Large-scale spring deposits on Mars?

Large-scale spring deposits on Mars?

Angelo Pio Rossi,1,2 Gerhard Neukum,3 Monica Pondrelli,4 Stephan van Gasselt,3

Tanja Zegers,1 Ernst Hauber,5 Agustin Chicarro,1 and Bernard Foing1

Received 15 December 2007; revised 18 March 2008; accepted 3 June 2008; published 22 August 2008.

[1] We present a large-scale spring hypothesis for the formation of various enigmaticlight-toned deposits (LTDs) on Mars. Layered to massive LTDs occur extensively inValles Marineris, chaotic terrains, and several large craters, in particular, those locatedin Arabia Terra. Most of these deposits are not easily explained with either a single processor multiple ones, either in combination or occurring sequentially. Spring deposits can havea very wide range of internal facies and exhibit complex architectural variations. Wepropose the concept of large-scale spring deposits for explaining LTDs on Mars.Stable volcano-tectonic settings, such as the ones typical on Mars, are compatible with alarge-scale, long-term, multistage formation of spring deposits. The large-scale springdeposit model can explain the formation of LTDs with a common process, although activein different times and locations, compatible with coeval local or regional processes anddeposits, such as volcaniclastic ones. LTDs, if formed as spring deposits derived fromsubsurface fluids, could potentially offer favorable conditions both to life and to thefossilization of past life forms.

Citation: Rossi, A. P., G. Neukum, M. Pondrelli, S. van Gasselt, T. Zegers, E. Hauber, A. Chicarro, and B. Foing (2008), Large-scale

spring deposits on Mars?, J. Geophys. Res., 113, E08016, doi:10.1029/2007JE003062.

1. Introduction

[2] Mars appears layered at all scales [Malin and Edgett,2000; McEwen et al., 1999; Picardi et al., 2005]. Many ofthe layered rocks cropping out on Mars are most probablyof volcanic origin [McEwen et al., 1999], neverthelessdiscrete but extensive layered sedimentary or sediment-likedeposits are present at several different locations [Arvidsonet al., 2003, 2005; Glotch and Christensen, 2005; Lucchittaet al., 1992; Malin and Edgett, 2000]. Several occur in theValles Marineris canyon system [Lucchitta et al., 1992] andseveral others in various depressions, mostly in craters(where they form characteristic central crater bulges) andin a few areas within the chaotic terrains of, e.g., Iani, Aramand Aureum Chaos (Figure 1).[3] A number of interpretations have been proposed for

Interior Layered Deposits (ILDs) in Valles Marineris[Lucchitta et al., 1992]: the proposed formation mecha-nisms in the literature include sedimentary genesis [Malinand Edgett, 2000], volcanic origin [Chapman and Tanaka,2001], salt diapirism [Milliken et al., 2007], or mechanismsrelated to eolian processes or pyroclastic events [Peterson,

1982]. Despite the various hypotheses proposed during lastdecades, the process of formation of ILDs is still debatedand theories about the extensive hydrated sulfate-rich out-crops in Valles Marineris ILDs [Gendrin et al., 2005] arebeing put forward. In some cases mineralogical variationshave been documented [Murchie et al., 2007] within thelayered sequences, e.g. in Candor Chasma.[4] Also, an origin as evaporite deposits, responsible for

the formation of outflow channels has been proposed[Montgomery and Gillespie, 2005]: this theory would re-quire that the ILDs are older than the Valles Marineriscanyon system. In this paper the discussed light-toneddeposits are designated LTDs, including the ones in VallesMarineris. However, because of the custom in the literature[Lucchitta et al., 1992], those particular deposits are alsomentioned as ILDs in the text.[5] In this study we investigate these features in more

detail by testing the spring deposit hypothesis [Rossi et al.,2006, 2007a] for these layered deposits, discussing thegeological and geometrical characteristics of a number ofLTDs. Although each LTD has its unique features in termsof scale, stacking pattern, and composition, a significant setof characteristics are shared by LTDs described here. Theircommon characteristics led us to propose a generic scenariofor their formation.[6] Spring deposits have been proposed for different

settings on Mars, mainly for small to medium-scale features[Allen and Oehler, 2008; Andersen et al., 2002; Bourke etal., 2007; Crumpler, 2003; Crumpler et al., 2007; Oehlerand Allen, 2008; Ori and Baliva, 1999], such as kilometer-sized spring mounds or single crater bulges. Spring depositsconsist of chemical precipitates which form as a result of

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, E08016, doi:10.1029/2007JE003062, 2008

1European Space Agency, Research and Scientific Support Department,European Space Research and Technology Centre, Noordwijk, Netherlands.

2Now at International Space Science Institute, Bern, Switzerland.3Institut fur Geologische Wissenschaften, Freie Universitat Berlin,

Berlin, Germany.4International Research School of Planetary Sciences, Universita

d’Annunzio, Pescara, Italy.5German Aerospace Center, Institute of Planetary Research, Berlin,

Germany.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JE003062

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emerging water (spring) carrying dissolved species whichreach super-saturation upon emergence of the water to theambient atmospheric conditions [Pentecost, 2005]. Suchsprings may be driven by thermal convection of water:hydrothermal or hot springs, or they may result fromhydrologic gradients [Andersen et al., 2002; Bock andGoode, 1996; Crumpler, 2003].[7] Spring deposits on Earth exhibit an extremely wide

range of features, morphologies and facies associations[Ford and Pedley, 1996; Pentecost, 2005] and variable size,

morphology and internal structure. The typical size forterrestrial spring deposits ranges from several meters toseveral hundred meters, up to few kilometers, while thelocal relief can usually top several tens of meters.[8] This extremely high variability makes complicated

the use of environmental or morphological classifications[Pentecost, 2005]. Among several classification schemes, insitu (autochthonous) and clastic (allochthonous) can bedistinguished [Pentecost and Viles, 1994]. The first groupincludes morphologies such as mounds, fissure ridges,paludal, cascade, dam and different sorts of cementedcrusts. The allochthonous ones comprise alluvial bars andlake or valley fills, thus subject to more extensive transport.[9] The range of actual morphologies that can occur on

terrestrial spring deposits is extremely wide (Figure 2):spring mounds on their own can occur in a variety ofdifferent types, according to local conditions, includingregular mounds, terraced mounds (Figures 2a, 2b, and 2c),almost flat subhorizontal ones, with or without a hostedlake; in addition spring mounds can be characterized byparasite submounds, positioned eccentrically with respect tothe main structure. The possible combination of many endmember morphologies can make the resulting assemblageof different depositional events, over a certain time span,rather complex.[10] The local topography can also affect the final shape of

a spring mound: deposits forming on a slope would have apreferential progradation direction downslope, with an in-creased complexity in the geometry of themound (Figure 2a).[11] Some spring deposit morphologies, such as fissure

ridges (Figure 2d) are formed along structurally controlledlineaments, such as faults, fractures or joints [Pentecost,2005] and the resulting spring deposits can be aligned,although possibly at discrete locations along the alignments,because of local variation of the local fracture/faultinggeometry.[12] Notably terrestrial spring deposits can show strong

and rapid changes of facies and geometry, both at the micro-,meso- (Figures 2e and 2f) and macro-scale [Guo and Riding,1998, 1999; Pentecost, 2005], often because of the rapidlychanging hydrodynamic and chemical conditions over shortdistances from the source areas.[13] All those characteristics in nature can occur com-

bined in a variety of ways, determining a large and complexset of final morphologies, facies association and extents ofthe spring deposits.[14] The role of groundwater in Martian geological history

and its hydrological cycle is being increasingly recognized inrecent years. Groundwater upwelling and evaporation mayhave played an important role in the deposition and subse-quent alteration of rocks and soils at Meridiani Planum[Andrews-Hanna et al., 2007; Squyres et al., 2004]. AlsoValles Marineris shows evidence of groundwater activityduring its evolution [Okubo and McEwen, 2007; Treiman,2008]. There is various evidence pointing toward diageneticactivity on Mars, on the basis of the analogy of Martian‘‘blueberries’’ found in Terra Meridiani [Squyres et al., 2004]to concretions on Earth [Chan et al., 2004].[15] We propose a common formation scenario for some

light-toned deposits (LTDs) in Valles Marineris, neighbor-ing chaotic terrains and crater bulges, interpreting the large,

Figure 1. (a) Global distribution of light-toned depositsindicated by dots of different colors: (LTD) in VallesMarineris (yellow), chaotic terrain (green), and craterbulges (red). (b) Mars Odyssey GRS-derived H2O (red:higher; no data displayed above 45� latitudes) concentrationat low latitudes, showing higher hydrogen concentrationin Arabia Terra, more likely due to mineral hydrationrather than ground ice. Data from Feldman et al. [2004].(c) Hydrated mineral distribution (light blue: sulfates, red:phyllosilicates, yellow: generic hydration). Data fromBibring et al. [2006].

E08016 ROSSI ET AL.: LARGE-SCALE SPRING DEPOSITS ON MARS?

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Figure 2. Examples of morphological variability in terrestrial spring deposits. (a) Travertineterraces in the Mammoth Hot Spring area (Yellowstone National Park, Wyoming); source: K. Bargar,USGS Photographic Library (width of several hundred meters). (b) Terraces in the Yellowstone NationalPark; source J. R. Stacy, USGS Photographic Library (width of several hundred meters). (c) Terrace top atMammoth Hot Springs, with a travertine fan in the foreground; source W. H. Jackson, USGSPhotographic Library (width of several tens of meters). (d) Fissure ridge in the Mammoth Hot Springsarea; source K. Bargar, USGS Photographic Library (width of several tens of meters). (e) Spring moundin Guettaia (Chott el Djerid, Tunisia): its internal structure is visible; the picture is several meters wide;courtesy of G. G. Ori. F. Fine structures at the edge of a pool in the Solfatara hydrothermal area (Pozzuoli,Italy): layering and terracing appear at multiple scales, the lateral extent of the photo is around 10 meters,the vertical around 3 meters; courtesy of M. Glamoclija.


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thick outcrops of layered material as complex assemblage ofspring mounds of varied age.

2. Description

2.1. Global Distribution, Setting, and CommonFeatures

[16] LTDs in Valles Marineris, chaotic terrains and craterbulges share a relative low topographic setting, from thefloor of several chasmata to the low portions of chaoticterrains, or on crater floors (Figure 1); similar deposits inMeridiani Planum are also present, in flat, plateau-likeareas. Globally, most thick bulge deposits in craters arealso at moderate to low absolute elevation. Several commonfeatures can be traced among these thick, architecturally andstructurally complex deposits: the range of thickness isvariable, being greatest in Valles Marineris, with the max-imum thickness exceeding 6–7 kilometers. Thicknessesare strongly correlated with the local relief differences(Figure 3), with lesser thicknesses in Capri Chasma, andgreater thicknesses in Hebes and Ganges chasmata(Figures 3 and 4) [Lucchitta et al., 1994]. Quasi-tabular(Aram) or mound-like (Iani) deposits have a variablevisible/apparent thickness, with a maximum of fewhundreds of meters (Figure 5). Crater bulges, regardlessthe size of the host crater, are up to 2.5–3 km thick(Figure 6), in most cases not exceeding 1.5 km. The levelof erosion is varied for both crater bulges and the impactcrater structures, suggesting a wide range of formation ages,although the degree of induration could also affect theobserved level of erosion.[17] Deposits in Valles Marineris [Quantin et al., 2006]

range in elevation of the base from about�4000 to�1500 m(with respect to MOLA (Mars Orbiter Laser Altimeter)aeroid-referenced topography) (Figure 3). The floors ofcrater bulges, concentrated in Arabia Terra and surroundings(Figure 1), have a slightly wider elevation distribution, withminimum elevations as low as �4000 m, and maximumvalues not exceeding 0 m.[18] Moreover, a significant number of these mound-

shaped deposits occur in closed depressions, such as HebesMensa and most deposits in crater bulges, which are

surrounded by high plateaus. In only a few cases thebounding plateaus or rims are partially eroded (e.g., Galecrater northern rim).

2.2. Setting-Dependent Features

[19] The LTDs in Valles Marineris, being the thickest andmost extensive ones among those treated in the presentwork, are also the ones with the largest morphological andfacies variability. A wide range of forms is present, rangingfrom thick, massive deposits, to finely layered sequences(Figures 4a–4d), with very different possible subenviron-ments present within the same chasma (e.g., Candor, Melas,etc.). Extensive unconformities have been documentedwithin the LTD in Valles Marineris (e.g., Candor Mensa)by various authors [Fueten et al., 2006; Mangold et al.,2007]. At regional scale, some of the deposits in VallesMarineris, such as those in Ophir and Candor Chasma and,slightly less, in Capri Chasma show a streamlined surfacegeometry in plan view, suggesting the action of fluid during/after their emplacement. At a multikilometer scale, LTDs inValles Marineris show a tabular to mound-like morphology.Both vertical and horizontal stacking pattern variationsare visible at HRSC (High Resolution Stereo Camera)[Jaumann et al., 2007; Neukum et al., 2004a] scale (10–20 m/pixel). Both albedo and architectural variability is stillhigh at MOC (Mars Orbiter Camera)/HiRISE (High Reso-lution Imaging Science Experiment) scale in certain areaswithin LTDs (Figure 4).[20] Valles Marineris ILDs, although sharing several

common features, locally show local morphological andapparent facies variation over a very short spatial distance,e.g., quasi-horizontal finely bedded sequences bounded byunconformities.[21] Chaotic terrains host relatively small (in terms of

volume) LTD patches or extended cover, e.g., in AramChaos. Their thickness to extent ratio is among the lowest.They show a certain variability of surface texture, beingoften affected by eolian erosion, with possible yardangs[Sowe et al., 2007]. At a scale of tens of meters they tend toshow flat, subtabular morphologies, although even theflattest looking deposits in Aram Chaos have a slightly

Figure 3. Valles Marineris elevation range map (MOLA) in kilometers. One-kilometer-spaced contoursare smoothed and not highlighted within Valles Mariners for clarity. Interior layered deposits are markedby a white outline. Plateaus surrounding the Valles Marineris are always higher than the top of LTDs,providing necessary hydraulic heads for eventual spring deposits on the floor of chasmata.


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bulged expression [Glotch and Christensen, 2005; Masse etal., 2007].[22] Their overall appearance is rather mound-like, re-

gardless the amount of erosion. Remarkable light-toneddeposits are also present, e.g., in Iani Chaos, as two maindistinct bodies, a few hundred kilometers distant from eachother (Figures 5a and 5b). In Aram Chaos, both thethickness and elongation of the light-toned deposits showevidence of structural control [Oosthoek et al., 2007]. Also,geological and geomorphological mapping [Glotch andChristensen, 2005] in Aram Chaos showed evidence oftilting and bulging of the LTDs within Aram [Glotch andChristensen, 2005]. Possible structural control (doming/bulging) on LTDs in Valles Marineris is also emerging from

attitude analysis over large areas where those deposits cropout [Fueten et al., 2006].[23] It is clear at all scales, from HRSC to HiRISE, that

LTDs in chaotic terrains are onlapping on knobs and mesas(Figures 5a and 5c), thus postdating chaotic terrain forma-tion [Sowe et al., 2007].[24] Crater Bulges appear as pristine to heavily eroded

tabular to mound-like structures located in a number ofcraters with diameters above several tens of kilometers.[25] Deposits in crater bulges have the widest range of

latitudinal and longitudinal distribution, although most ofthem are concentrated in Arabia Terra (Figure 1). Theirelevation distribution is less wide, being usually at moderateelevations (��2000 ± 2000 m) above MOLA datum.

Figure 4. Valles Marineris LTDs: examples of candidate spring deposits, all in perspective view, drapedon HRSC-derived DTM, except for Figures 4e and 4f. (a) Candor Mensa: the eroded mound-like reliefshows festoon or terrace-like layering, indicated by white arrows. Internal unconformities are presentwithin the deposits (HRSC nadir from orbit 2116). (b) Hebes Mensa (HRSC nadir from orbit 2138).(c) Line drawing of material deposited from a lateral source in Hebes Mensa, showing multiple stages indeposition. (d) Candor Mensa viewed from SW (THEMIS V03561002 draped on HRSC stereo-derivedDTM from orbit 3217). (e) Light-toned deposits from Candor chasma (HiRISE red channel imagePSP_001390_1735). The dark material in the top left portion of the image appears like loose (or slightlyindurated) eolian cover, not bedrock, consistently with a post-canyon formation emplacement of LTDs.(f) Schematic section representing Valles Marineris LTDs both in terms of relative thickness andgeomorphological setting.


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[26] All LTDs in crater bulges mentioned and analyzedhere share a lack of signs of relevant lacustrine activity intheir respective basin. This includes evidence of terraces,deltas, fan deltas [Pondrelli et al., 2005] and significantdrainage basins associated with the crater itself.[27] Also, there is no or little evidence of fluvial activity

in the immediate surroundings of the craters hosting bulges

and within their rim. A few non-eroded crater bulges appearto be mantled, with low exposure of underlying material.High-resolution observations, such as with HiRISE imagesover Gale Crater (Figure 6c) show finely layered brightmaterial resembling sedimentary rocks. LTDs in craterslocally show evidence of structural control in their deposi-tion/emplacement [Bridges et al., 2007].

Figure 5. LTDs in chaotic terrain: examples. (a) Perspective view of Iani Chaos northern light-toneddeposits (HRSC nadir from orbit 934). Example of contacts between LTDs and knobs in Iani Chaos ishighlighted by white arrows. (b) Iani Chaos: perspective view of southern light-toned deposits (HRSCnadir from orbit 934 and MOC NA E1202584, outlined in black, draped on HRSC stereo-derived DTMfrom orbit 934). (c) HiRISE red image (PSP_002892_1760) showing convoluted highly eroded LTDs inAureum Chaos and their relationship with a small knob, similarly, but at a different scale, to what isvisible in Figure 5a. (d) Schematic section of LTDs in chaotic terrains.


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2.3. Volumes, Composition, and Internal Geometry

[28] Volume determinations show a wide range of values:the largest ones belonging to thicker, low-lying deposits inValles Marineris. The volume of most ILDs in VallesMarineris is of the order of 104 – 105 km3 (althoughsmaller ones are present), with a total volume of 105 –

106 km3 in the Valles Marineris interior [Lucchitta et al.,1994]. This is roughly one order of magnitude lower thanthe total eroded or removed volume: i.e., the missingvolume ideally filling the canyon system to the top of thesurrounding plateau; it is not implied that this missingvolume has ever been filled with deposits, we simply

Figure 6. Examples of LTDs in crater bulges. (a) Gale crater (HRSC nadir mosaic from orbit 1916,1927, and 1938). Solid black lines indicate bedding; dashed black lines indicate possible unconformitieswithin the mound. Qualitative representation of attitude is indicated. The bedding in the upper centralportion of the mound is clearly dipping, while the lower crescent-shaped stack of light-toned material ishorizontally layered. The white circle indicates approximate location of HiRISE image in Figure 6c.(b) Crommelin Crater, a few hundred kilometers north of Meridiani Planum [Andrews-Hanna et al.,2007] (HRSC nadir mosaic from orbit 3253 and 3264). (c) HiRISE view, centered at 5.33�S and138.33�E (in the background portion of the perspective view in Figure 6a) of the basal subhorizontalportion of Gale crater light-toned deposits (HiRISE image PSP_001897_1745, red channel). (d) Idealizedsection of a crater bulge.


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compare the differences in magnitude between LTDs andmaximum potential available volume [Lucchitta et al.,1994].[29] The following volume estimates have been con-

ducted using Geographic Information System-based (GIS)tools in order to calculate thicknesses and volumes fromdigital raster topography data. The volumes have beencalculated assuming a base level of the deposits derivedfrom interpolation of available bedrock outcrops at thebottom of the layered sequences [Oosthoek et al., 2007];uncertainties in the measurements are lower for LTDslocated in rather flat areas (e.g., Gale crater floor) andhigher for LTDs in more rugged terrains, such as Iani Chaos(characterized by small-scale mesas and knobs).[30] LTDs in chaotic terrains such as Aram or Iani are

much smaller, starting from few tens to a few hundred km3,e.g., above 30–40 km3 (uncertainty because of unknownbasal topography) in Iani southernmost LTDs, as measuredfrom the HRSC Digital Terrain Model (DTM) [Gwinner etal., 2005; Jaumann et al., 2007; Neukum et al., 2004a].Volumes in crater bulges vary from a few tens of cubickilometers in small bulges, up to 103–104 km3 in largeones. For Gale and Henry craters total volumes of about3000 km3 and of about 6000 km3, respectively, have beenmeasured. Within the bulge, its upper, steeper and appar-ently clinostratified portion (Figure 6) contains approxi-mately 700 km3, as derived from both MOLA and HRSCDTMs.[31] Quantitative measurements of bedding attitude in

Valles Marineris ILDs show outward dipping layers [Fuetenet al., 2006; Gaddis et al., 2006; Hauber et al., 2006],consistent with a mound-like structure. Several depositsshow central (crater bulges) or elongated symmetry (Hebesand Gangis Mensae). In particular, Hebes Mensa is E–Woriented, showing consistency with some of the regionaltectonic trends, e.g., [Schultz, 1998].[32] Evidence of tectonic deformation within LTDs in

different settings, both in ILD [Fueten et al., 2006;Mangoldet al., 2007; Okubo and McEwen, 2007] and craters[Bridges et al., 2007] have been found by various authors,suggesting synsedimentary deformation of LTDs.[33] Several unconformities have also been documented,

qualitatively and quantitatively [Fueten et al., 2006]. Evenhorizontal or subhorizontal layers, such as those in thedeposits of Hebes Mensa, are difficult to trace over longdistances, showing more or less low lateral continuity. TheGale crater bulge [Rossi et al., 2007b] is characterized byinternal unconformities, the largest and most evident beingthe one separating the horizontally bedded crescent-shapedbasal mound and the steeper, less cratered upper mound(Figure 6). Apparent facies variations are visible in mostdeposits, both laterally and vertically (Figure 6). Thebedding dip and direction and the attitude of the contactsbetween different subunits have been observed and mea-sured [Rossi et al., 2007b] using high-resolution topographyand coregistered ortho-rectified imagery, in order to observethe 3D location of the interfaces (bedding, unconformities,etc.) and their respective relationships. In this way angularunconformities could be easily spotted and mapped.[34] Gale crater central mound is locally higher than the

current rim, which is differentially eroded (mostly thenorthern side), but the highest point actually corresponds

to what is probably the central peak of this 150 km widecrater; the northern rim, coinciding roughly with the Mar-tian dichotomy boundary, is located in an area whichappears affected by degradation, collapses and incipientchaotic terrain formation [Watters et al., 2007].[35] It is debated [Lucchitta, 2001; Malin and Edgett,

2000] whether all LTDs are exhumed or not and to whichextent. In any case LTDs with enigmatic origin across Marsrange in age from possibly Noachian [Malin and Edgett,2000] to Amazonian [Scott and Tanaka, 1986].

2.4. Crater Size-Frequency Dating

[36] For several characteristic surfaces crater size-fre-quency measurements have been performed in order toconstrain surface ages. This has been done using the lunarcrater production, adapted to Martian rates [Hartmann andNeukum, 2001; Ivanov, 2001; Neukum and Hiller, 1981].Available cratering models have been used in order toobtain isochrones [Hartmann and Neukum, 2001; Ivanov,2001]; a discussion on the errors in the method is providedby Neukum et al. [Neukum et al., 2004b].[37] Dating based on crater size-frequency analysis

(Figures 7 and 8) has been performed using a variety ofdata sets (Figure 7) on various examples among the light-toned deposits presented in this study, covering a wide agespan (Figure 8). Nevertheless, all dated deposit surfaceshave a relatively young exposure age, being in generalyounger than 0.5 billion years (Gyr).[38] A few age measurements have been performed on

ILDs in Valles Marineris, dated in the literature as UpperHesperian – Amazonian [Lucchitta et al., 1992; Lucchitta,1999].[39] Ages as young as 10 Myr have been determined for

Hebes Chasma (Figure 4b) and these very young ages aretherefore likely to be erosional ones: this age should becarefully evaluated and is among the less reliable amongthose presented here. The ILDs in Juventae Chasma havebeen dated using both HRSC and MOC data [Neukum et al.,2007] providing ages as young as about 400 Myr.[40] LTDs in chaotic terrains such as Iani or Aureum

Chaos appear moderately eroded, with low crater densities.Measured ages on the slightly bulged LTD in Aram Chaosprovide values of around 300million years (Myr) and 70Myr(Figure 8) in its eastern portion (Figure 7). Also, a resurfac-ing event around 100 Myr seems likely, as visible from thecharacteristic kink in the distribution of the crater size-frequency measurements (Figure 8).[41] All young ages derived from crater size-frequency

measurements may indicate the timing of deposition of theuppermost LTD layer. In most cases it is rather more likelythat either erosional effects have lead to a general underes-timation of true ages or that deposition of, e.g., wind-blownmaterial have masked true ages which are consequentlysupposed to be significantly younger. A decision on whichprocess is more likely cannot be taken.[42] Iani Chaos is characterized by two main distinct

bodies of light-toned deposits, both embedding chaosknobs: a northern outcrop (Figure 5a) and a southern one(Figures 5b and 7). Dating of both deposits providesconsistent ages, around 10 Myr, in terms of the lastresurfacing event (Figure 8). These isolated, small, easily


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erodible deposits within Iani Chaos show remarkably com-parable ages, although they are separated by almost 200 km.[43] LTDs in Gale and Crommelin Crater bulges have

been dated using crater counting (Figures 7 and 8). Cratersize-frequency analysis for Gale crater central bulge wascarried out using HRSC, CTX (Context Camera) and MOCdata (Figure 7). The Gale basal tabular unit (Figures 6 and7) has a measured age of about 100 Myr; the younger agelimit obtained for the uppermost part of Gale mound isaround 5 Myr. This apparent range is consistent with thegeological and geomorphological observations on the var-iably eroded bulge (Figure 6). This age is not necessarily adepositional one, but the age range could provide anestimate of the time interval of activity during the formationof the bulge itself.[44] Therefore two main age groups emerge from our

dating on 3 different classes of deposits (LTDs in Valles

Marineris, chaos deposits, crater bulges): an older set ofdepositional/resurfacing events at a few hundred Myr (e.g.,Aram, Crommelin) and a younger one close to 10–20 Myr(Iani, Hebes, Gale, etc.).[45] In general, the ages cover a certain time span, but

most of them are relatively young. Some, e.g., in GaleCrater, are surprisingly recent, suggesting some kind ofgeological activity in the very recent geological past[Neukum et al., 2004b, 2007]. This activity could be likelyrelated to eolian erosion, but it would not necessarily be theonly active process/agent, in the case of relatively younglate-stage depositional activity.

3. Discussion

[46] The presence of different geometries, stacking pat-terns and multiple unconformities co-existing often in the

Figure 7. Context maps used for crater counting of selected LTDs. (a) Aram Chaos, layered deposits,image center located at 349.1�E, 1.5�N, image width 52 km, image HRSC 0967_0000. (b) CrommelinCrater, crater bulge, image center located at 349.9�E, 5.0�N, scene width 45 km, MOC image M0302716,background images HRSC 3253_0002 and 3264_0000. (c) Hebes Chasma, layered deposits, imagecenter located at 283.3�E, 0.9�S, image width 9 km, MOC image E1700518, background image HRSC2138_0000. (d) Gale Crater, crater bulge, image center located at 73.5�E, 2.9�S, image width 95 km,CTX images T01_000881_1752_XI_04S223W, P02_001897_1747_XI_05S221W,P02_001752_1753_XI_04S222W, P01_001620_1749_XI_05S222W, P01_001554_1745_XI_05S221W,P01_001422_1747_XN_05S222W, and P01_001356_1747_XN_05S221W, background images HRSC1916_0000, 1927_0000, and 1938_0000. (e) Southern Iani Chaos, layered deposits, image center locatedat 355.7�E, 18.5�S, image width 12 km, MOC images E0301845 (measurements area), M1401230, andR1001264, background image HRSC 0934_0000. (f) Northern Iani Chaos, layered deposits, image width5.5 km, image center located at 341.4�E, 0.8�S, MOC images R1600246, R0904228, R090025,R0802256, and R0200623, background image HRSC 0934_0000; numbered labels refer to measurementareas shown in Figure 8.


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same chasma or crater suggest variations in the sedimentaryevolution (Figures 4, 5, and 6). Spatial and temporalvariations in spring mounds could lead to vertical andlateral variations in sedimentary environments, facies andinternal geometries [Pentecost, 2005]. A complex internalarchitecture, developed during multiple formation stages, iscommon on spring mounds on Earth [Martin-Algarra et al.,2003]: Terrestrial spring deposits do show the co-existenceof several different sedimentary subenvironments within thesame assemblage of spring deposits [Guo and Riding, 1999;Martin-Algarra et al., 2003; Pentecost, 2005].[47] LTDs in Valles Marineris postdate the canyon for-

mation, and although this has been previously challenged[Catling et al., 2006; Malin and Edgett, 2000; Montgomeryand Gillespie, 2005; Schultz, 1998] there are strong evi-dences of a post-canyon formation of ILDs [Lucchitta et al.,1992; Lucchitta, 1999; Mangold et al., 2007, 2008;Quantin et al., 2006]. Also, LTDs in chaotic terrains suchas Iani and Aram Chaos clearly postdate chaos formation[Glotch and Christensen, 2005], as visible from the strati-graphical relationships with knobs and mesas (Figure 5).This is evident in most crater bulges, too (e.g., Figure 6).[48] Tectonic, possibly synsedimentary, deformation of

LTDs [Bridges et al., 2007; Fueten et al., 2006; Mangold etal., 2007; Okubo and McEwen, 2007] is apparent indifferent relatively large outcrops of LTDs, such as VallesMarineris ILDs and crater bulges. Such tectonic structures,if active during the emplacement of LTDs, could supplysubsurface fluids [Okubo and McEwen, 2007]. Some

authors have presented evidence of groundwater flow alongfault lines in Valles Marineris [Treiman, 2008], increasingthe evidence of a potentially structurally controlled supplyof subsurface fluids [Okubo and McEwen, 2007; Treiman,2008], thus supporting the idea of spring deposits forming atthe surface. This is in agreement with the evidence ofstructural control in the emplacement and stratigraphicalevolution of various LTDs on Mars, from chaotic terrains[Glotch and Christensen, 2005; Masse et al., 2007;Oosthoek et al., 2007] to crater bulges [Bridges et al.,2007]. Structurally controlled lines could have providedsuitable pathways for subsurface fluids to circulate in thesubsurface of Martian upper crust [Rodriguez et al., 2003]and to reach the surface and lead to the formation ofspring deposits.[49] The stable volcano-tectonic settings on Mars could

potentially provide long-lived fluid supply to the surface,especially in Valles Marineris: this could justify, possiblytogether with the lower Martian gravity, compared to Earth,much larger spring deposit buildups. The still very largescale difference between spring deposits on Earth and thecandidate spring deposits on Mars discussed here might bemore easily explained given the longer time span availablefor their progressive formation, with multiple discreteevents in the same area over long time. No spring depositon Earth has had as much time (and endogenic power)available as on Mars: in this perspective the lifetime ofTharsis volcanic activity, exceeding 3.5 billion years (Gyr.)[Neukum et al., 2004b], would provide a reasonable engine

Figure 8. (Left) Stratigraphical context for LTDs in Valles Marineris, chaotic terrain, and crater bulges,and (right) crater size-frequency measurements or selected areas (see Figure 7). The stratigraphical rangeswith dark outlines refer to work done in earlier work: (N) [Neukum et al., 2007], (1) [Lucchitta et al.,1992], (2) [Scott and Tanaka, 1986], (3) [Lucchitta, 1999], (4) [Witbeck et al., 1991], and (5) [Rotto andTanaka, 1995]. For a discussion on methodology and errors, see Neukum and Hiller [1981] and Neukumet al. [2004b].


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for driving groundwater over large distances [Andrews-Hanna et al., 2007] for a long time. Regional slopes wouldalso provide an important control on groundwater flow[Andrews-Hanna et al., 2007] in the surroundings of ArabiaTerra and Meridiani Planum: this may also explain theconcentration of LTDs in Arabia Terra [Allen and Oehler,2008; Oehler and Allen, 2008].[50] The extent of this Earth-Mars spring deposit scaling

is not well constrained and should be further investigated.Moreover, a few of the (less extensive) LTDs are not veryclose to major tectonic provinces, although they could behave been affected by more local effects. Among craterbulges, Gale is located in a more important tectonic region,being positioned on the dichotomy boundary.[51] Smaller, thinner deposits such as LTDs in chaotic

terrains (e.g., Iani) are apparently less affected by tectonicdeformation: they could relate to supplies of subsurfacefluids active for shorter periods of time, possibly derivingfrom smaller and/or more local sources.[52] The age estimates of these deposits vary consider-

ably (Figure 8) in the literature, from Noachian to Hesperian[Malin and Edgett, 2000; Scott and Tanaka, 1986] and insome cases Upper Hesperian to Amazonian [Lucchitta etal., 1992] (Figure 8). Moreover, because of the relativelyhigh erodibility of LTDs, the crater counting age couldrecord portions of the craters embedded in the depositsduring their formation, which might have been in multiplestages, as we suggest in the present work. Therefore theactual age of the uppermost portions of some LTDs (e.g.,Gale upper mound, Figure 7d) might be much younger thanthat derived from all embedded craters.[53] In particular, the assumed age of several crater bulges

is Noachian [Malin and Edgett, 2000], but a wider agedistribution is present [Greeley and Guest, 1987; Scott andTanaka, 1986]. Notably, several bulges show very lowcrater densities, both for pristine and exhumed impactcraters.[54] Our new age measurements show relatively (locally

very) young ages for LTDs in different geological settings.Although these new ages should be taken with some care,they definitely show that the geological activity, and possi-bly not only the erosion, might be sensibly younger thanNoachian and possibly Hesperian as indicated by theabsence of large craters.[55] Absolute measurements ages show some correlation

among a few of the LTDs described in the present work.Among all dated deposits, some clustering of absolute agesappears at a few million years and a few hundreds ofmillions of years. Interestingly, they share ages of the sameorder of magnitude with recent volcanic, fluvial and glacialactivity on Mars [Neukum et al., 2004b, 2007].[56] Alleged Noachian crater bulges [Malin and Edgett,

2000] in Arabia Terra, similarly to what is proposed forMeridiani Planum soils [Andrews-Hanna et al., 2007] couldbe possibly related to earlier Tharsis activity, while ChaoticTerrains and Valles Marineris could be linked to later stages(Figure 9). Hematite bearing LTDs in Aram Chaos areconsidered to be substantially younger than those in Mer-idiani Planum [Glotch and Christensen, 2005].[57] New extensive dating would be needed for constrain-

ing the age of several crater bulges, since most currentlyavailable age estimates are based on Viking-era data or are

poorly constrained: our first attempts, presented here,showed very young ages that do not necessarily reflectthe true ages of formation [Malin and Edgett, 2000],however further focused dating efforts would provide betterconstraints for the formation of LTDs in crater bulges.[58] Links could be drawn between multiple depositional

events visible in LTDs, both in Valles Marineris and largebulges, such as Gale, and multistage Tharsis activity[Andrews-Hanna et al., 2007; Neukum et al., 2004b].Indeed, Tharsis activity dates back to the Noachian anddeveloped during Hesperian and Amazonian [Scott andTanaka, 1986], but the volcanic, and likely hydrothermaland enhanced hydrologic activity, has persisted possibly upto recent times [Neukum et al., 2004b, 2007].[59] Eroded, extensively layered bulges such as those

visible in Crommelin (Figure 6) and its unnamed southernneighbor are indeed just a few hundred kilometers fromMeridiani Planum, where there is clear evidence for ground-water activity [Andrews-Hanna et al., 2007; Chan et al.,2004; Grotzinger et al., 2005; Squyres et al., 2004]. There isa clear spatial correlation between the assemblage of LTD-bearing craters close to Crommelin and the findings ofsulfates within the soil [Andrews-Hanna et al., 2007;Grotzinger et al., 2005] explored by one of the MarsExploration Rovers (MERs) (Opportunity).[60] Hydrated minerals have been detected in various

areas of Mars [Bibring et al., 2006], and mono and poly-hydrated sulfates are widespread in Valles Marineris[Gendrin et al., 2005]. A complex association of sulfatesand hematite in various segments of the canyon system,such as Ophir and Candor chasmata is present [Weitz et al.,2008], similarly, although with differences possibly becauseof the different erosion history and physiographic setting[Weitz et al., 2008], to what has been observed by variousauthors in Aram Chaos [Glotch and Christensen, 2005] andMeridiani Planum [Hynek, 2004].[61] The sulfate-bearing outcrops within the LTDs in

Meridiani Planum show evidence of aqueous processes[Grotzinger et al., 2005, 2006]. It has been suggested thatthe original sedimentary environment for the formation ofthese LTDs could have been a playa/sebkha [Grotzinger etal., 2006].[62] In principle, putative playas in Meridiani Planum

would be possibly benefit from groundwater availability[Andrews-Hanna et al., 2007] and both rover and orbiterdata analysis show that a rise of the water table could havebeen responsible at least for the alteration of the volcanicbedrock [Arvidson et al., 2006]. In addition, a fumarole-driven hydrothermal alteration of volcanic and volcaniclas-tic material in Meridiani Planum has been invoked in orderto explain the diagenetic features observed by the MERrover Opportunity [McCollom and Hynek, 2005]. The‘‘playa’’ interpretation for the deposits in Meridiani Planum,possibly associated with groundwater movement and evap-oritic pumping is in principle compatible with springactivity and the formation of spring mounds [Komatsu etal., 2007]. Therefore Meridiani playa-like LTDs and Crom-melin mound-like LTDs could be the two faces of the samemedal.[63] There is also a strong correlation between other

LTDs described here and the detection of hydrated mineralsin Arabia Terra (Figure 1c), where regional high abundances


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Figure 9


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of hydrogen have been detected [Dohm et al., 2007;Feldman et al., 2004] (Figure 1b).[64] The mineralogy of Martian spring deposits, appar-

ently dominated by sulfates, would be the biggest differencewith terrestrial counterparts, being in most cases carbonates[Pentecost, 2005]. However, locally sulfate-dominatedspring deposits are known on Earth as well [Pentecost,1995; Drake et al., 2004]. Still, this major difference isprobably related to the geological and geochemical evolu-tion of Mars at large and the lack of large deposits ofcarbonates [Chevrier et al., 2007; Halevy et al., 2007].Sulfates have been found to be locally very abundant onMars from both orbiter [Gendrin et al., 2005] and rover[Grotzinger et al., 2005; Squyres et al., 2004] missions,therefore putative subsurface fluids would be sulfate-richand likely under acidic conditions [Chevrier et al., 2007;Tosca et al., 2005].[65] Variations on the hydrological, physical and chemi-

cal regime through time could justify variations in compo-sition and mineralogy [Murchie et al., 2007] along thestratigraphical sequence of LTDs (Figure 9). Moreover,the possible detection of mafic minerals interbedded withsulfates in Valles Marineris ILD [Murchie et al., 2007], forexample in Hesperian to Amazonian units [Lucchitta, 1999]could be related to the presence of tephra trapped inside theLTDs produced by Tharsis volcanic activity (Figure 9), assuggested in the case of Hebes Chasma [Peterson, 1982].[66] Tharsis volcanic activity might have been in part

coeval with the emplacement of ILDs in Valles Marineris[Lucchitta et al., 1992; Lucchitta, 1999; Neukum et al.,2004b], regardless the origin of the light-toned depositsthemselves. Volcanic activity from Tharsis might well havedispersed volcaniclastic material over large distances [Headand Wilson, 1998; Wilson and Head, 1994].[67] Estimates of maximum hydraulic heads for Valles

Marineris are compatible with the present maximum topo-graphic relief of ILDs, since none is higher than immedi-ately surrounding plateaus (Figure 3). Those theoreticalmaximum values would require the latest spring depositsto be formed at the top of the LTDs in the canyon systemand to have the presence of confined aquifers, with thepossibility of a temporary water fill of the basins. Lateralaccretion rather than vertical over multiple generations ofspring mounds would require smaller hydraulic heads andlocal aquifer confinement, which is likely to be achievedwith cementation within the mounds themselves.[68] Gale crater appears to be an exception, but its

position at the very edge of the crustal dichotomy boundary,where extensive erosion, degradation and chaos formationhas occurred [Watters et al., 2007], could justify thesignificant erosion and reduction of relief of its rim.[69] The thickness and base level of ILDs show a strong

correlation with both plateau elevation and location(Figure 3), with inferred hydraulic heads diminishing east-

ward (e.g., Capri Mensa is less thick than Candor, OphirMensae) in the Valles Marineris canyon system [Lucchittaet al., 1994]. This correlation would strongly suggest the roleof regional and local hydrogeology in the emplacement andshaping of ILDs (Figure 3). A certain amount of erosion ofLTDs should also be taken into account, depending on thesetting: likely minor in a completely closed basin (e.g., HebesChasma), but eolian erosion (not easily constrained) on theseapparently not very resistant materials should also be takeninto account. The topographic setting of Valles Marineriswith respect to the Tharsis bulge (Figure 3) is consistent withthe presence of (at least in the past) aquifers beneath theTharsis bulge and in the high plateaus surrounding thecanyon region [Harrison and Grimm, 2007].[70] The possibility of such a large volume of spring

deposits would require the presence of a significant amountof water in the subsurface. The presence of such waterreservoirs has been proposed for the ancient pre-Tharsisdevelopment of the Valles Marineris area [Dohm et al.,2001] (Figures 9b and 9c). Although terrestrial springdeposits are mostly related to meteoric waters [Pentecost,2005], Martian ones could be more related to juvenilesources. The extremely long lived Tharsis super-plumecould have led to a significant supply of water from deepmantle sources [Farmer, 1996].[71] In the case of Valles Marineris ILDs, the formation of

spring deposits would generally postdate the canyon for-mation itself, consistently with an emplacement of ILDswithin an already formed canyon [Chapman and Tanaka,2001; Lucchitta et al., 1992].[72] The fluted or streamlined shape of various LTDs in

Valles Mariners, mainly Ophir, Candor and Capri Mensae,indicates at least a morphological record of fluid flow anderosion, possibly related to large-scale outflow and/orsmaller-scale spring activity. If the flow were syn-postValles Marineris formation, this would put some constrainton the relative formation/modification age for these depos-its. Ganges Mensa, which is located in a semi-open chasma,appears much less streamlined. More obviously, HebesMensa does not show major signs of unidirectional flowmodifications, being in a closed basin.[73] The scenario of spring deposits formed after the

emersion of subsurface fluids would be consistent with apseudokarst-like mechanism [Otvos, 1976], between Tharsisrise and the plateaus surrounding Valles Marineris, compat-ibly with subsurface cavernous systems as proposed bysome authors [Rodriguez et al., 2003].[74] Thus, summarizing, we propose a scenario for the

large-scale spring deposit hypothesis on Mars: Valles Mari-neris LTDs are interpreted as an association of large-scalespring mounds (in a broad sense) formed over varyingdurations with discrete steps of activity, possibly related toperiods of increased volcanic activity. This activity, likelypartially or totally contemporaneous to the formation of

Figure 9. (a) Conceptual model showing the possible complexity of multistage spring mound formation interacting withpreexisting morphology and Tharsis volcanic activity (SM: spring mounds, V: volcaniclastic layers). Thick dashed blackline symbolizes current erosion level. (b) Proposed model relating Valles Marineris, close cavi, Tharsis, and LTDs (not toscale). (c) Speculative scenario for the development of large collapses and Valles Marineris within the proposed hypothesis,with progressive development of collapses (possibly combined with tectonic deformation), which also serve as the basin formultistage large coalescing spring mound produced LTDs, such as Hebes Mensa.


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LTDs would have been recorded within the sedimentaryrecord, possibly as tephra (Figure 9a). The amount oferosion which occurred after the formation and emplace-ment of the LTDs is not easily constrained (ideally indicatedby the thick dashed line in Figure 9a). Especially in the caseof Valles Marineris, the close presence of Tharsis and itslong-lasting activity and possibly large aquifers beneath theplateaus [Dohm et al., 2007] would offer a reservoir ofsubsurface fluids (Figure 9b), which could have supported:(1) subsurface erosion and mass transfer, producing collap-ses and formation/enlargement of cavi/chasmata [Rodriguezet al., 2003] and (2) provided dissolved sulfates to bedeposited within the depressions/collapsed areas as springdeposits formed over multiple stages (Figure 9c). Anassociation of collapses and tectonic deformation duringinitial and later stages of Valles Marineris development[Schultz, 1998] is also possible: such a complex combina-tion of processes would provide even more pathways forsubsurface fluids to reach the surface [Treiman, 2008]. Thediagram in Figure 9 summarize the large-scale springdeposit scenario, including the structure and interplaybetween ILDs formation and the Tharsis volcaniclasticcontribution (Figure 9a), the relationship between the Thar-sis province, Valles Marineris and smaller cavi and chas-mata (Figure 9b), and the possible extrapolation throughtime of the scenario presented here (Figure 9c). The possiblelong-term role of tectonic extension in Valles Marineris isalso highlighted (Figure 9c).[75] Large, coalescing spring-mounds, with local varia-

tions of facies, environment and geometry, active in differ-ent times as a response to increased activity, linked toTharsis volcanism [Andrews-Hanna et al., 2007; Neukumet al., 2004b] and related to its products, would explain thegeological variability of LTDs in Valles Marineris and moreperipheral deposits in chaotic terrains. Smaller-scale wide-spread activity, not only related to Tharsis, could be linkedto the peculiar mounds in large craters, mostly in Arabia.The need for long-term strict Earth-like conditions [Baker etal., 1991] would not be a too stringent requirement in orderto explain the deposition of LTDs in Valles Marineris andchaotic terrains as spring deposits: still, liquid water shouldbe stable, but the climate would not have to be as warm andwet as that needed to develop and sustain a full hydrologiccycle. In particular, ILDs in Valles Marineris, which dateback to Hesperian or Amazonian [Lucchitta et al., 1992;Lucchitta, 1999], were deposited during possibly unfavor-able climatic and environmental conditions [Bibring et al.,2006], which would be too harsh to support the develop-ment of lacustrine basins. In this respect, some of theadvantages and the attractive features of the subice volcanohypothesis [Chapman and Tanaka, 2001] are shared by thelarge-scale spring deposits scenario presented here, with theadvantage of not requiring a large stable presence of iceoccupying the canyons and chasmata.[76] The present and future robotic missions to Mars

could offer the possibility of testing, up to a certain extent,the large-scale spring deposit hypothesis presented here.[77] Not all the specific characteristics of spring deposits

on Earth could be used to uniquely identify their Martiancounterparts, including micro- or meso-facies, in the ab-sence of lander/rover close-up data over candidate springdeposits. A combination of features that could possibly be

of help in testing the spring deposit hypothesis for LTDsinclude those listed below. In brackets the spatial scale ofthe feature is indicated, classified into mega- (at the scale ofthe entire deposit: as visible from medium-high resolution,wide-swath satellite imagery, e.g., HRSC) macro- (at thescale of several km to few hundreds of meters, e.g., asvisible on high-resolution images, e.g., MOC, CTX), meso-(at the scale of hundreds to few meters, e.g., HiRISE),micro- (at the scale of the hand specimen/micro-imager,e.g., with imaging instruments on rovers):[78] 1. Sedimentary-like appearance of the deposits at

regional, local and outcrop scale (mega-/macro-/meso-/micro-).[79] 2. General mound-like morphology (mega-/macro-/

meso-).[80] 3. Coexistence of different textures (e.g., on remotely

sensed data), facies, morphologies over short distances(macro-/meso-).[81] 4. Local compositional homogeneity or systematic/

periodical compositional variations along the stratigraphicalsequences. (macro-/meso-/micro-).[82] 5. Evidence of fault or fracture-controlled deposition

of the sedimentary-like layers (macro-/meso-).[83] 6. Presence of possible source areas within the extent

of the deposit, for autochthonous spring deposits (macro-/meso-).[84] 7. Directional variation of thickness/facies over short

distances ascribable to discrete sources (macro-/meso-).[85] 8. Correlation between LTD extent/elongation and

local/regional structural trends (mega-/macro-).[86] 9. Morphology and internal structure (from both

orbital images and outcrop observations, e.g., with rovers)suggesting draping, accretion or progradation over bedrockor other LTDs (macro-/meso-/micro-).[87] 10. Evidence for non-volcanic origin (e.g., compo-

sition compatible with sedimentary deposits, as derivedfrom imaging spectrometers on board orbiters and rovers).[88] A few of the features listed above could be detected

and studied with remote sensing instruments, either mediumto high-resolution imagery and high-spatial (and spectral)-resolution images and spectrometers; some others could bemore reliably addressed with detailed observation at themeso- and micro-scale, which would make current andfuture landing missions more suitable.[89] Single processes could not easily produce such a

large morphological, geometrical, textural and composition-al variability in the resulting LTDs. Alternative mechanismsable to produce sedimentary deposits and morphologiessomehow similar to those produced by spring activityinclude: volcanism [Chapman and Tanaka, 2001] or fluvio-lacustrine processes [Pondrelli et al., 2005]. Locally facies oflacustrine or spring origin could be confused, but lacustrineones would have a much wider lateral continuity and theywould occur within a traceable basin. Of course spring-feddeposits can include lacustrine or paludal facies as well[Pentecost, 2005], but in the context of local sources ratherthan with a basin-wide occurrence. A point of discrimination,as mentioned above, would then be the stronger vertical andlateral facies variation in any of the LTDs, if formed as aspring mound or as the result of the coalescence of severalindividual mounds over a large period of time.


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[90] Therefore the key for testing the spring-deposithypothesis on Martian LTDs is the co-occurrence of severalfeatures (like those listed above) at multiple scales, fromboth orbiter and rover platforms, using multiple data sets(such an approach is indeed useful for most geologicalproblems).[91] Although the task of uniquely identifying spring

deposits within sedimentary material on Mars is challengingand not easily solvable by robotic missions orbiting orlanding, in particular future high-mobility rover missions,such as Mars Science Laboratory (MSL) or ExoMars, couldprovide both the detailed morphological, geological, min-eralogical information together with the local/regional out-line needed to constrain the nature of eventuallyinvestigated LTDs (if any of the several LTDs mentionedhere are indeed selected as landing sites).[92] In the meantime, further work on comparing the

stratigraphical, compositional, geometrical and structuralfeatures of Martian LTDs with terrestrial spring deposits isneeded in order to better constrain the combination of keyfeatures to be matched.

4. Conclusions

[93] We propose a large-scale spring mound origin forvarious light-toned deposits on Mars. Given the wide rangeof possible architectural styles, facies, morphologies andsurface composition possibly resulting from spring mounds,this concept can explain in a comprehensive way severalMartian light-toned deposits. Although precipitation fromhydrothermal waters has been proposed for certain LTDs,for the hematite associated with them [Catling and Moore,2003; Chan et al., 2004; Glotch and Christensen, 2005;Weitz et al., 2008] and for several deposits in craters, thescenario proposed here explains the actual buildup of majorlight-toned deposits on Mars.[94] The spring deposit scenario particularly suits the

materials with strong analogies with sedimentary rocks,but not easily fitting with lacustrine or volcanic genericinterpretations, unlike clearly fluvio-lacustrine basins suchas Holden [Pondrelli et al., 2005]. If light-toned deposits,unevenly distributed on the surface of Mars, are indeed,even just in part, spring deposits; this would have severalimplications for the geological evolution of Mars and thevariety of its sedimentary environments and their possiblepotential for life. Also, spring deposits locally as young asAmazonian [Lucchitta, 1999] would have offered a suitablesetting for life to sustain and fossilize even in a very ‘‘dry’’scenario for Martian post-Noachian hydrological evolution[Bibring et al., 2006], being a possibly protective sheltereven during harsh surface and atmospheric conditions.Testing of the large-scale spring deposit scenario for LTDson Mars with future landing missions, could offer valu-able new information on Mars’ still enigmatic geologicalfeatures.

[95] Acknowledgments. We thank the HRSC Experiment Teams atDLR Berlin and Freie Universitat Berlin as well as the Mars ExpressProject Teams at ESTEC and ESOC for their successful planning andacquisition of data as well as for making the processed data available to theHRSC Team. We acknowledge the effort of the HRSC Co-InvestigatorTeam members and their associates who have contributed to this investi-gation in the preparatory phase and in scientific discussions within the team.

We thank MOLA, MOC, THEMIS, CTX, and HiRISE respective teams formaking data available to the public through PDS. Thanks for insightfulcomments and discussion to Nicolas Mangold, Vincent van Hinsberg, andPaul Mason. We greatly thank Aileen Yingst and an anonymous reviewerfor their thorough and constructive comments and suggestions that providedvaluable information and substantially improved the manuscript. Ourwarmest thanks to Greg Michael for reading the final manuscript andproviding precious comments. Many thanks to Mihaela Glamoclija andGian Gabriele Ori for providing useful pictures. This work was carried outduring a research fellowship at the European Space Agency (ESA).

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��A. Chicarro, B. Foing, and T. Zegers, European Space Agency, Research

and Scientific Support Department, European Space Research andTechnology Centre, Keplerlaan 1, PO Box 299, 2200 AG Noordwijk,Netherlands.E. Hauber, German Aerospace Center, Institute of Planetary Research,

Rutherfordstrasse 2, D-12489 Berlin, Germany.G. Neukum and S. van Gasselt, Institut fur Geologische Wissenschaften,

Freie Universitat Berlin, Planetologie und Fernerkundung, Malteserstrasse74-100, D-232, D-12249 Berlin, Germany.M. Pondrelli, International Research School of Planetary Sciences,

Universita d’Annunzio, V. le Pindaro, 42 65127 Pescara, Italy.A. P. Rossi, International Space Science Institute, Hallerstrasse 6,

CH-3012 Bern, Switzerland. ([email protected])


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