuuuu wucuuutlier 39 39 g up u and this inhibits dynamo activity Alternatives to convective driving for example precession still 1 L 439 L 39 1 1 t I 39 if f enniigh or39 039 allow innerecore nucleation then a dynamo occurs but ifthe mantle is too hot or fails to cool then there is no dynamo Possible histories of the martian core Three possible scenarios for the history of the martian dynamo are presented in Figure 3 The first is the simplest the planet starts out veryhnt 391 1 39 111 L T 39 39 39 I J NATURE lVOL 412 l 12 JULY 2001 l wwwmtunecom 2001 Macmillan Magazines Ltd insight review articles Monotonically cooling mantle With stagnant lid fully liquid core convecting thermally Mantle cooling declines r convection stops e no Inner core no dynamo Outer core liquid Inner core solid Monotonically cooling mantle with stagnant lid soild inner core and liquid outer core Outer core becomes too 7 L 4 u J Possible Inner core 4 and dynamo no dynamo Figure 3 Possible scenarios of martian core evolution a Hignrsulpliur model b lOWrSLllpl lUl model c plalerlECIOl39ilC model works for all sulphur contents throughout As the cooling rate declines a point is reached at which the heat ow out of the core can be accommodated by conduction alone At that epoch the dynamo turns off in a very short time geologically perhaps as little as a few thousand years and no further field generation is possible provided an inner core never develops This model requires that the core of Mars is sulphurerich perhaps 10 or more by mass It also requires tuning ofthe parameters so that the dynamo turns off as early in Mars history as some arguments suggest Most published models are ofthis kindmg T in is almost 39 ofthe rst It has not been modelled in detail although it is implicit in the early work of Young and Schubert3 who considered the possibility of complete core freezing In this model the sulphur content for the core is suf ciently low that an inner core develops early and grows rapidly iquid outer core becomes progressively more sulphurerich and evolves towards the eutectic composition The experimental data31 yield a eutectic of around 1400 K at the top ofthe core For realistic models of Mars mantle convection the expected presenteday coreimantle boundary temperature is at least 1850 K so there is no 217 insight review articles Boxl What is a dynamo The essehce of a dyharho Mes 1h eiectrorhaghetrc 1hductroh 7 the creatroh of currehts ahd assocrated he d through the rhotroh of cOhductrhg urd across rhaghetrc he d irhes Nurherrcai ahd ahaiytrcai work 1hdrcate thata dyharho er exrst 1f the urd rhotrohs have certarh desrred features ahd the rhaghetrc Reyholds hurhber Rm exceeds about i 0 Here RmEtLA AE1uoo39 where vs a characterrstrc urd veiocrty L 1s a characterrstrc 1ehgth scaie of the rhotrohs or he d for exarhpie the core radrus t 1s caHed the rhaghetrc drffusrvrty yo 1s the perrheabrhty of free space ahd ms the e1ectrrca1 cOhductrvrty 1 uhrts itseerhs irkeiythat urd rhotrohs of the desrred character arrse haturaHy 1h a cOhvectrh fur 1rrespectrve of the source of urd buoyahcy provrded the Corrohs force hasa targe effectOh the ow that 1s vlt lt i Where Its the piahetary rotatroh rate Thrs 1s easHy satrshed for ahy piausrbie urd rhotroh of 1hterest Reahstrc dyharhos ofteh requrre a somewhat targer vaiue thah 10 for R especraHy 1fdr1veh by secuiar colog wrthoutah 1hher core ahd attehtroh rhustaiso be pard to the drffereht krhds of rhotrohs ofreievahceforexarhplevertrca1rhotrohs ahd drfferehtrai rotatroh may have drffereht arthtudes ahd scaies of varratroh prospect that the outer core will completely freeze However it is con ceiva ble thatthe outer core will become sufficiently thin that dynamo activity can no longer be sustained This would seem implausible L quot 39 I 39 hm i1 rnigm 39 era 33 r tunytun J I 1Lr J 1 cup u tuuteut of the martian core than most would consider plausible perhaps no more than a few per cent even less than typical estimates for Earth r r unmet T 1 a hannnin 9 quot totrige 39m 39 ger the death of the martian dynamo It is assumed that early Mars the parameters that define their chronologies are not known with sufficient accuracy However the presence or absence or size of the inner core is clearly a crucial variable and may eventually be deter mined by a combination of geodesy and seismology Numerical quot39 Willa obeimpnrtam39 39 Cyean Consequences oi the manian core and dynamo Core cooling dictates the presence of a thermal boundary layer at the base of the overlying mantle Plumes can detach from this layer and may be a cause ofhotispotvolcanism Harder and Christensen35 have proposed that Mars may be in a regime where a single plume dominates because of the effect of a major endothermic phase transition near the base of the martian mantle the same phase transition that 39 Fr y Earth This plume might be stable for a long period oftime perhaps billions of years and may be responsible for the Tharsis volcanic province This hypothesis provides the exciting prospect of linking core thermal history with martian volcanic history However itleaves unanswered several questions If the core heat flow is so low as required by the absence of a dynamo throughout much of Mars history then is it reasonable to suppose that it is responsible for the dominant volcanic activity on Mars Why would a deepiseated plume happen to produce volcanism at a locationjust northward of the principal geological feature the crustal dichotomy Why is the q t ari quot thesurface uu r of Mars rather than in the core histor The history of the atmosphere36 may also be influenced by the magnetic field history through the effect of the field on atmospheric sputtering The history of martian magnetism might even be linked to the history of life on Mars Perhaps the strongest argument for a biological effect in ALH84001 lies in the singleidomain magnetite grains whose presence in biological organisms is useful only while Mars has a field This might also push martian magnetism back to the earliest epoch The future Although martian core studies can benefit from work in all areas of planetary science including geochemistry the greatest contribu7 tion is likely to arise from seismological and geodetic efforts In particular the Mars Netlander mission38 and subsequent followaups are likely to have the greatest role It may also be essential to better On Earth this is accomplished by plate tectonics and 39 d be the case on Mars It is however the recycling of the lithosphere that matters not the form of the recycling so there is no need o assume that Mars did exactly what Earth does At some time perhaps after only a few hundred million years this process ceased and Mars evolved slowly into the stagnant lid regime that it and all terrestrial bodies except Earth currently occupies If this regime ollows one of lithospheric recycling then the mantle must heat up because the elimination o heat is less efficient In other words the coldesttime for the martian mantle was early in Mars history despite the inexorable monotonic decline of radioactive heat sources in the mantle and the crust This scenario has the advantage that it may work for all possible sulphur contents in the core as the presence of an inner core will not rive a dynamo if the mantle minimum temperature was reached early in Mars history An inner core drives a dynamo only while it is growing and it can grow only ifthe core is cooling One problem with this scenario is that it invokes an adhoc timing for the cessation of plate tectonics it also implies the ability Jul 1 in LU u r a any 39 a uenlnoiral time In all these scenarios the beginning of dynamo activity may be delayed after Mars accretion until a thermal boundary layer builds up in the lowermost mantle depending on the uncertainty in the initial temperature difference between the core and mantle But this is unlikely to produce a delay ofmore than 100 Myr There may be other scenarios not yet considered Unfortunately none of these scenarios can be tested with great confidence because 218 2001 Macmillan Magazines Ltd LildldLLEIILL the u something that no currently 1 u 1 1 m Y c r ments in our understanding of dynamos Mars core is at least as interesting as Earth s core for our general understanding of planet evolution El 1FolknerWN r1ch now tnthM PvtnP 7 R 1 4971752 1997 F 1 Y terrestrialplanet Science 231 183871840 1998 onD 1 Steven r a H v H extremes 39lt 1 9 1 1 r ern D h m m h m A R7 15 72177 1998 5 ChenH vl39wv r r UrThersystematrc Geachr m Camachr mActa50 9557988 1988 8 1 D r H 1M N 2547257 1997 7 Wetherrll G W Provenanoe ofthe terrestrialplanet Geochr m Camachr mActa58 451374520 1994 8 Chamber E SLWetherrll G W Making the terrestrialplanet Nrbodyintegrationsofplanetary embryo mthree dimension Icarw1363047327 1998 n v r r r 1 n H r mm A xx late stage otterrestnalplanet formation Icarus14221972371999 10 Matm1T h effectofammpactrmduced atmosphere EarthMaan Planet 34 2237230 1988 11m vFM th H 1973 x 51016103 mm M Surveyor MAGER experiment ScienceZM 7907793 1999 3 r nn m 1 F v 1 7m 794792 1999 CealagyZE 1A 111mm F 391394 2000 NATURE VOL 412 1 12 JULY 2001 Wwwnaturacom insight review articles 15 SchubertGRuellC T ampMooreW B 4quot M13788 m A n by P h 11 1 7301995 2000 28 StevensonD J Planetarymagneusm Icau224037415 1974 t emmttt a huh n r H H 11 1 R M n a r M NH M 17 Mernll R TMcElh1nney M W StMcFadden P L TheMagnetic eldaftheEaxth Academic NEW 1477183Unw AnzonaPressTucon199Z York1998 0 un P F huh V r I R F H A F 39 1 1577159 1974 32323408 2000 Fe1Y W Bertka c W ampF1ngerL W Hughrpressure Honesulfurcompound F9232 and me1tmg 1g Robertep H 1 t m t m At 7 1 m u u t 551 7 5181716230997 1081711232000 W F n 1 n 1 n t n F r L Geophys 12533 eld ofMar Geophys 125105 11989711979 2000 596175970 1978 huh V r P M N t n n Dj gLSpohn r 4297450 21 Gubb1n3DMaterT G ampawb A 39 M A Hm v1 Pt ucgonJgBB Sat 59 57799 1979 34 Sleep N H Maman plate tectonlc Geophys Res 99 583975855 1994 22 Stevemono spohnm huh n r H m v H mm t n n H p Kquot ham 54 4887489 1983 5077509 1998 23 Tyburczyl F11 v l V 38 BralnD A ll R M ed AhremT J 1857208 Am Geophy Unlon1995 In a m 100 24 L1l r R 37 Thomaexeprtm L eta1Truncated hexaroctahedralmagneutecrystalsmALH84001 preeumptwe core Geophys Res Lett23 81784 2001 blongnamresthcNat1Acad 5 USA 93 218472189 2001 25 GemannC K WoodB I Ruble f V1ll39wm M P n t H 11 AR 114 714Z02000 P h 1M 0 Wm 1 v M n n a Geophys 3877378 2001 Res Left 27 244972452 2000 A0 Kmquot v r rm t n quotH H 25 StevensonD J Planetary magnetlcfleld RepngPhy46 555520 1923 27 Listen R ampBuffettB A Theetrengthandetmency ofthermalandcomposmonalconvecuonln NATURE lVOL 412 1 12 JULY 2001 1 wwwmtumcom Lett 27 29732 2000 2001 Macmillan Magazines Ltd articles Recent ice ages on Mars James W lleadlI John F Mustardl Mikhail A Kreslavskyl39zI Ralph E Milliken1 amp David R Marchant3 IDepartment of Geological Sciences Brown University Providence Rhode Island 02912 USA ZAstronornical Institute Kharkov National University Kharkov 61077 Ukraine 3Department ofEarth Sciences Boston University Boston Massachusetts 02215 USA A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters Recent Mars exploration has revealed dusty waterice rich mantling deposits that are layered metres thick and latitude dependent occurring in both hemispheres from midlatitudes to the poles Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 21 to 04 Myr ago The deposits were emplaced symmetrically down to latitudes of 30 equivalent to Saudi Arabia and the southern United States on the Earth in response to the changing stability of water ice and dust during variations in obliquity the angle between Mars pole of rotation and the ecliptic plane reaching 3035quot Mars is at present in an interglacial period and the icerich deposits are undergoing reworking degradation and retreat in response to the current instability of nearsurface ice Unlike the E h martian ice a es are characte rIzed by warm r polar climates and enhanced d art g equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to midlatltu es Of the planets in the Solar System the climate of Mars is most similar to that of the Earth Small quasi periodic variations in the Earth s orbit and spin parameters over timescales of 10 105 years drive large scale changes in climate Similarly Mars exhibits variability of its orbital and axial elements with timescales compar able to the Earth s however the ranges of Mars variations are signi cantly greater Over the past 10 Myr Earth s obliquity has ranged from 22 to 245 while for Mars3 5 the range was 14 to 48 Similarly the eccentricity of Earth s orbit varied from 0 to 006 while for Mars the range was 0 to 012 The consequent changes to insolation and seasonality at middle to high latitudes on Mars have inevitablyran ed imi rznt rhah e in fcarbon dioxide water and dust Thus Mars may have experienced the most signi cant quasi periodic variations in its climate over the past 10 Myr of any planet in the Solar System Analysis of data from spacecraft missions before Mars Global Surveyor MGS identi ed laminations and layers in the north and south polar caps whose origin was attributed to quasi periodic climate change Theoretical modelling of water vapour diffusion in near surface soils predicted that ground ice stability in the mid latitudes would vary in concert with the orbital and axial forcing An array of landforms in the mid latitudes of Mars were cata logued that have analogues to ice rich terrains on Earth However their ages appeared to be signi cantly greater than 10 Myr and their origin was not linked to quasi periodic climate change Here we use data from the recent Mars missions MGS and Odyssey to show multiple lines of evidence for surface deposits that formed as a result of recent quasi periodic climate change on Mars The observations span a large range of scales metres to kilometres to hundreds of kilometres are of diverse nature morphology topography and chemistry and are remarkably consistent with models of current and past ground ice stability These results all point to the presence of a succession of metres thick latitude dependent surface deposits that are young ice rich when formed and whose deposition and removal is driven by climate change that is induced by spin axis tilting MGS observations The subkilometre scale roughness derived from global Mars Orbiter Laser Altimeter MOLA data revealed smoothing at subkilometre scales above about 30 latitude in both hemispheres 2 Fig 1a This topographic smoothing was attributed at least partly to a surface deposit several metres thick that was superposed or draped on older geological units Fig 2 and referred to as a mantle 2 4 The NATURE lVDL 425 1825 DECEMBER statistics of the kilometre and subkilometre scale topography of Mars are in uenced by such a metres thick mantle because the typical topographic slopes at these scales are of the order of a degree or less this means that the typical vertical scale of topography is two orders of magnitude shorter than the spatial scale Another promi nent latitudinal trend is seen in topographic concavity at hundreds of metres baseline Figs 1b and 3c a measure of the relative balance of concave and convex segments of topographic pro les Analysis of Mars Orbiter Camera MOC images revealed the distribution of many geological features that also exhibit a latitude dependenceIS 23 A systematic study ofthe presence or absence ofa speci c morphology representing a metres thick but partially degraded and discontinuous surface deposit Fig 2c f showed that this terrain was limited to the 30 60 north and south latitude zone Figs 1c and 3b This deposit was interpreted as a recent formerly ice rich dust mantle that originated as a thin blanketing airfall layer and was now undergoing dissection and removal There was no evidence of this deposit in equatorial and low latitude regions between 30 N and 30 S 5 Poleward of 60 this mantle was continuous and commonly characterized by a unique bumpy texture at the scale of several tens of metres that often resembled the surface of a basketball Fig 2a and lineated and wrinkled versions of this terrain type 2 22 Evidence for multiple layers Fig 2b within the mantle unit has been documented indicating several generations of deposition and removal Different types of polygons and patterned ground were also noted to occur preferentially above 30 north and south latitude 7 suggesting the presence of a dynamic layer in which near surface ice rich material was undergoing thermal cycling Startling images show what appear to be very recent water carved gullies that occur preferentially in this region and have been interpreted to represent groundwater sapping melting of ground ice or snowpack during higher obliquity or local microclimate conditions Viscous ow features2D and related deposits articles Furthermore it shows a very high correlation with the present stability of near surface ground ice Fig 1d as modelled from the diffusive exchange of water vapour between the regolith pore space and the martian atmosphere10 32 Latitude is the single variable with which all of these diverse observations correlate and climate is the only process known to be latitude dependent Degradation of the youthful mantle in the mid latitudes suggests a very recent change in climate The remarkable correspondence Fig 1 between the character of the terrain smoothness the continuity of the mantle the high hydrogen Latitude degrees 80 Longitude degrees Figure 1 Maps showing important latitudinal trends on Mars simple cylindrical projection a Subkilometre scale topographic roughness Brighter shades denote a rougher surface The roughness parameter mapped is the interquartile width of the frequency distribution of the differential slope12 at 06 km baseline b Subkilometre scale topographic concavity Brighter shades denote higher prevalence of concave topography The parameter mapped isthe median curvature of topographic profiles at06 km baseline normalized by the interquartile width of the curvature frequency distribution Most of the geologic units in the equatorial zone have some concave topography positive concavity The high latitude zones have higher positive concavity Fig 3c Smooth filling of local lows by the mantle here eliminates smaller scale topographic variations and in this way amplifies the concavity The mid latitude zones 30 60 exhibit a distinctive low concavity that differs from both unmantled equatorial and high latitude mantled regions This is interpreted to be related to dissection of the mantle13 15 small scale irregular topographic variations due to dissected and patchy mantle occurrence in these zones cause randomly alternating concave and convex profile segments and concavity values close to zero All data in a and b are derived from statistics of all along track 798 2003 Nature Publishing Group abundance and the theoretical stability of ice in the near surface soil provide compelling evidence for climate driven water ice and dust mobility emplacement and dissection What do the charac teristics of these regions and deposits tell us about the nature and timing of such activity Interpretations of deposits One explanation for the correlation between the observations and models is that we are simply observing an outcome of climate related diffusive exchange of water vapour between the regolith pore Ice table depth m m 00 02 04 06 08 10 e Water abundance wt 012345678910 120 60 0 60 120 180 Longitude degrees topographic profiles obtained by MOLA c Distribution of examined MOC images Yellow circles indicate MOC images with dissected mantle terrain red circles indicate images with no apparent dissected terrainleizo The mantle is interpreted to be present poleward of 60 but is not dissected An albedo mosaic is used as a background d Map of ice table depth for an annual mean of 10 precipitable micrometres of atmospheric water vapour based on a near surface ice stability model In this model ice in the near subsurface is unstable at low latitudes equatorial regions no ice rich mantle terrains observed and stable at high latitudes polar regions thick intact ice rich mantles observed The mid latitude regions are transitional zones in which ice is currently unstable corresponding to areas where we observe degraded previously ice rich deposits such as the dissected mantle terrain This model of ice stability shows a remarkable correlation with the observations of ice rich deposits from Odyssey27 30 and M08 data e Mars Odyssey GRSNS experiment data showing interpreted water abundance in weight per cent White areas above 60 latitude are interpreted as ice buried a few centimetres below the surface NATURE 1 VOL 426 1 1825 DECEMBER 2003 lwwwnaturecomnature space and the martian atmosphere The depth to ice below the surface is predicted to have varied vertically in the regolith and with latitude as Mars experienced various orbital cycles in its most recent history In this scenario the latitude dependent surface textures in the 30 60 zones would be due to reworking of the surface caused by repeated diffusive precipitation and subsequent sublimation of water ice during recent excursions in ground ice stability The 30 60 zones would thus represent a zone of dynamic exchange between regolith and polar water reservoirs while regions poleward of 60 represent a remnant stable deposit of water ice in the regolith under current conditions In this scenario no signi cant amounts of new material would be deposited on the surface There is however abundant evidence for deposition of dust ice mixtures and layering Fig 2 Superposition relations are com mon dissection in the 30 50 latitude range shows removal and exposure of underlying units and multiple layers are seen 739 K r 7 v gt3 Figure 2 Characteristic tens of metre scale textures of the mantle and dissected terrain as seen in MOC high resolution images1539l 3 Scale bars at the bottom of each image are 1 km north is at the top Arrows show the illumination direction a Homogeneous mantle with basketball texture covers well preserved crater and its ejecta Portion of MOC image E02 01380 61 N 210 W b A mantle layer with the basketball texture partly removed Co registered MOLA profile shows a 3 4 m height for the topographic step across the mantle layer boundary Two circularfeatures are probably old heavily degraded mantled impact craters Portion of MOC image M02 01316 69 N 150 W e Example of dissected mantle terrain with internal layering found in the mid latitudes The mantle is smooth and has been removed from the pole facing slopes The removal of the mantle on the shaded side of the rightmost knob has revealed an underlying mantle layer Portion of MOC image M20 00144 35 8 174 W d f Examples of dissected mantle found in the mid latitude regions where the mantle has been preferentially removed from pole facing slopes d Portion of MOC image M04 02856 47 8 218 W e Portion of MOC image FHA 01450 44 8 240 W f Portion of MOC image M04 02289 43 S 240 W NATURE lVOL 426 l 1825 DECEMBER 2003 l wwwmaturecomnature 2003 Nature Publishing Group articles particularly toward the poles The latitudinal trend of subkilo metre scale roughness is readily explained by the deposition of a metres thick smooth layer on top of underlying terrain Such a deposit explains the observed increase of topographic concavity above 60 latitude13 while the decrease in concavity in the 30 60 Zones is interpreted to be related to dissection of the mantle The extreme morphological homogeneity of the unit above 60 latitude and the manner in which it drapes subjacent deposits22 also supports deposition of a mantle The very low abundance of superposed impact craters13 16 23 supports a very youthful age lt10 Myr and the paucity of subkilometre scale degraded impact craters23 supports deposition mantling and obscuration rather than in situ reworking The very high near surface water ice content inferred from the Odysseyy ray and neutron spectrometer data27 30 is consistent with a model of deposition of a relatively uniform ice rich dust layer from the atmosphere Other evidence for the ice rich nature of the deposit includes the correspondence of its continuous extent with modelled ground ice stability10 32 its apparent mechani cal strengthzo evidence for creep on steep slopes20 39 and its relation to water and ice related features such as viscous ow and poly gons19 213435 The weight of the geological observations points to airfall deposition Roughness degrees Dissection Concavity 0 o m I I I Latitude degrees Figure 3 Latitudinal dependence of statistical characteristics of subkilometre scale surface topography roughness and concavity and percentage of images with dissected terrain Southern latitudes are shown as negative values a Roughness parameter plotted is the interquartile width of the frequency distribution of the differential slope12 at 06 km baseline The peak at 10 8 is related to the Valles Marineris complex the difference in roughness between 20 40 8 and 20 40 N reflects the geological dichotomy of Mars with prevalence of rough heavily cratered highlands in the southern hemisphere and smoother lowlands in the northern hemisphere The decrease of roughness poleward from 40 N and 40 8 does not correlate with large geomorphological features and reflects the latitudinal smoothing trend b Percentage of high resolution MOC images having the characteristic dissected morphology among 15000 images surveyedle in 25 latitude bins There are two peaks in the presence of the dissected mantle at 41 N and 41 S which correspond to the latitude in a where smoothing begins c Topographic concavity The parameter plotted is the median curvature of topographic profiles at 06 km baseline normalized by the interquartile width of the curvature frequency distribution A strong decrease of concavity is observed within the 30 50 zone in both hemispheres and correlates well with the occurrence of dissected terrains 799 articles Integration of observations and climate models We propose a model that explains the observations by the depo sition and removal of mixtures of dust and water ice and is controlled by climate variations resulting from quasi periodic variations in orbital parameters During periods of high obliquity gt30 increased summer insolation on the polar regions results in greater water vapour release from the polar caps increasing the atmospheric water vapour content and thus the humidity33 34 Higher humidity moves the latitude of subsurface ice stability closer to the equator and within the soil pro le closer to the surface Climate models predict that ice removed from the poles is deposited on the surface at these lower latitudes where it persists throughout the year35 38 Mars global circulation model GCM simulations also predict strong winds favourable for dust lifting from global reser voirs and redistribution during such times35 36 This dust is readily incorporated into the growing ice deposits These three factors increased atmospheric water vapour content increased atmos pheric dust content and shifting of the surface ice stability zone toward the equator are thus predicted to act in concert at high obliquity to produce the latitude dependent deposits that we observe Owing to the higher year average and summer average polar temperatures and denser atmosphere the transport of water at high obliquities is predicted to be rapid High obliquity periods favourable to this process are of the order of 40 kyr Fig 45 long enough to provide deposition of at least several metres of ice rich material Thus in our model the deposited layers should be geologically recent latitude dependent dust ice mixtures super posed on underlying terrain and varying in their character as a function of latitude a Accumulation and Desiccation and modification degradation quot39IquotquotIquotquotlquotquotlquotquot U I 39D 0 g l n l l 8 Tn E u 9 a E 639 E S E 439 5 O l quotu 3 ID 2 39 5 15 IllllllIIIIIIIIIIIIIIIIIIIIII 0 O5 1 15 2 25 3 Time Myr ago b A 70 it Desiccation and 5 6 39 degradation 50 tn 5 39 9 40 O 9 3o a E J l n I u I n l 0 02 04 06 08 1 Time Myr ago Figure 4 Orbital forcing of climate in the past a Obliquity variations5 for the past 3 Myr with glacial accumulation and modification dark grey and interglacial desiccation and degradation pale grey periods marked Low amplitude line between 22 and 24 represents the obliquity range on Earth during the comparable period of history5 b Maximal insolation ot the north and south poles5torthe past1 Myr During the past300 kyr there is an asymmetry in insolation but before 300 kyr ago insolation is primarily driven by obliquity and is symmetric 800 2003 Nature Publishing Group As obliquity decreases the surface ice stability zones shrink toward the polar caps and ice sublimes from the uppermost layer of the mantle creating a protective ice poor crust analogous to the sublimation of ice from loess on Earth At lower obliquity water remains sequestered in cold polar regions and the atmospheric humidity is decreased Under this drier atmosphere calculations suggest poleward shrinking of this ice rich mantle and widespread desiccation10 between 30 and 60 As the ice sublimes it leaves behind a weakly cemented porous surface lag analogous to the lags observed in the FOX permafrost tunnel that retards sublimation If dessication is severe and the remaining ice poor regolith lacks a cohesive surface crust then deposits will become susceptible to eolian disruption and destruction For high latitudes gt60 ground ice is stable near the surface with obliquity as low as 20 and thus over the past 10 Myr dissection would be most important in the mid latitudes 30 60 The depth of desiccation is predicted to vary as the square root of time and thus the characteristic timescale of water loss increases with time and decreasing obliquity Thus low obliquity periods are likely to be too short to desiccate and remove the mantle completely at all latitudes Evidence for the present desiccation and removal of the mantle layers includes the dissected terrain in the 30 60 north and south latitude zone16 20 Fig 2e f and Fig 3 the decrease of subkilometre scale topo graphic concavity Figs 1b and 3c in this same latitude range and the correspondence of this zone with the latitudes predicted to be at present desiccated Fig 1e f in the near surface10 32 A variety of surface features at higher latitudes gt60 for example basketball textured terrain and its variants and polygons are interpreted to represent the net effect of long term orbital variations on relatively stable debris covered ice rich deposits In the Antarctic dry valleys a cold polar desert that represents a terrestrial analogue for regions with buried ice on Mars such landforms are related to sublimation and thermal cycling of ice rich sediments40 41 In Beacon Valley sublimation of debris rich ice produces a dry surface lag that insulates and slows loss of remaining ice Ground temperature cycling between lt0 C and ltlt0 C creates tensile stresses that result in a network of hexagonal cracks extending upward from buried ice to the ground surface and vice versa A lag of coarse grained sublimation till forms on top of the ice where nes sift into open thermal contraction cracks Owing to these spatial variations in till texture rates of sublimation vary across the ice surface High rates occur below coarse grained lags that cap contraction cracks and low rates are found at polygon centres beneath ne grained debris of low porosity and per meability The high centred sublimation polygons of Beacon Valley may be analogous to the latitude dependent basketball textured terrain on Mars If so the basketball terrain probably re ects a combination of long term ice sublimation coupled with thermal cycling on timescales both short seasonal and long for example obliquity A close analogue for such modi cation on Earth includes long term changes in the distribution and character of permafrost regions of the high Arctic 43 and Antarctic Estimates of the age of the mantling unit and its contained layers come from several sources The number of small fresh craters is very low and the crater retention age is correspondingly very young possibly as young as 150 kyr but most certainly less than 10 Myr Stratigraphic relations show a complex history of successive epi sodes of deposition and removal suggesting that the mantle consists of multiple depositional cycles Regional stratigraphic evidence at high latitudesg 14 shows outliers of polar cap deposits as well as dark dunes locally superposed on the mantle Finally many of the surface textures of the mantle for example polygons and basketball textured terrain are consistent with morphologies formed over several glacial cycles in the Arctic and Antarctic periglacial environments suggesting that changing environmental conditions over extended time periods are required Thus taken together the characteristics of the mantle suggest that the deposit is NATURE VOL 426 1825 DECEMBER 2003 wwwnaturcc0mnaturc geologically very young but that it is composed of numerous sublayers and features requiring multiple depositional cycles Integrating the geologic evidence with the most recent orbital history of Mars results in a d amic depositional erosional model for ice in the mid high latitudes Fig 4 The orbital and spin elements of obliquity eccentricity and the areocentric longitude of the Sun at perihelion LP Fig 4 all affect water ice migration on Mars 137 but with varying timescales and degree Over the past 300 kyr the obliquity has remained relatively stable within a range of 22 26 Fig 4 During this period high eccentricity causes differences in seasonality of the northern and southern hemispheres for example the southern hemisphere today experiences shorter warmer summers and longer winters than the northern hemi sphere while retarding of LF causes the seasonality difference to reverse every 25 kyr If the retardation of LF were the dominant driver of the deposition and erosion of the mantle then we would expect the geologic evidence to be asymmetric in its preservation re ecting the last such state Fig 4b On a global scale however the geologic evidence is remarkably symmetric Figs 1 and 3 But earlier than 30 yr ago obliquity regularly exceeded 30 totalling 15 discrete excursions over the past 2 Myr during which each excursion lasted of the order of 20 40 kyr During this period the high amplitude obliquity oscillations dominate the insolation regime of the polar regions When obliquity exceeds 30 water ice is stable in the near surface down to the lower mid latitudes and climate models predict that water ice is ef ciently moved from polar cap reservoirs by sublimation and deposited in the mid latitudes According to Mars climate models each high obliquity excursion would result in tens ofmetres of ice being removed from t e poles this material would then be transported and deposited in the mid latitudes with a water ice content equivalent to a few metres together with dust also eroded from the polar layered terrainsg As the obliquity changes to values lt30 ice is sublimed from the mid latitude deposits creating a surface lag of dust analogous to that seen in Earth environments39 4 Such a lag deposit strongly retards further sublimation The lower year average and summer temperatures at high latitudes at low obliquity decrease the diffu sion rate of water vapour in the pores Furthermore as obliquity declines below 21 the atmospheric pressure drops signi cantly with the sequestering of C02 in the polar caps further decreasing the diffusion rate in small pores and preventing eolian erosion of the lag The effects of a retarding loess like lag allows for all or part of the mantle deposited during a high obliquity period to be preserved during the low obliquity excursion It is during the extended periods of moderate obliquity such as for the past 300 kyr that geologic processes have suf cient time to produce large scale degradation of the mantle in mid latitude zones Before 5 Myr ago martian obliquity regularly exceeded 45 and the mean obliquity over this time was gt35 compared to a mean of 26 for the past 4 Myr ref 5 With obliquity gt45 gt6 Myr ago the polar sublimation rate is very high and ice is predicted to persist at the equator Given the relatively sharp boundary of the mantle at mid latitudes and the lack of evidence of an extensive remnant mantle in equatorial regions we consider this time period a less likely option for formation of the present day high latitude mantle If however mantle deposition and preservation is sustained by the mean obliquity rather than the excursions then the period before 5 Myr ago may be a more likely candidate Ice ages on Mars Integrating the observations with global climate models results in a martian ice age hypothesis Fig 4a Between about 2100 and 400 kyr ago obliquity regularly exceeded 30 and water ice was removed from the polar reservoirs and transported to mid latitudes where it nucleated on dust particles in the air andor at the surface and was deposited as a mantle Though obliquity varied greatly during this time the periods with lower obliquity were too short to allow for NATURE lVDL 426 l 1825 DECEMBER articles complete erosion of the sublimation retarding surface lags preserv ing much of the mantle intact We refer to such periods of net deposition for example between 2100 and 400 kyr ago as glacial periods During the past 300 kyr obliquity has been near 25 with comparatively little variation We refer to these times as interglacial periods Fig 4a Water ice in the mid latitudes has been slowly and steadily removed from this reservoir by diffusion sublimation and atmospheric transport processes it was deposited in the polar regions creating the uppermost layers in the polar cap and also resulting in the degradation of the surface mantle in the 30 60 latitude bands Because of the thinness of the deposit and the presence of admixed dust the total volume of water is of the order of a metre global equivalent layer and thus only a few per cent ofthe present volume ofthe polar caps Ifthe dissected portion of the deposit 30 60 latitudes contained 500o by volume water ice and half of this was removed by desiccation processes and deposited at the poles it would form a layer several tens of metres thick on the polar caps We thus conclude that the emplacement of the ice and dust rich mantle extending from polar regions down to low mid latitudes represents the equivalent of an ice age on Mars If such an ice cover ad o urred on Earth it would have reached southward to latitudes equivalent to Saudi Arabia North Africa and the southern United States Poleward migration of the equatorward limit of the ice dust rich mantle partial desiccation of the mantle in the latitudinal belt between 30 and 60 and modi cation via sublima tion and thermal cycling of long lived ice rich deposits poleward of 60 represent some of the changes that occur during interglacial times Fig 4 Martian ice ages as proposed here differ considerably from those on Earth7 47 First terrestrial oceans are large heat reservoirs and distribution systems and sinks for dust Second the major reservoir response during climate cycles on Earth is the amount of water stored in the polar caps the major atmospheric gas components are not thought to change signi cantly Third orbital forcing factors thought to be the pacemaker for the timing of Earth s recent ice ages48 are much less extreme than on Mars6 Fig 4a owing to the stabilizing presence of the Earth s Moon On Earth lower polar insolation causes the deposition of snow and the formation and lateral spreading of continental ice sheets Mars on the other hand is characterized by a lack of oceans in its recent past an abundant and mobile dust supply and extreme orbital forcing factors Its major atmospheric gas C02 is in dynamic equilibrium with its solid phase resulting in the potential for signi cant changes in atmospheric COZ abundance and press ure Glacial periods on Earth are characterized by colder tempera tures at the poles on average while on Mars the reverse is true In cold polar deserts on Earth evidence has been cited for the preservation of glacial ice gt8 Myr old at depths just below the urface5 Mars is characterized by an extremely cold polar desert environment and we anticipate that the record of recent climate change on Mars is similarly preserved in the shallow subsurface over wide latitude expanses and thus readily accessible to future robotic and human exploration D m Recerved 15 May accepted 8 Dereber 2003der101ossna1ureoz114 1 V l l V l K D U D h r M I9 5 zaehes1eraz a we 4 L 39 39u o 68676932001 Teuma j arWrsdem 1 The ehaerre obliquity ofMarsSc12v1ce 259 129471297 1993 1 l l 12 1 rel an N a n 75121993 Laskar 1 er a1 Orbital articles Kl rll 4 34lllvRMHrl M MT Iquot Lhamanlan I Geophys Res 10526695726711 2000 elrrnalel Geaphy Re 1110 157971584 1995 V l 1vM H r l Marv DI rrelarnande Geophy Re Left 29dor1u10292002lt3m153922002 39 l 39 39 4 39 J Geophys Re 10 14 Kreslavsky M A amp Head 1 w Stratlgraphy ofyoung deposus 1n dial1011113111 erreurnpolar reg1on 36 Haberle R A 2121 Orbltal elaange Expenments wlth a Mars general erreulaoon model Iowa 161 arsL1mmPlrzv12r 5a xxxlv abstr 1476 2003 66789 2003 15 Malrn M Fr e11 1C M 1 M 1 Ll39lmughpnmarymlssmn amply Re 106 2342923570 2001 studywldlsunpll ai volaulesdaernes J Geophys Re 108do110102920031E0020512003 Is Mu an I F r l l 38 Rlchardson M 1 ml Obllqulty1 sheets andlayeradsedlments on ars atspacecra nearrsurface ground ree Name 412 4117414 2001 observauons and dlmatemodels are 1ellrng usL1mmPlrzv12r 5n xxx1v abstr 1281 2003 1 mhnNM w 11 11 111 1215 water Geaphy Re 1211218997903 2001 mamanpolarlayeredlerram Geophys Res Left 27 27692772 2000 18 Mango d araz 1 4n Bodd39lelm 1 Han w 1 p 1 1 1 1 1 surfare ground ree Lunar Planer 5n xxx111 abstr 1219 2002 conunental Anrareura s Afr 1 So 914 82790 2002 19 am F di AI hv p I Sclevlce 2111 233072335 2000 Beaeon Valleysoud39lernV1rtoria 1an Antarctlca 6201 Soc Am Bull 114 7187730 2002 n M D F 7 42 WHlamsP ampSm1d1MWTheFvazenEmrh Alden OxfordUK1989 MOCrrnages Geophys Res 1014 do1lD1029ZDDZIEDDZDDS 2003 43 n 1 1 1 n 21 1 IN 11 IntSerNo22WllzyandSOnsdud1231211992 4 l 44 1 1 1 1112 Planer 5a xxx111 abstr 1058 2002 past 1mm 111 3057316 1994 2 Carr ars Global Surveyor observauons ofMamall medterram J Geophys Re 106 45 T 1 w 1 a n 14 1 MlstnvesLSar 23571523593 2000 US Geologreal Survey RestonVA 1987 23 Kostamarvrp 7 M V W 46 KolbEampTal1a1lta V l 51 XXXN abstr 1340 2003 data7IAmazon1al1penod learns 154 22739 2001 24 Comm 1 47 1 1 111 1 n 1 A hlgh obllqulty Sclevlce 295 110711312002 1W 50408422119821 25m v1 411 1 1 040422 dapos1tsNamve 422 4548 lt2uu3 19821 26 HadAt M H Metastablllty ofl1qu1d water on Mars km 156 37L386 2002 49 LEM Q wad and ghmm 0 MM Name 412 245249 2001 2780 1onw 1 n a n1 1 a m dep031ts5aenc229681785 2002 281 anwcaraz 1 1 1 1 297757782002 L a W 1571 9 Mquotquot quot I J M Mazch 2003 We also thank 1 Dumn A C t and 11 Nelvezt fox asslstance wth manuscupt SEY SC Equot 29778 8 an pzepazatlon 11 H IF M MAM 1lt1am1 m N 1 may 30 e1 an w 9 er al The global d1smbuuon ofnearrsurfacehydmgen on Mars 6111M Mm Canf 110st DEM Abst 5218 2003 31 1 ar R I 1 I a r 12 1 a 2002 TL L 1 32 Mellon M T Theory of ground ree on Mars and rrnplreauons 1o Lhe neutron leakage ux Lunar 1111mm ler 5a xxxlv abstr 1916 2003 33akoskyRM mu 1 me 11 1 H hlgh obllqulty Narm 315 5597561 1985 IameLHeadbmwnedu 2003 Nature Publishing Group NATURE VDL 425 1 1825 DECEMBER 2003 wwwmatuzexomnatuze


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