Evidence for Amazonian acidic liquid water on Mars

12 Pages·2009·1.14 MB·English

ARTICLE IN PRESS
PlanetaryandSpaceScience57(2009)276–287
ContentslistsavailableatScienceDirect
Planetary and Space Science
journal homepage: www.elsevier.com/locate/pss
Review Article
Evidence for Amazonian acidic liquid water on Mars—A reinterpretation of
MER mission results
Alberto G. Faire´na,(cid:2), Dirk Schulze-Makuchb, Alexis P. Rodrı´guezc, Wolfgang Finkd, Alfonso F. Davilaa,
Esther R. Ucedae, Roberto Furfarof, Ricardo Amilsg, Christopher P. McKaya
aNASAAmesResearchCenter,SpaceScienceandAstrobiologyDivision,MoffettField,CA94035,USA
bSchoolofEarthandEnvironmentalSciences,WashingtonStateUniversity,Pullman,WA99164,USA
cPlanetaryScienceInstitute,Tucson,AZ85719,USA
dCaliforniaInstituteofTechnology,VisualandAutonomousExplorationSystemsResearchLaboratory,Pasadena,CA91125,USA
eNASAAmesResearchCenter,BiosciencesDivision,MoffettField,CA94035,USA
fAerospaceandMechanicalEngineeringDepartment,UniversityofArizona,Tucson,AZ,USA
gCentrodeAstrobiolog´ıa(CSIC–INTA).28850-Torrejo´ndeArdoz,Madrid,Spain
a r t i c l e i n f o a b s t r a c t
Articlehistory: TheMarsExplorationRover(MER)missionshaveconfirmedaqueousactivityonMars.Herewereview
Received21February2008 theanalysesofthefield-basedMERdata,andconcludethatsomeweatheringprocessesinMeridiani
Receivedinrevisedform PlanumandGusevcraterarebetterexplainedbylatediageneticwater-rockinteractionsthanbyearly
19November2008 diagenesisonly.AtMeridiani,thediscoveryofjarositebyMER-1Opportunityindicatesacidicaqueous
Accepted20November2008
activity,evaporation,anddesiccationofrockmaterials.MER-basedinformation,placedintothecontext
Availableonline6December2008
of published data, point to local and limited aqueous activity during geologically recent times in
Keywords: Meridiani.Pre-Amazonianenvironmentalchanges(includingimportantvariationsinthenear-surface
Mars groundwater reservoirs, impact cratering, and global dust storms and other pervasive wind-related
Liquidwater
erosion)aretooextremeforpulverulentjarositetosurviveoverextendedtimeperiods,andtherefore
Amazonian
we argue instead that jarosite deposits must have formed in a climatically more stable period. Any
MarsExplorationRovers
depositsofpre-existentconcretionaryjarositesurvivinguptotheAmazonianwouldnothavereached
MeridianiPlanum
Gusevcrater completioninthehighlysalineandacidicbrinesoccurringatMeridiani.MER-2Spirithasalsorevealed
evidenceforlocalandlimitedAmazonianaqueousenvironmentalconditionsinGusevcrater,including
chemical weathering leading to goethite and hematite precipitation, rock layering, and chemical
enhancementofCl,S,Br,andoxidizedironinrocksandsoils.Theestimatedrelativeageoftheimpact
cratermaterialsinGusevindicatesthattheseprocesseshavetakenplaceduringthelast2billionyears.
Weconcludethatminoramountsofshallowacidicliquidwaterhavebeenpresentonthesurfaceof
MarsatlocalscalesduringtheAmazonianPeriod.
&2008ElsevierLtd.Allrightsreserved.
Contents
1. Introduction...................................................................................................... 277
2. JarositedatesAmazonianliquidwateratMeridianiPlanum................................................................ 277
2.1. Meridianijarositeispulverulent.................................................................................278
2.2. Meridianijarositeisconcretionary...............................................................................280
2.3. TheaqueoushistoryofMeridianiPlanumduringtheAmazonian ......................................................280
3. Amazonianwater-surfaceinteractionsintheGusevcraterrocksandsoils .................................................... 281
3.1. EarlyMERinvestigations...................................................................................... 282
3.2. EvidenceforbrinesonthesurfaceofGusev ...................................................................... 282
3.3. ThehistoryofwateratGusev.................................................................................. 283
4. SettingMERresultsinthecontextofotherdatasets:amodelforgeologicallyrecentacidicbrinesonMars ......................... 284
(cid:2)Correspondingauthor.Tel.:[phone];fax:[phone].
E-mailaddress:[email protected](A.G.Faire´n).
0032-0633/$-seefrontmatter&2008ElsevierLtd.Allrightsreserved.
doi:10.1016/j.pss.2008.11.008
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A.G.Faire´netal./PlanetaryandSpaceScience57(2009)276–287 277
5. Conclusions ...................................................................................................... 284
Acknowledgements................................................................................................ 285
References ....................................................................................................... 285
1. Introduction presenceofverylimitedamountsofliquidwateronthesurfaceof
MeridianiintimesasrecentastheLateAmazonian.
Aside from safety considerations, landing sites for the Mars Gusev impact crater (lat.14.51S, lon.184.61W) has long been
ExplorationRovers(MER)OpportunityandSpiritintheMeridiani hypothesizedtobethesiteofdebouchmentofMa’adimVallis,a
Planum and Gusev crater regions, respectively, were selected large structurally controlled outflow channel, which includes
because theywerepreviouslyidentifiedas potentiallocations of ponding to form lake deposits (Cabrol et al., 1998). Ma’adim is
pastwaterandrelatedsedimentarydepositionbasedondistinct interpretedtohaverecordedeithermultipleNoachian–Hesperian
morphologicandmineralogicindicators(Golombeketal.,2005). floods that sourced from grabens of Sirenum Fossae and chaotic
Water is regarded as a key prerequisite for life on Earth, and as terrain of the Ariadnes Colles (Cabrol et al.,1998; Kuzmin et al.,
such,answeringthequestionofwhenandwhereliquidwaterhas 2004)orasinglelargefloodeventfromthereleaseofaputative
beenpresenton the surfaceof Mars isa first-orderobjectivefor upstream lake (Irwin et al., 2002). In either case, floodwaters
astrobiology-oriented planetary exploration. If liquid water had appear to have transected numerous ancient impact craters
been present during the Amazonian Period, especially the Late including Gusev, eventually debouching in the region that
Amazonian epoch (see Table 1), then microbial life could have straddles the northern margin of Gusev and the large shield
found a way to endure near the surface of Mars until recent or volcano, Apollinaris Patera (Scott et al., 1993), which is also a
evenpresenttimes. primecandidateforpotentialhydrothermalactivityonMars(e.g.,
The hematite signature in the Meridiani Planum region (lat. Schulze-Makuch et al., 2007). It has been proposed that surface
51S–101N, lon.121W–51E) was identified through the TES instru- flow along Ma’adim Vallis may have led to the formation of a
ment (Thermal Emission Spectrometer, on the Mars Global paleolakewithinGusevcrater(Cabroletal.,1998).TheSpiritrover
Surveyorspacecraft,seeChristensenetal.,2001).Itwasinterpreted investigations,atleastforthelimitedareascovered,suggestthat
to consist of Noachian and possibly Early Hesperian layered basalt and/or impact resurfacing has been important such as to
hematite deposits, based on MOC and THEMIS crater retention obscurethehypothesizedsedimentaryrecord.Here,wearguethat
ages (Mars Orbiter Camera, on the Mars Global Surveyor, and limited amounts of water have been present on the surface of
Thermal Emission Imaging System, on the Mars Odyssey space- Gusev(ColumbiaHills)uptotheLateAmazonian.
craft; see Hartmann et al., 2001; Lane et al., 2003). The deposits It has been suggested (Squyres and Knoll, 2005; Golombek
have a relief of (cid:2)600m and an areal extent of about 3(cid:3)105km2 etal.,2006)that MERobservationsindicateveryancientweath-
(Hynek, 2004). This outcrop of hematite, which has been eringofsurfacematerials,followedbybillionsofyearsofabsence
interpreted to consist of basaltic rock materials with (cid:2)10–15% ofliquidwateronthesurface.Particularly,thepresenceofjarosite
crystalline grey hematite deposits (specularite, Fe O ), may have inMeridianiPlanumhasbeentakenasevidenceforanextremely
2 3
beenprecipitated from groundwater upwelling and/or hydrother- dry Amazonian period on Mars (Elwood Madden et al., 2004).
mal circulation (Christensen et al., 2001; Hynek et al., 2002; Here,thelatestfield-basedinformationacquiredfromtheMERis
Squyresetal.,2004a,b;Hynek,2004;Andrews-Hannaetal.,2007), analyzed and placed into the context of existing orbital-based
among other possible mechanisms which also include a volcanic published geologic information showing that limited aqueous
(McCollomandHynek,2005)orimpact(Knauthetal.,2005)origin. activity occurred at local scales during the Amazonian Period at
Opportunity data has also shown the presence of hematite least up until recent geologic times. The Amazonian aqueous
(confirmed by TES, Mossbauer and APXS), ferrihydrite, poorly activityproposedinthispaperreferstothecollectionofmoisture
crystalline goethite, and schwertmannite in Meridiani outcrops onrocksurfacesandtheflow,infiltration,and/orpossibleponding
(Farrand et al., 2007). An extended diagenetic history in the of localized brines. Our conclusions indeed support veryancient
presence of an episodic water system of high ionic strength deposition of surface materials in water-enriched environments
(McLennanetal.,2005;Toscaetal.,2005)aptlyexplainstheorigin during the early times of Mars history, but also indicate the
ofhematite:byheatinganddehydroxilationofgoethite,whichin presence of scarce water at diverse locations throughout the
turnprecipitated from the breakdown of jarosite. Jarosite formed Amazonian Period, promoting geologically very recent (referring
when acidic liquid water infiltrated the Meridiani ferric sulfatic to the Late Amazonian epoch), and possibly even continuing
soils,dissolvingancientbasalticandsulfidephasesandpromoting geochemical alteration at present times, as we discuss in the
evaporitic sequences. The timing of all these events is unknown, followingsections.
andweargueherethat,althoughtheyessentiallyoccurredinthe
early history of Mars, the latest of them could be related to the
2. JarositedatesAmazonianliquidwateratMeridianiPlanum
Table1 The only place where jarosite has been found on Mars is in
AbsoluteageestimatesforthesurfaceofMars.
bedrockoutcropsinMeridianiPlanum,whereupto28%oftheFe
Epoch Absoluteagerange(Ga) isintheformofthishydratedferricsulfatemineral(Klingelho¨fer
et al., 2004), as revealed by the exploration rover Opportunity
LateAmazonian 0.6–0.3topresent along the traverses to Eagle and Victoria craters (Fig.1). Jarosite
MiddleAmazonian 2.1–1.4to0.6–0.3
hasaratherdistinctiveMo¨ssbauersignature,soitiseasytodetect
EarlyAmazonian 3.1–2.9to2.1–1.4
LateHesperian 3.6to3.1–2.9 and to place an upper limit on its abundance. But Mo¨ssbauer
EarlyHesperian 3.7to3.6 spectroscopy is not sensitive to Fe-free sulfate minerals, so the
LateNoachian 3.82to3.7 detected jarosite is possibly a mixture of sulfates and mafic
MiddleNoachian 3.95to3.82
siliciclastic grains (debris). This mineralogical assemblage is
EarlyNoachian 43.95
indicativeofacomplexdiagenetichistoryintheMeridianiplains,
CondensedfromFig.14ofHartmannandNeukum(2001). which are Noachian in origin, but also record modification
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The aqueous alteration of sulfides also leads to nanocrystalline
jarosite, with crystals o500nm in size (Chevrier et al., 2004,
2006). Interestingly, relatively large sizes and well developed
crystal forms of jarosite suggest that it is abiotic in origin (Akai
et al.,1999), while pulverulent jarosite can be abiotic (Chevrier
etal.,2006)orhaveabiologicallyinducedorigin(Amilsetal.,2007).
Todate,themorphologyofMeridianijarositeisstillunknown,and
thuswewillconsiderbothpossibilitiesinouranalysis.
2.1. Meridianijarositeispulverulent
RioTintoisanacid–sulfatedominatedfluvialsystemwithnear
constant acidic pH (about 2.3) along its 100km long course.
A variety of hydroxylated and hydrated sulfate minerals form
annually in the river as evaporite and efflorescent concretions
during the summer dry season, and are destroyed in the wet
season due to the highly seasonal rainfall. Sulfate-bearing
mineralsfoundalongtheriverderivefromaqueousalterationof
iron-rich sulfide minerals of the Iberian Pyrite Belt, the largest
pyritic province in the world, and include jarosite, copiapite,
coquimbite, schwertmannite, gypsum and epsomite. Similarly to
Rio Tinto, if soft jarosite formed on Mars billions of years ago,
during the Noachian or Hesperian time periods, not only the
chemical but also the physical environmental parameters at
Fig.1. Opportunity’sexplorationaroundVictoriacrater,inMeridianiPlanum.To Meridianimusthaveremainedstrictlyconstant,andsimilartothe
seeanuptodatelocationofOpportunityinMeridiani,see/http://marsrover.na- arid current conditions, during the entire time for pulverulent
sa.gov/mission/traverse_maps.htmlS.ImagecourtesyNASA/JPL/Cornell.
jarosite to be stable. This scenario seems unlikely, as any pre-
Amazonian assemblage of jarosite would have faced complete
processes which have been dated as Amazonian in age (Grant destructionduetocontinuingandpowerfulweatheringprocesses
et al., 2006). After jarosite precipitation, an arid environment of surface materials operating in Meridiani during the Noachian
must have prevailed, as jarosite becomes unstable and rapidly andtheHesperian,whichinclude
decomposestoferricoxyhydroxidesinhumidclimates.However,
it still remains an open question (Elwood Madden et al., 2004; (a) Important changes in the near-surface water reservoirs
Squyres et al., 2004b) whether jarosite was formed in ancient (Boynton et al., 2002), and in atmospheric composition,
times,withnosubsequentaqueousalterationforbillionsofyears, pressure, and temperature due to (i) large excursions in
orinamorerecentperiodoflocalliquidwaterstabilityembedded planetaryobliquity (Touma and Wisdom,1993), (ii) continu-
in the long-term dry Amazonian climate. Several factors, listed ing extensive volcanism (Dohm et al., 2001; Neukum et al.,
below, illustrate problems withthe consideration that Meridiani 2004),and/or(iii)impacteffects(Seguraetal.,2002).Alarge
jarositeisanancientmineralassemblage. number of evidences indicates climate change on Mars over
On Mars, where aqueous acidic conditions may have existed its entire geological history, including ocean-related land-
and have been widespread (Faire´n et al., 2004), at least during forms (Parker et al.,1993), massive layered outcrops (Malin
some periods (Bibring et al., 2006), jarosite could have had a and Edgett, 2000), valley networks and other valley forms
global distribution as a common evaporitic and/or diagenetic (Scott et al.,1995; Mangold et al., 2004), anastomosing and
mineral (Tosca et al., 2005). On Earth, under the oxidizing and meandering rivers and deltas (Malin and Edgett, 2003), and
nearneutralconditionsofthesurface,andbecauseofthelackof mineralogies indicating ancient aqueous environments over
iron and sulfur compared to other elements, jarosite is by regional scales (Hynek, 2004). For these features to have
necessity of limited distribution in typical weathering environ- formed, increased wetter conditions must have prevailed
ments.Itisusuallyfoundintheoxidizedzonesofsulfidedeposits, (e.g.,Baker,2001).Suchsubstantialvariationsinatmospheric
andformsthroughthereactionofdilutesulfuricacidderivedfrom pressure, temperature and humidity would have destroyed
theoxidationofpyrite.Theoxidationofpyrite(BurnsandFisher, pulverulentjarositeassemblagesinrelativelybriefperiodsof
1990,1993) and pyrrhotite(the usual primarysulfide in basaltic time.OnlyifMeridianiwaterwasinequilibriumwithjarosite,
rocks, e.g. Lorand et al., 2005) lead to jarosite, as shown by it could have come into contact with the bedrock with no
experimental weathering in simulated Martian environment chemicalalteration.Ifthiswasthecase,watermusthavealso
(Chevrieretal.,2004,2006). beeninequilibriumwithMg-sulfates,asthereisverystrong
Jarositeisabrittlesecondarymineral,withaMohs’hardnessof evidencethatthesemineralsexistinthebedrocks(Clarketal.,
21to31,andisusuallyfoundasamber-yellowtobrowncrustsor 2005),andthisfamilyofmineralsisamongthemostsoluble
2 2
coatings of minute crystals, as larger crystals are rather rare. sulfates. Any aqueous solution in chemical disequilibrium
Natural jarosite occurs in two different morphologies: (1) withjarositeandMg-sulfatesexistinginMeridianiatanytime
pulverulent, in nodules or fibrous masses, earthy, including wouldhavedissolvedthehypothesizedancientjarositeright
granular crusts where indistinguishable crystals are made up of away. Even reaction with atmospheric water vapor can
fibersandformuniformlylargemasses,bestexemplifiedinmine convert jarosite to ferric oxide when exposed at an outcrop
drainage waters, such as in Rio Tinto (Amils et al., 2007; see surface. In Rio Tinto, jarosite assemblages are seasonal,
Fig.2);or(2)concretionary,asdistinctfine-grainedcrystals,first formingeverysummeranddissolvingafterthefirstrainfall.
describedin1852inElJarosoHydrothermalSystem,whichisthe (b) Impact cratering, especially before the Amazonian, resulting
worldlocalityof jarosite(Martı´nez-Frı´as et al., 2007; see Fig. 2). inanumberofimpactcratersinvaryingstatesofdegradation,
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Fig.2. Jarositelocalitiesandtypes.Above:SatelliteimageofSpainshowingwithdotsthelocationofRı´oTinto(left)andElJaroso(right).Below:pulverulentjarositefrom
Rı´oTinto(left)andconcretionaryjarositefromElJaroso(right).
including deeply modified rims and interiors through wind probably deposited from the air during a dust storm. In
and mass wasting processes, as revealed by Opportunity’s Meridiani, the major components of bright dust and dark
traverse in Meridiani Planum. Impact cratering results in basaltic sand are supplied by wind, and various episodes of
enormous transient increases in local (or even global for dustdepositioncovertheprevioussoil(Yenetal.,2005).The
Noachian impacts) temperatures and pressures which affect brightlocaldustdepositsonMarsareinterpretedtobepartof
jarosite, as it begins dehydroxylation at over 700K. Also, aglobalunit(Yenetal.,2005),indicatingmobilizationoflarge
excavationoftargetmaterials,bedrockdeformation,exposure amountsofdustandtheassociatedburialand/orexposureof
offracturedrocks,emplacementoffreshejectablanketswith rockysurfaces,aswellasthedramaticchangeoftheMartian
widely varying grain sizes, formation of perched coarser landscapeandalbedoondecadalscales(Geissler,2005),allof
fragments, deposition of ejecta blocks on the surface, and which collectively mark the significant influence of global
impact-relatedcrushedlayersmantlingpreviousdeposits,are circulating winds across the planet. The surface is thus
usual consequences of bolide impacts possibly affecting coveredbydustfromannualglobalstorms,andthendeflated
jarosite assemblages. Water may have also contributed to by winds capable of completely stripping away the dust,
crater modification, including the local redistribution of exposingtherockoutcrop(Soderblometal.,2004),aswellas
fragmentstoformpavements(Fig.3)andsimilarsmall-scale resultinginerosionofthebedrockandreorientingubiquitous
landforms. smallsinuouscrestedeolianbedforms(Sullivanetal.,2005).
(c) Wind activity, in this case through the entire geological Theseprocessesofsandmovementandbedformdevelopment
history of Mars, which includes rock abrasion by saltating are recorded as occurring even nowadays (Bridges et al.,
sand grains impacting the very soft bed rock, dust particle 2007).Delicateassemblagesofpulverulentjarositeembedded
transportincludingjarosite and othersulfatesintheform of in very soft bedrock cannot survive wind abrasion, removal
very fine particles highly diluted in the global wind- and redistribution of debris, global mixing, and other wind-
transporteddust (as there areno detection of jarosite in the relatedweatheringprocessesduringbillionsofyears.
soils), and wind streak, dune, and fine-grained dust mantle
formation blanketing the soil, all reported for Meridiani:
bright wind streaks seen in MGS-MOC and Opportunity It is therefore difficult to imagine how assemblages of
descentimagesindicateongoingpervasivewinderosion.Seen pulverulent jarosite formed before the Amazonian could have
from orbit, Eagle, Fram and Endurance craters have bright survived the intense weathering of the early history of Mars,
streaks to their southeast, and surface observations confirm leaving as the only plausible explanation for its presence in
that the streaks derive from bright dust-sized particles, Meridiani that the synthesis of jarosite occurred during the
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Fig.3. EvidenceforrecentliquidwateronMars.(A)Opportunitypicture1N143189015EFF3243P1961L0M1,obtainedonSol169insideEndurancecrater.Thechannelled
featuresmayhavebeenformedbyoverlandwaterflowingthroughfractures.Centimeter-sizedfinshighstandinginthevein-likefeature,highlightedintheinset,arerichin
hematite,andseemtobesodelicatethattheirpresencesuggestsaveryrecentepochofformation.(B)CompositeimageofseveralOpportunitypicturestakenonSol208,of
plate-likerocksonthesouthwesternslopesofEndurancecrater.Thepatternsofpolygonsandcracksappeartohavebeenformedafterthefinaldesiccationofarecentwet
environment,leavingcrackedmudbehind.Thiskindofpolygonaltextureisseenonallsidesoffree-standingrocks,forexampleWopmay,alsoinEndurancecrater,around
Sols250–264.Moreover,theplate-likerockshavebeenchemicallyalteredtoagreaterextentthantherockslocatedhigherinthestratigraphiccraterrim,asindicatedby
greaterlevelsofchlorine(seeFig.4).Althoughalternativeinterpretationscanbeproposed,suchasaeolianerosionorrecentexposureofancientfeatures(forA),and
fracturingbytheforceofthecrater-causingimpact,thedryupoftheoriginalwaterthatformedtherocks,ordehydrationofhydratedsulfatesonexposureforming
dehydrationcracks(forB),arecentoriginforbothinawateryepisodehavingoccurredlongafterthewetperiodwhentherocksfirstformedisthemostlikelyhypothesis
aftercompellinggeologicalandgeochemicalanalyses.AninterestingterrestrialanaloguefortherecentformationofthesefinsandcracksinMartiansulfatesandis
providedbyChavdarianandSumner(2006),andalsoSquyresetal.(2006b)describerecentmodificationinMeridianioutcroprocksformingthefins.Imagescourtesy
NASA/JPL/Cornell.
climatically stable Amazonian. Any alternative explanation con- pressure and temperature changes over time. As Meridiani has
sidering these same physical weathering mechanisms as trigger beenadeflationareaforquitesometime,inthisunlikelyscenario
agents for environmental changes leading to the exhumation of concretionary jarosite would be currently exhumed, exposing a
ancient (i.e., pre-Amazonian) buried assemblages of pulverulent freshsurfaceofjarosite-containingrockoutcrops.Suchanancient
jarosite, must convincingly resolve how this extremely delicate rock outcrop would not necessarily require Amazonian aqueous
form of jarosite could have been first buried without being conditions. But the stripping of buried concretionary jarosite in
destroyed. And, when exposed, how it can resist the fundamen- Meridianidoesnotimplypersetheabsenceofliquidwateronthe
tally perturbing processes of exhumation, which must have Martian surface during the Amazonian. Any solution interacting
included at least one of the following destructive mechanisms: with concretionary jarosite in Meridiani during the Amazonian
surface or subsurface liquid water-related chemical weathering, wouldlikelyhavebeenintheformofepisodic,highlysalineand
impacttemperaturesandpressures,impactmantling,ordestruc- acidic brines, derived from intense evaporation and/or sublima-
tive global dust storms removing uppermost deposits. No other tionprocessesthroughtransientmigrationofgroundwatertothe
mechanisms exist on Mars to expose ancient sediments, and surface.In theanticipatedrecent aqueousphases,thehydroliza-
sandblastingandweatheringwouldratherburyanddestroythan tionofconcretionaryjarositewouldnothavereachedcompletion
exhumeanypulverulentjarositeexposedatthesurface.Thesame (Barro´n et al., 2006). Instead, concretionary jarositeassemblages
can be argued to explain the survival of any soluble salts in wouldhaveremainedstableinthebottompartofimpactcraters,
exposed outcrops. Therefore, pulverulent jarosite in Meridiani where they may interact with these cold and acidic brines.
wouldlikelybetheresultofalate(i.e.,Amazonian)sequenceof Therefore, previous investigations that consider jarosite as an
precipitation events in ponded brines, where jarosite would be indicatorofwater-limitedchemicalweathering(ElwoodMadden
dissolved when water is available and reprecipitated in the arid et al., 2004) no longer qualify as an argument to rule out the
epochs,inasimilarwayasisrecordedinRioTinto. presenceofwateronthesurfaceofMarsduringtheAmazonian.
Assuming a prolongeddiagenetichistory in the Burns formation
(McLennanetal.,2005),atleastsomeoftheconcretionaryjarosite
2.2. Meridianijarositeisconcretionary may have been a relatively late precipitate of the brines, rather
thananearly-formingmineral.Jarositemineralizationcouldeven
We explore here the possibility that jarosite assemblages in be a much later feature of the Burns formation, such as during
Meridiani are concretionary, although terrestrial jarosite is most transientaqueousactivity(Barro´netal.,2006).
ofthetimepulverulent,orformingverysmallcrystals.Terrestrial
concretionary jarosite from El Jaroso is genetically linked to the
calc-alkaline shoshonitic volcanism of the SE Mediterranean 2.3. TheaqueoushistoryofMeridianiPlanumduringtheAmazonian
marginofSpain,datingfromtheUpperMiocene(Martı´nez-Frı´as
etal.,2004,2007),amodeofformationsofarnotveryrelevantto AfterthepreviousanalysisofthemineralogyinMeridiani,we
Mars (and especially not relevant for Meridiani) compared to conclude that jarosite formed in a geologically recent, transient
otherprocesssuchasaqueousweathering. wetenvironment,asscenariosunderwhichjarositemightpersist
Onlyin thecase that jarositein Meridiani was concretionary, for several billion years on Meridiani are very unlikely. Further-
formed in wetter conditions during the Noachian or the more, these unlikely scenarios do not imply an absolutely dry
Hesperian,andthemineraldepositwasimmediatelyburiedafter Amazonianclimate.Insummary,weproposethatthepresenceof
formation, jarosite would have been protected from humidity, pulverulent jarosite in Meridiani would imply the activity of
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acidicliquidwaterduringtheAmazonian,whiletheexistenceof (2008) have concluded that the alteration history of Meridiani
concretionary jarosite could not be used as an argument to outcrop rocks did notend with the earlygroundwater-mediated
exclude the presence of Amazonian acidic brines. The important diageneticfeaturesdescribedinpreviouspublications.
difference is that, in such acidic brines, pulverulent jarosite Meridiani outcrops show sulfur concentrations as much as a
would be physically dissolved and precipitated repeatedly factorof4to5timeshigherthansoilmaterialsintheotherfour
responding to the variations in water availability, while concre- Martian landing sites, as well as a very high ferric to total iron
tionary jarosite would resist complete chemical hydrolization ratioinexcessof0.84(Klingelho¨feretal.,2004).Accordingtoour
in such a variable brine regime. Therefore, ponded acidic brines model, modest acidic liquid water has infiltrated the Meridiani
may have been associated to both types of jarosite during the ferric sulfatic soils during the Amazonian, dissolving ancient
Amazonian. olivine basaltic and sulfatic phases and promoting evaporitic
The actual contribution of water on Meridiani during the sequences,whichledtotheformationofjarositeinlocalsurficial
Amazonian was not a continuous body the size of the sulfate- depressions.Duringperiodsof verylimitedaqueousactivity,the
bearingunitthatextendsoverregionalscales(upto3(cid:3)105km2) breakdownofjarositecausedgoethiteprecipitation,andposterior
(Hynek,2004;Gendrinetal.,2005;Arvidsonetal.,2005).Rather heating, dehydroxilation and crystallization forming coarse-
it comprised shallow and heterogeneously localized ponded grained hematite. Alternatively, jarosite could have transformed
brines remaining liquid at temperatures below 273K (Kuzmin directly to hematite (Barro´n et al., 2006), as both goethite and
andZabalueva,1998;Gendrinetal.,2005;Faire´netal.,2008),in hematite are direct products of the hydrolysis of jarosite. This
theformofanopenacid–sulfateweatheringregime(Goldenetal., extendeddiagenetichistoryfinallyresultedintheassemblageof
2005).Thewaterwouldhavebeenacidicandoxidizing,reacting hematitic spherules (Squyres et al., 2004b; Zolotov and Shock,
withthebasalticsurfacestoproducesulfatesandironoxides.The 2005). Hematite may also be an early mineral phase, due to its
diageneticprocesslikelyprolongedintimeduringtheAmazonian, verylowsolubility,althoughitcandissolvequiteeasilyinacidic
includingepisodicinundationsbyshallowacidicandsaltysurface conditions.Suchaqueousactivityappearstohavebeenrecurrent
water followed by evaporation/sublimation, and desiccation. For duringtheAmazonian,intheformofepisodesofshallowsurface
example, rocks located deep within Endurance crater display a acidicwater,infiltrationandsubsequentdesiccation.Asaresultof
higherdegree ofchemicalalterationwhencomparedtotherock therecurrentinfiltrationoflimitedamountsofwater,jarositewas
outcrops located at higher stratigraphic positions within the formed and coexist along with hematite and goethite, probably
crater (Fig. 4), which could be interpreted as an indication of even forming mixtures of the three minerals. Also, hematitic
enhanced weathering by acidic liquid water in the past (e.g., spherules are located at varying stratigraphic sequences in the
Noachian/Hesperian,seeArvidsonetal.,2006)comparedtomore rockoutcrops,indicatingthatthespheruleshavebeenformedin
geologicallyrecenttimes(Amazonian).Alternatively,wefavorthe wateryenvironments occurring in different times prior to crater
idea that this vertical gradation in chemical alteration may excavation, and that this diagenetic process can be still active
indicate occasional acidic groundwater rising to the surface and today. The most recent of such watery episodes is hypothesized
leavingafilmatthebottomofthecraterinrecenttimes.Thisis here to have contributed to jarosite formation observed both in
alsosupportedbytheerosionalfeaturesdescribedatthesurface Eagle,Fram,andEndurancecraters,aswellasinrocksexposedat
of the crater, evidencing the activity of minor amounts of post- Anatoliatrough.Also,variousstratificationstyles(planarlamina-
crater formation surface water (see Fig. 3, and Chavdarian and tion, low-angle cross-stratification, cross-bedding, ripple cross-
Sumner, 2006), and suggesting that this is a recent evaporite lamination, and crinkly and undulatory lamination) have been
deposit.Small-scale,shallowcracksystemsobservedonoutcrops identified at Meridiani, indicating deposition from subaqueous
of the Burns formation can equally be originated in the dunesinanenvironmentcharacterizedbyepisodicinundationby
precipitationanddissolutionofsalts,moisturechangesorpartial surfacewaterviatheriseofanaquifertoshallowdepths,followed
dehydrationofsomeminerals(Chanetal.,2008).AlsoKnolletal. by evaporation/sublimation, exposure and desiccation. In fact,
minor amounts of water appear to have been flowing recently
between rocks and ponding deep into craters in Meridiani, as
suggested by the MER Opportunity investigations in Endurance
100
crater(Fig.3).AstheoutcropwasinfiltratedbysurfaceliquidH O,
2
that water likely became hypersaline, with soluble sulfates and
Surface chlorides,orenrichedinNaandCl(Knolletal.,2008).
Brushed
a post-RAT
ni
gi 3. Amazonianwater-surfaceinteractionsintheGusevcrater
Vir rocksandsoils
er /
h
sc 101 Decades of Mars exploration and derived photogeological
E
interpretations have illuminated an evolutionary sequence for
Gusevcrater,whichincludesMa’adimVallis-relatedfloodingand
associatedpondingandsedimentationonatleastmorethanone
occasion during the Noachian and Hesperian (Scott et al.,1993;
Grin and Cabrol,1997; Cabrol et al.,1998; Kuzmin et al., 2004).
O2 gO O3 O2 O5 O3 CI O2 aO O2 O3 O2 O3 Ni Zn Br Themulti-levelledterracesinthecraterinteriorwereinterpreted
Na M Al2 Si P2 S K C Ti Cr2 Mn Fe2 tomarkepisodicflood-relatederosionalevents,whilethesmooth
surface was interpreted to be fine-grained sediments deposited
Fig. 4. Chemical alteration of rocks located deeper (Escher) and shallower into the crater by the floodwaters (Cabrol et al., 1998). This
(Virginia)intoEndurancecrater.Escher’slevelsofchlorinerelativetoVirginia’s proposedpaleohydrologichistory,revealedthroughgeologicand
wentup,andsulfurdown,implyingthatthesurfaceofEscherhasbeenchemically
geomorphicmapping,wasthebasisforselectingGusevasthefirst
alteredtoagreaterextentthanthesurfaceofVirginia.DataobtainedbytheMars
ofthetwoMERtargets,withthetwofoldobjectiveof(1)collecting
ExplorationRoverOpportunity’salphaparticleX-rayspectrometer.Imagecourtesy
NASA/JPL/Cornell/MaxPlanckInstitute. in situ evidence to test the hypothesis of the existence of an
ARTICLE IN PRESS
282 A.G.Faire´netal./PlanetaryandSpaceScience57(2009)276–287
ancientpaleolake,and(2)determiningthecharacteristicsofthis to the previously hypothesized wetter impact crater paleoenvir-
putativeancientdepositionalenvironment. onment,notedparticularlyfortheWestSpurareaoftheColumbia
Hills, where the record preserved in the ancient rocks points to
3.1. EarlyMERinvestigations substantial water activity (Cabrol et al., 2006; Squyres et al.,
2006a), and where the extent of alteration of rocks and soils
Initial investigations by the rover Spirit indicated that the rangesfrommoderatetoextensive(Cabroletal.,2006;Mingetal.,
plains-forming materials (encountered at the landing site and 2006).
along the traverse to the Columbia Hills, see Fig. 5) within the Theinteractionbetweenthevolcanicrocksandminoramounts
Gusev impact crater did not support the purported flood- of water has been highlightedby Spirit’s in situ local reconnais-
lacustrine environment, but rather a mostly volcanic surface sance. Aqueous-related activity within Gusev impact crater is
mainlycomposedofdark,fine-grained,vesicularrocksinterpreted accentuated in the Columbia Hills through the identification
asbasalticlavaflows(Squyresetal.,2004a).Furthermore,impact of bedrock containing goethite and hematite, layering of rock
and eolian activity appeared to dominate the geologic history materials, rounded blocks, rock interiors with enhanced soft-
recordedintheplains-formingmaterials.Thecompositionofthe ness, and high concentrations of sulfur, chlorine, and bromine
fine-grained component of the plains-forming materials, as (elements easily mobilized by water and known to form highly
revealed by Spirit, displays similar characteristics to those solublesalts)inrocksandsoils(Farrandetal.,2006;Gellertetal.,
encountered at the Viking and Pathfinder landing sites, inter- 2006).Inaddition,Haskinetal.(2005)reportedhighconcentra-
pretedtobethepossibleresultofglobalmixinganddistribution tionofS,Cl,andBrinbothmulti-layeredcoatingsandinteriorsof
by dust storms (Gellert et al., 2004). Thus, the elemental rocks. In the case of some of the rocks of the plains-forming
composition of the bright surface dust at the scale of the Gusev materials, such as those provisionally dubbed ‘‘Mazatzal’’ and
crater floor is remarkably uniform. Moreover, the basaltic lava ‘‘Humphrey’’, enhanced Br concentrations in their interiors
flows appear to be the most pervasive of the plains-forming (mm deep, relative to their exteriors) have been observed to be
materials,androcksareangularanddispersedacrossthesurface, associated with alteration zones or veins, suggesting that water
suggestingthattheyareby-productsofnearbyimpacts(McSween playedamajorroleinCl/Brfractionationwithinthecrater(Gellert
et al., 2004). This led to the initial interpretation that the et al., 2004). ‘‘Mazatzal’’ shows high concentrations of sulfur,
evolutionofthecratersurfacewasdominatedbyphysicalrather chlorine, and oxidized iron, and is coated by materials with an
than chemical weathering processes, such as impact events elevated concentration of sulfur, oxidized iron, chlorine, and
followed by eolian modification (Squyres et al., 2004a; Grant bromine, suggesting post-crystallization interaction with acidic
etal.,2004;Morrisetal.,2004)overalargelyvolcanicsurface. water and subsequent evaporation. ‘‘Humphrey’’ shows bright
materialininteriorpores,crevices,andcracksthataresimilarto
mineralscrystallizedoutofwaterflowingthroughvolcanicrocks
3.2. EvidenceforbrinesonthesurfaceofGusev
on Earth. The identification of goethite in rocks in the Columbia
Hills, such as significant concentrations in the rock ‘‘Clovis’’,
Furtheralongthetraverse,Spiritencounteredamajorgeologic
confirmsaqueousprocessesintheWestSpur(Morrisetal.,2006;
contact near the base of the Columbia Hills that separates the
see Fig. 6), as goethite contains the hydroxide anion as an
plains-forming materials from the more diverse suite of rock
importantpartofitsstructure((cid:2)10%H Obyweight).Hematiteis
assemblages observed in the Columbia Hills (Crumpler et al., 2
alsoabundantinbothrockoutcropsandmantlesoftheColumbia
2005;Mingetal.,2006).Here,aftersome160Solsfollowingtouch
Hills (‘‘Pot of Gold’’, ‘‘Watchtower’’, and ‘‘Keel’’). Jarosite along
down onto the surface of Mars, the ‘‘dry’’ view based largely on
with other ferric sulfates (coquimbite and copiapite) has been
unequivocal evidence of limited chemical weathering in the
detected in the regolith (Johnson et al., 2007; Lane et al., 2008).
plains-forming materials (e.g., Haskin et al., 2005) reverted back
Hematite
5 Silicate
Goethite
4 Nanophase-oxide
(Goethite ?)
sity 3
n
e
nt
I 2
1
0
0
Velocity
Fig.5. Spirit’straversethroughColumbiaHills.TofindtheexactlocationofSpirit,
see /http://marsrover.nasa.gov/mission/traverse_maps.htmlS. Image courtesy Fig.6. Mo¨ssbauerspectrum(200–220K)showingthepresenceofhematiteand
NASA/JPL/Cornell/MRO-HiRISE/New Mexico Museum of Natural History and goethiteinarockcalled‘‘Clovis’’intheColumbiaHills.Thisisdefinitiveevidence
Science. forwateractivityinGusev.ImagecourtesyNASA/JPL/UniversityofMainz.
ARTICLE IN PRESS
A.G.Faire´netal./PlanetaryandSpaceScience57(2009)276–287 283
Optically thick coatings of fine-grained ferric iron-rich dust associatedpondingintopographicdepressionsduringtheMiddle
dominatemostbrightsoilandrocksurfacesatGusev. Noachian Period, prior to the formation of Ma’adim Vallis.
Subsurfacematerials,exposed by trenchingand wheelmove- GeochemicalconditionsduringthismoreancientperiodofMars
ments of the rover, show additional evidence for aqueous point to moderately to highly acidic waters, due to the CO -
2
processes. Spirit measured higher Mg, S, and Br concentrations dominatedatmosphereandthesteadysupplyofsulfateandiron
in rock materials exposed in ‘‘Boroughs’’ trench compared to to the water accumulations (Faire´n et al., 2004). Subsequently,
surfacematerials.Anotherexamplearerockmaterialsexposedby episodic flooding ensued during the Late Noachian and into the
the movement of Spirit’s wheels referred to as ‘‘Paso Robles’’, Hesperian,resultingintheformationofMa’adimVallis,aswellas
whereFeisincreased,SO representsmorethan30%wt,andthe multi-tiered terraces cut intothe crater floor and rim materials;
3
sulfateandphosphatecontentsarethehighestmeasuredsofaron water input into the crater included relatively small flows that
Mars(Gellertetal.,2006;Mingetal.,2006;Morrisetal.,2006). dissectedHesperianterraintoformalargenumberofsmallvalley
Interestingly, Mg-sulfates in trenches appear to be in a higher networks.Impactcrateringandwindactivitydominatethepost-
hydration state than is potentially stable at the surface under floodrecord.
current Mars atmospheric conditions (as speculated by Wang MinoraqueousactivityextendedintotheAmazonian,followed
etal.,2006),suggestingthattheymayhaveformedveryrecently, bywind-relatedactivityprevalentatpresent-day,whichinvolves
perhaps during the most recent period of high obliquity (Head mantling and etching of flood-related deposits and landforms.
etal.,2006),anepochduringtheLateAmazonianthatmayhave One of the more important processes that may suggest
providedelevatedhumidityandsomewhatwarmertemperatures. Amazonian acidic waters is acidic alterations on rock surfaces
In general, salts exposed by trenching appear to have been (Hurowitz et al., 2006). The identification of a compositional
separatedaccordingtosolubility. Bromineisalsoobservedtobe equivalent of the smectite montmorillonite (Clark et al., 2007)
elevatedinmaterialsthatfillthevugsandveinsofplains-forming couldargueforahigherpH;butasitseemstobeanamorphous
basaltic rocks where chemical mobility and separation are formofpre-orpost-montmorillonite,wefavortheinterpretation
indicated by decoupling of salt concentrations between rock that it indicates smectite formation process not going to
materials exposed in the trenches and at the Martian surface. completionin thelow-pH environmentthat characterized water
Positive correlations between salt components have also been occurrencesatGusev.
identified in the trenches (Haskin et al., 2005). In addition, the TracesofwaterbasedondatafrominstrumentsonboardSpirit
solubleelementsSandClareexposedatthesurface,suggesting point to post-Noachian water events (Gellert et al., 2006),
transport to the surface by liquid water. Other soluble species including episodic aqueous fluid movement altering multiple
such as an evaporitic MgSO phase have been mobilized and geologic units and resulting in the formation of surficial thin
4
concentratedinnear-surfaceintercraterplains-formingmaterials crusts through recent evaporation of brines (Herkenhoff et al.,
exposed by rover activities (Crumpler et al., 2005). Wang et al. 2006).TheredistributionofCl,S,Br,andoxidizedironmayhave
(2006) concluded that the geochemical trends observed in the beenmobilizedbythinfilmsofliquidwater;thesemobilephases
trenchesarebestexplainedbyiontransportationviafluids,fluid may penetrate into fractures and cracks in rock surfaces and
evaporation, and sequential salt deposition as a result of open- translocate in soils. A plausible mechanism for such transient
system hydrologic behavior. As discussed above, the latest liquid water on the surface of Mars and associated thin crusts
aqueousactivitycouldhaveoccurredduringtheLateAmazonian (Herkenhoffetal.,2006)observedonthetoplayerofsomesoils
epoch. and rocks may include atmosphere/surface interactions, as the
atmosphericpressureatGusevisabovethetriple-pointpressure
of water and the melting point of surficial water could be
3.3. ThehistoryofwateratGusev substantially depressed by the high ionic content of the surface
layers (Cabrol et al., 2006). This is consistent with Spirit data
Takentogether,thegeologicalandgeochemicaldatacollected showing generallyenhanced concentrations of S, Mg, Br, Cl, and
inthefieldatGusevindicateanevolutionarysequenceconsistent Fe3+/Fe in the trenched soil materials when compared to the
total
with Viking-based geological mapping (Cabrol et al., 1998; materials observed at the upper soil layer (Haskin et al., 2005),
Kuzmin et al., 2004; see Fig. 7). Some of the earliest geologic suggestinginitialaqueousalterationofvolcanicrocksinanacidic
andpaleohydrologicactivitiesrecordedintheGusevimpactcrater environment followed by mostly physical weathering. Elevated
and surroundings include significant widespread flooding and and highly variable Br concentrations in stratigraphically young
soilmaterialsofGusevcraterargueforepisodicbrinedeposition
andevaporation(Haskinetal.,2005),evenduringtheAmazonian
Period,withthewaterandsoilinteractionsoccurringatunknown
water/rock ratios. Alternatively, water may have been recently
present in Gusev as hydrothermal and fumarolic condensates,
ultimately forming the unconsolidated and near-surface soils
exposedbytheactionsoftheroverwheels(Yenetal.,2008).
The minimum exposure time of surface rocks to acidic liquid
water in Gusev is 22ky (Hausrath et al., 2008), likely due to
episodicaqueousalteration;buttheageoftherocksandthetotal
time they have been placed at their current locations is still
unknown,soitispossiblethatsomeofthesealterationepisodes
took place recently. The coating, which characterizes most rock
surfaces explored by Spirit, may have beenproduced by varying
amountsofwater,thoughonlysmallquantitiesoftransientdaily
toseasonalacidicliquidwaterarenecessarytoexplaintheabove
Fig.7. Stratigraphicdiagramshowingtheagesoffivedifferentgeologicunitsin
mentioned geochemical signatures. Consequently, the long-term
theMa’adim/Gusevcomplexhydrologicsystem.DurationoftheAmazoniantimeis
netchangeofthesurfaceatGusevatglobalscaleshasbeenvery
well in accordancewith all published Martian cratering geochronologies. After
Cabroletal.(1998). small(Golombeketal.,2006).
ARTICLE IN PRESS
284 A.G.Faire´netal./PlanetaryandSpaceScience57(2009)276–287
4. SettingMERresultsinthecontextofotherdatasets:amodel remixed the fragile sediments into the planetary homogeneous
forgeologicallyrecentacidicbrinesonMars soil. GRS has also shown variations of potassium relative to
thorium (Taylor et al., 2006), typical indicators of water transfer
MuchoftheexposedMartianrocksurfacesgenerallyconsistof and weathering. Also the CRISM instrument onboard the Mars
volcanic materials, over which chemically different waters have Reconnaissance Orbiter has revealed the presence of H O- and
2
weathered the late-stage basaltic surfaces through time. Aside SiOH-bearingphasesandhydratedFe-sulfates,includingjarosite,
fromtheMeridianiplains,hydratedsulfateshavebeendetectedin on late Hesperian to Amazonian deposits on and surrounding
extensivedunefieldsinthenorthpolarregions,aswellaswithin VallesMarineris(Millikenetal.,2008),implyingextensiverecent
Valles Marineris and in Aram Chaos (Gendrin et al., 2005). This wateralterationonthesurfaceofMars.
suggeststhatintheseplacesacidicandoxidizingwaters,provided SNCmeteoriteinformationisalsoconsistentwithliquidwater
byvolcanogenicSO3andHClandatmosphericCO2andH2O,have onMarsduringtheAmazonian:Bakeretal.(2000)concludethat
reacted with the mafic–ultramafic surface, dissolving rocks and meteorite-sourcingMartianrockswereintermittentlyexposedto
producinghydratedsulfatesandironoxides,whilenocarbonates water, Swindle et al. (2000) argue that the Lafayette meteorite
are evident as a result of the acidity of the early water masses wasweatheredbyliquidwaterduringthepast650millionyears,
(Faire´netal.,2004).Infact,Meridiani-likegroundwatersystems Borg and Drake (2005) suggest episodic flooding at the surface
seemtohavebeenwidespreadandrepresentativeofanextensive duringalmosttheentiregeologicalhistoryofMars,andRaoetal.
acid-sulfate aqueous system (Bibring et al., 2007). Also, the (2008)foundthatCl-richfluidsseemtohaveinfiltratedintothe
discovery of phyllosilicates (Poulet et al., 2005) indicates the nakhlite source region (cid:2)600Ma ago, whereas SO -rich fluids
3
interactionofslightlyacidictoneutralliquidwaterwithigneous likelypercolatedintotheshergottitesourceregionat(cid:2)180Ma(or
mineralsovergeologicaltimescales(Chevrieretal.,2007). less). Taken together, these studies on meteorites provide
Afteranactivelossofatmosphere(MeloshandVickery,1989; evidenceofsubsurfacewateratverylowwater/rockratiosduring
Brain and Jakosky,1998; Lundin et al., 2004) and surface water the Amazonian period. About the surface, Haberle et al. (2001)
(Clifford and Parker, 2001; Lammer et al., 2003) that occurred provideadetailedexaminationofwhereandforhowlongliquid
duringthelatterpartofthewetterNoachianPeriod,thesolutions water may be present in contemporary times, highlighting that
became more and more evapoconcentrated. In a later stage, a theregionswhereliquidwatermayremainstableforperiodsup
highly saline cryolithosphere derived from the disappearance of to70Martiandayscovernearlyathirdoftheplanet’ssurfacearea.
the ionic supersaturated hydrosphere, especially in the northern ThiscorrelatesremarkablywellwithreportedAmazonianpaleo-
lowlands and in the bottom of craters and depressions. Particu- lakes (Scott et al., 1995; Cabrol and Grin, 2001), and include
larly,HesperianandAmazonianplains-formingunits(e.g.,Tanaka Elysium(theregionthatstraddlestherisetonearGusev)andthe
etal.,2005)covertheNoachianbasementoftheMartiannorthern plainsofArabiaTerra(whichincludesMeridiani).
lowlands(Freyetal.,2002)withrelativelyrecentsecondaryand Realizing that atmospheric changes may have been regional
water-relatedminerals,indicatingextensivesedimentdeposition and even global, brine evolution and subsequent evaporite
occurringwellintotheAmazonianera.ConditionsonMarsmust assemblages in different Martian locations (i.e., Meridiani and
have also beenwetter than today during transient episodes into Gusev)wouldhavebeensomewhatdifferentduetovaryingrock/
geologically very recent times (Faire´n et al., 2003), when minor soil compositions and other local distinctions buffering water to
amounts of water are likely to have covered and infiltrated differentpHlevels. Minoramountsof recent (Amazonian)liquid
individual rocks and unconsolidated rock materials, as well as acidic water, transientlyweathering the sulfate-rich materials in
ponded to form puddles within closed depressions at sub-meter Meridiani, would have contributed to the formation of jarosite
scale.Somethermokarstprocesses,includingtheepisodiclossof outcrops detected by Opportunity. Similarly, minor amounts of
pondedwaterbyevaporationordrainage,arenotedevenforthe acidic water (from precipitation, condensation, or melting of
Late Amazonian (Soare et al., 2008). Also described are quite groundice;e.g.,Haskinetal.,2005)onthemostlybasalticGusev
young dry lake beds and sediments, displaying features such as cratersurfacemayhaveproducedthesulfatecoatingofrocksand
deltas,shorelinesandterraces,whichrequiretheactionofliquid soilmaterialsashighlightedbySpirit.
water over extended periods of time (Cabrol and Grin, 2001). Thefactthatsomeofthesegeochemicalsignatures(jarositeon
Diminished liquid accumulations during the Amazonian Period Meridiani and sulfates on Gusev, see Squyres et al., 2004b;
occurred over terrains cemented with different salts, sulfur, and Stockstilletal.,2005,respectively)andvaryingsurfacecomposi-
iron, thereby increasing the density and acidity of the water tionsdowntothemeterscale,havegoneundetectedintheorbital
solutions, and making the global Martian hydrologic system datasets because of the small scale of the occurrences and
persisting into the Late Amazonian as a heterogeneous and obscuration by mantling with soil and dust, and the fact that
chemically evolving one, with enhanced acidity through time. landers cannotoffera broad geomorphological perspective,both
Thus, the Amazonian Period can be viewed as persistently cold reinforcetheimportanceofamulti-tieredreconnaissancemission
and dry, recording small impact events and pervasive wind architecture for future science-driven and distributed planetary
activity,withprobablecontinuedslowdigestionofbasalticrock/ surfaceandsubsurfaceexploration,whichincludessimultaneous
regolith and silicate rocks by supercold brine films and low-pH orbital, airborne, in situ (surface/subsurface) and sample return
ponds that are episodically liquid. Gas/solid reactions cannot be investigations(Finketal.,2005).Samplereturnwillcontributeto
assumedasthesolealterationprocessintheAmazonian. a first approximation of real age dates for surface features or
TheabovescenariofortheAmazonianPeriodissupportedby alteration, the current lack of which leads to speculation about
gamma ray spectrometer (GRS) and neutron spectrometer potentialagesofmodificationevents,witherrorbarsofabillion
investigationsshowingthepresenceofwaterinequatorialregions yearsormore.
of Mars (Boynton et al., 2002; Jakosky et al., 2005), eventually
related to some hydrated sulfates (Feldman et al., 2004). In
addition,theGRSidentifiedalargeanomalyinchlorineinGusev 5. Conclusions
crater, about 2–3 times the average value (Keller et al., 2006;
Boynton et al., 2007; Newsom et al., 2007), which validates our ThescenarioofAmazonianwateralterationsproposedinthis
hypothesisaboutrecentbrineactivityinthisregion,sincefurther paperhasprofoundimplicationsfortheongoingdebateaboutthe
eolian and other physical alteration would have definitively age of the water-related geology at Meridiani (Squyres et al.,
ARTICLE IN PRESS
A.G.Faire´netal./PlanetaryandSpaceScience57(2009)276–287 285
2004a,b;Christensenetal.,2004)andGusev(Haskinetal.,2005). Bridges,N.T.,Geissler,P.E.,McEwen,A.S.,Thomson,B.J.,Chuang,F.C.,Herkenhoff,
We have argued that both the presence of jarosite deposits in K.E.,Keszthelyi,L.P.,Martı´nez-Alonso,S.,2007.WindyMars:adynamicplanet
asseenbytheHiRISEcamera.Geophys.Res.Lett.34,L23205.
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(Noachian–Hesperian) aqueous alteration and later (Amazonian) Burns,R.G.,Fisher,D.S.,1993.RatesofoxidativeweatheringonthesurfaceofMars.
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episodic occurrence of small quantities of liquid water on the
Cabrol,N.A.,Grin,E.A.,2001.TheevolutionoflacustrineenvironmentsonMars:is
surface of Mars, rather than by early diagenesis only. Therefore, Marsonlyhydrologicallydormant.Icarus149,291–328.
wesuggestthatpartofthemineralogicalhistoryofMeridianiand Cabrol,N.A.,Grin,E.A.,Landheim,R.,Kuzmin,R.O.,Greeley,R.,1998.Durationof
Gusev,asderivedfromdataprovidedbytheMERrovers,reflects the Ma’adim Vallis/Gusev crater hydrogeologic system, Mars. Icarus 133,
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