ARTICLE IN PRESS

Planetary and Space Science 57 (2009) 276–287

Contents lists available at ScienceDirect

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´n a,Ã, Dirk Schulze-Makuch b, Alexis P. Rodrı´guez c, Wolfgang Fink d, Alfonso F. Davila a, Esther R. Uceda e, Roberto Furfaro f, Ricardo Amils g, Christopher P. McKay a a NASA Ames Research Center, Space Science and Astrobiology Division, Moffett Field, CA 94035, USA b School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164, USA c Planetary Science Institute, Tucson, AZ 85719, USA d California Institute of Technology, Visual and Autonomous Exploration Systems Research Laboratory, Pasadena, CA 91125, USA e NASA Ames Research Center, Biosciences Division, Moffett Field, CA 94035, USA f Aerospace and Mechanical Engineering Department, University of Arizona, Tucson, AZ, USA g Centro de Astrobiologı´a (CSIC–INTA). 28850-Torrejo´n de Ardoz, Madrid, Spain article info abstract

Article history: The Mars Exploration Rover (MER) missions have confirmed aqueous activity on Mars. Here we review Received 21 February 2008 the analyses of the field-based MER data, and conclude that some weathering processes in Meridiani Received in revised form Planum and Gusev crater are better explained by late diagenetic water-rock interactions than by early 19 November 2008 diagenesis only. At Meridiani, the discovery of jarosite by MER-1 Opportunity indicates acidic aqueous Accepted 20 November 2008 activity, evaporation, and desiccation of rock materials. MER-based information, placed into the context Available online 6 December 2008 of published data, point to local and limited aqueous activity during geologically recent times in Keywords: Meridiani. Pre-Amazonian environmental changes (including important variations in the near-surface Mars groundwater reservoirs, impact cratering, and global dust storms and other pervasive wind-related Liquid water erosion) are too extreme for pulverulent jarosite to survive over extended time periods, and therefore Amazonian we argue instead that jarosite deposits must have formed in a climatically more stable period. Any Mars Exploration Rovers Meridiani Planum deposits of pre-existent concretionary jarosite surviving up to the Amazonian would not have reached Gusev crater completion in the highly saline and acidic brines occurring at Meridiani. MER-2 Spirit has also revealed evidence for local and limited Amazonian aqueous environmental conditions in Gusev crater, including chemical weathering leading to goethite and hematite precipitation, rock layering, and chemical enhancement of Cl, S, Br, and oxidized iron in rocks and soils. The estimated relative age of the impact crater materials in Gusev indicates that these processes have taken place during the last 2 billion years. We conclude that minor amounts of shallow acidic liquid water have been present on the surface of Mars at local scales during the Amazonian Period. & 2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 277 2. Jarosite dates Amazonian liquid water at Meridiani Planum...... 277 2.1. Meridiani jarosite is pulverulent ...... 278 2.2. Meridiani jarosite is concretionary ...... 280 2.3. The aqueous history of Meridiani Planum during the Amazonian ...... 280 3. Amazonian water-surface interactions in the Gusev crater rocks and soils ...... 281 3.1. Early MER investigations ...... 282 3.2. Evidence for brines on the surface of Gusev ...... 282 3.3. The history of water at Gusev ...... 283 4. Setting MER results in the context of other datasets: a model for geologically recent acidic brines on Mars ...... 284

à Corresponding author. Tel.: +1650 604 3842; fax: +1650 604 3992. E-mail address: afairen@arc.nasa.gov (A.G. Faire´n).

0032-0633/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2008.11.008 ARTICLE IN PRESS

A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287 277

5. Conclusions ...... 284 Acknowledgements ...... 285 References...... 285

1. Introduction presence of very limited amounts of liquid water on the surface of Meridiani in times as recent as the Late Amazonian. Aside from safety considerations, landing sites for the Mars Gusev impact crater (lat. 14.51S, lon. 184.61W) has long been Exploration Rovers (MER) Opportunity and Spirit in the Meridiani hypothesized to be the site of debouchment of Ma’adim Vallis, a Planum and Gusev crater regions, respectively, were selected large structurally controlled outflow channel, which includes because they were previously identified as potential locations of ponding to form lake deposits (Cabrol et al., 1998). Ma’adim is past water and related sedimentary deposition based on distinct interpreted to have recorded either multiple Noachian–Hesperian morphologic and mineralogic indicators (Golombek et al., 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, answering the question of when and where liquid water has 2004) or a single large flood event from the release of a putative been present on the surface of Mars is a first-order objective for 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 even present times. prime candidate for potential hydrothermal activity on Mars (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 paleolake within Gusev crater (Cabrol et al., 1998). The Spirit rover Surveyor spacecraft, see Christensen et al., 2001). It was interpreted investigations, at least for the limited areas covered, suggest that 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 obscure the hypothesized sedimentary record. Here, we argue that 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 (Columbia Hills) up to the Late Amazonian. 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 600 m and an areal extent of about 3 Â105 km2 et al., 2006) that MER observations indicate very ancient weath- (Hynek, 2004). This outcrop of hematite, which has been ering of surface materials, followed by billions of years of absence interpreted to consist of basaltic rock materials with 10–15% of liquid water on the surface. Particularly, the presence of jarosite crystalline grey hematite deposits (specularite, Fe2O3), may have in Meridiani Planum has been taken as evidence for an extremely been precipitated 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, the latest field-based information acquired from the MER is Squyres et al., 2004a, b; Hynek, 2004; Andrews-Hanna et al., 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 (McCollom and Hynek, 2005)orimpact(Knauth et al., 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 activity proposed in this paper refers to the collection of moisture crystalline goethite, and schwertmannite in Meridiani outcrops on rock surfaces and the flow, infiltration, and/or possible ponding (Farrand et al., 2007). An extended diagenetic history in the of localized brines. Our conclusions indeed support very ancient presence of an episodic water system of high ionic strength deposition of surface materials in water-enriched environments (McLennan et al., 2005; Tosca et al., 2005) aptly explains the origin during the early times of Mars history, but also indicate the of hematite: by heating and dehydroxilation of goethite, which in presence of scarce water at diverse locations throughout the turn precipitated 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, dissolving ancient basaltic and sulfide phases and promoting geochemical alteration at present times, as we discuss in the evaporitic sequences. The timing of all these events is unknown, following sections. and we argue here that, although they essentially occurred in the early history of Mars, the latest of them could be related to the 2. Jarosite dates Amazonian liquid water at Meridiani Planum

Table 1 The only place where jarosite has been found on Mars is in Absolute age estimates for the surface of Mars. bedrock outcrops in Meridiani Planum, where up to 28% of the Fe Epoch Absolute age range (Ga) is in the form of this hydrated ferric sulfate mineral (Klingelho¨fer et al., 2004), as revealed by the exploration rover Opportunity Late Amazonian 0.6–0.3 to present along the traverses to Eagle and Victoria craters (Fig. 1). Jarosite Middle Amazonian 2.1–1.4 to 0.6–0.3 Early Amazonian 3.1–2.9 to 2.1–1.4 has a rather distinctive Mo¨ssbauer signature, so it is easy to detect Late Hesperian 3.6 to 3.1–2.9 and to place an upper limit on its abundance. But Mo¨ssbauer Early Hesperian 3.7 to 3.6 spectroscopy is not sensitive to Fe-free sulfate minerals, so the Late Noachian 3.82 to 3.7 detected jarosite is possibly a mixture of sulfates and mafic Middle Noachian 3.95 to 3.82 siliciclastic grains (debris). This mineralogical assemblage is Early Noachian 43.95 indicative of a complex diagenetic history in the Meridiani plains, Condensed from Fig. 14 of Hartmann and Neukum (2001). which are Noachian in origin, but also record modification ARTICLE IN PRESS

278 A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287

The aqueous alteration of sulfides also leads to nanocrystalline jarosite, with crystals o500 nm 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 et al., 2006) or have a biologically induced origin (Amils et al., 2007). To date, the morphology of Meridiani jarosite is still unknown, and thus we will consider both possibilities in our analysis.

2.1. Meridiani jarosite is pulverulent

Rio Tinto is an acid–sulfate dominated fluvial system with near constant acidic pH (about 2.3) along its 100 km 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 minerals found along the river derive from aqueous alteration of 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’s exploration around Victoria crater, in Meridiani Planum. To Meridiani must have remained strictly constant, and similar to the see an up to date location of Opportunity in Meridiani, see /http://marsrover.na- arid current conditions, during the entire time for pulverulent sa.gov/mission/traverse_maps.htmlS. Image courtesy NASA/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 destruction due to continuing and powerful weathering processes 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 and the Hesperian, which include decomposes to ferric oxyhydroxides in humid climates. 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, with no subsequent aqueous alteration for billions of years, pressure, and temperature due to (i) large excursions in or in a more recent period of local liquid water stability embedded planetary obliquity (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 with the consideration that Meridiani 2004), and/or (iii) impact effects (Segura et al., 2002). A large jarosite is an ancient mineral assemblage. 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 near neutral conditions of the surface, and because of the lack of 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. It is usually found in the oxidized zones of sulfide deposits, (e.g., Baker, 2001). Such substantial variations in atmospheric and forms through the reaction of dilute sulfuric acid derived from pressure, temperature and humidity would have destroyed the oxidation of pyrite. The oxidation of pyrite (Burns and Fisher, pulverulent jarosite assemblages in relatively brief periods of 1990, 1993) and pyrrhotite (the usual primary sulfide in basaltic time. Only if Meridiani water was in equilibrium with jarosite, 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 chemical alteration. If this was the case, water must have also (Chevrier et al., 2004, 2006). been in equilibrium with Mg-sulfates, as there is very strong Jarosite is a brittle secondary mineral, with a Mohs’ hardness of evidence that these minerals exist in the bedrocks (Clark et al., 1 1 22 to 32, and is usually found as amber-yellow to brown crusts or 2005), and this family of minerals is among the most soluble 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) with jarosite and Mg-sulfates existing in Meridiani at any time pulverulent, in nodules or fibrous masses, earthy, including would have dissolved the hypothesized ancient jarosite right granular crusts where indistinguishable crystals are made up of away. Even reaction with atmospheric water vapor can fibers and form uniformly large masses, best exemplified in mine 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, as distinct fine-grained crystals, first forming every summer and dissolving after the first rain fall. described in 1852 in El Jaroso Hydrothermal System, which is the (b) Impact cratering, especially before the Amazonian, resulting world locality of jarosite (Martı´nez-Frı´as et al., 2007; see Fig. 2). in a number of impact craters in varying states of degradation, ARTICLE IN PRESS

A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287 279

Fig. 2. Jarosite localities and types. Above: Satellite image of Spain showing with dots the location of Rı´o Tinto (left) and El Jaroso (right). Below: pulverulent jarosite from Rı´o Tinto (left) and concretionary jarosite from El Jaroso (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 dust deposition cover the previous soil (Yen et al., 2005). The Noachian impacts) temperatures and pressures which affect bright local dust deposits on Mars are interpreted to be part of jarosite, as it begins dehydroxylation at over 700 K. Also, a global unit (Yen et al., 2005), indicating mobilization of large excavation of target materials, bedrock deformation, exposure amounts of dust and the associated burial and/or exposure of of fractured rocks, emplacement of fresh ejecta blankets with rocky surfaces, as well as the dramatic change of the Martian widely varying grain sizes, formation of perched coarser landscape and albedo on decadal scales (Geissler, 2005), all of fragments, deposition of ejecta blocks on the surface, and which collectively mark the significant influence of global impact-related crushed layers mantling previous deposits, are circulating winds across the planet. The surface is thus usual consequences of bolide impacts possibly affecting covered by dust from annual global storms, and then deflated jarosite assemblages. Water may have also contributed to by winds capable of completely stripping away the dust, crater modification, including the local redistribution of exposing the rock outcrop (Soderblom et al., 2004), as well as fragments to form pavements (Fig. 3) and similar small-scale resulting in erosion of the bedrock and reorienting ubiquitous landforms. small sinuous crested eolian bedforms (Sullivan et al., 2005). (c) Wind activity, in this case through the entire geological These processes of sand movement and bedform development 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). Delicate assemblages of pulverulent jarosite embedded transport including jarosite and other sulfates in the form 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- transported dust (as there are no detection of jarosite in the related weathering processes during billions of years. 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 descent images indicate ongoing pervasive wind erosion. 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 ARTICLE IN PRESS

280 A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287

Fig. 3. Evidence for recent liquid water on Mars. (A) Opportunity picture 1N143189015EFF3243P1961L0M1, obtained on Sol 169 inside Endurance crater. The channelled features may have been formed by overland water flowing through fractures. Centimeter-sized fins high standing in the vein-like feature, highlighted in the inset, are rich in hematite, and seem to be so delicate that their presence suggests a very recent epoch of formation. (B) Composite image of several Opportunity pictures taken on Sol 208, of plate-like rocks on the southwestern slopes of Endurance crater. The patterns of polygons and cracks appear to have been formed after the final desiccation of a recent wet environment, leaving cracked mud behind. This kind of polygonal texture is seen on all sides of free-standing rocks, for example Wopmay, also in Endurance crater, around Sols 250–264. Moreover, the plate-like rocks have been chemically altered to a greater extent than the rocks located higher in the stratigraphic crater rim, as indicated by greater levels of chlorine (see Fig. 4). Although alternative interpretations can be proposed, such as aeolian erosion or recent exposure of ancient features (for A), and fracturing by the force of the crater-causing impact, the dry up of the original water that formed the rocks, or dehydration of hydrated sulfates on exposure forming dehydration cracks (for B), a recent origin for both in a watery episode having occurred long after the wet period when the rocks first formed is the most likely hypothesis after compelling geological and geochemical analyses. An interesting terrestrial analogue for the recent formation of these fins and cracks in Martian sulfate sand is provided by Chavdarian and Sumner (2006), and also Squyres et al. (2006b) describe recent modification in Meridiani outcrop rocks forming the fins. Images courtesy 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 been a deflation area for quite some time, in this unlikely scenario 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 fresh surface of jarosite-containing rock outcrops. Such an ancient 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- Meridiani does not imply per se the absence of liquid water on the 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, would likely have been in the form of episodic, highly saline and impact temperatures and pressures, impact mantling, or destruc- acidic brines, derived from intense evaporation and/or sublima- tive global dust storms removing uppermost deposits. No other tion processes through transient migration of groundwater to the mechanisms exist on Mars to expose ancient sediments, and surface. In the anticipated recent aqueous phases, the hydroliza- sandblasting and weathering would rather bury and destroy than tion of concretionary jarosite would not have reached completion exhume any pulverulent jarosite exposed at the surface. The same (Barro´ n et al., 2006). Instead, concretionary jarosite assemblages can be argued to explain the survival of any soluble salts in would have remained stable in the bottom part of impact craters, exposed outcrops. Therefore, pulverulent jarosite in Meridiani where they may interact with these cold and acidic brines. would likely be the result of a late (i.e., Amazonian) sequence of Therefore, previous investigations that consider jarosite as an precipitation events in ponded brines, where jarosite would be indicator of water-limited chemical weathering (Elwood Madden dissolved when water is available and reprecipitated in the arid et al., 2004) no longer qualify as an argument to rule out the epochs, in a similar way as is recorded in Rio Tinto. presence of water on the surface of Mars during the Amazonian. Assuming a prolonged diagenetic history in the Burns formation (McLennan et al., 2005), at least some of the concretionary jarosite 2.2. Meridiani jarosite is concretionary may have been a relatively late precipitate of the brines, rather than an early-forming mineral. Jarosite mineralization could even 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 transient aqueous activity (Barro´ n et al., 2006). of the time pulverulent, or forming very small crystals. Terrestrial concretionary jarosite from El Jaroso is genetically linked to the calc-alkaline shoshonitic volcanism of the SE Mediterranean 2.3. The aqueous history of Meridiani Planum during the Amazonian margin of Spain, dating from the Upper Miocene (Martı´nez-Frı´as et al., 2004, 2007), a mode of formation so far not very relevant to After the previous analysis of the mineralogy in Meridiani, we Mars (and especially not relevant for Meridiani) compared to conclude that jarosite formed in a geologically recent, transient other process such as aqueous weathering. wet environment, as scenarios under which jarosite might persist Only in the case that jarosite in 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, and the mineral deposit was immediately buried after Amazonian climate. In summary, we propose that the presence of formation, jarosite would have been protected from humidity, pulverulent jarosite in Meridiani would imply the activity of ARTICLE IN PRESS

A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287 281 acidic liquid water during the Amazonian, while the existence of (2008) have concluded that the alteration history of Meridiani concretionary jarosite could not be used as an argument to outcrop rocks did not end with the early groundwater-mediated exclude the presence of Amazonian acidic brines. The important diagenetic features described in previous publications. 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 factor of 4 to 5 times higher than soil materials in the other four 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 ratio in excess of 0.84 (Klingelho¨fer et al., 2004). According to our 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, which led to the formation of jarosite in local surficial Amazonian was not a continuous body the size of the sulfate- depressions. During periods of very limited aqueous activity, the bearing unit that extends over regional scales (up to 3 Â105 km2) breakdown of jarosite caused goethite precipitation, and posterior (Hynek, 2004; Gendrin et al., 2005; Arvidson et al., 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 273 K (Kuzmin directly to hematite (Barro´ n et al., 2006), as both goethite and and Zabalueva, 1998; Gendrin et al., 2005; Faire´n et al., 2008), in hematite are direct products of the hydrolysis of jarosite. This the form of an open acid–sulfate weathering regime (Golden et al., extended diagenetic history finally resulted in the assemblage of 2005). The water would have been acidic and oxidizing, reacting hematitic spherules (Squyres et al., 2004b; Zolotov and Shock, with the basaltic surfaces to produce sulfates and iron oxides. The 2005). Hematite may also be an early mineral phase, due to its diagenetic process likely prolonged in time during the Amazonian, very low solubility, although it can dissolve quite easily in acidic including episodic inundations by shallow acidic and salty surface conditions. Such aqueous activity appears to have been recurrent water followed by evaporation/sublimation, and desiccation. For during the Amazonian, in the form of episodes of shallow surface example, rocks located deep within Endurance crater display a acidic water, infiltration and subsequent desiccation. As a result of higher degree of chemical alteration when compared to the rock the recurrent infiltration of limited amounts of water, jarosite was 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, see Arvidson et al., 2006) compared to more rock outcrops, indicating that the spherules have been formed in geologically recent times (Amazonian). Alternatively, we favor the watery environments 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 leaving a film at the bottom of the crater in recent times. This is here to have contributed to jarosite formation observed both in also supported by the erosional features described at the surface Eagle, Fram, and Endurance craters, as well as in rocks exposed at of the crater, evidencing the activity of minor amounts of post- Anatolia trough. Also, various stratification styles (planar lamina- 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, shallow crack systems observed on outcrops identified at Meridiani, indicating deposition from subaqueous of the Burns formation can equally be originated in the dunes in an environment characterized by episodic inundation by precipitation and dissolution of salts, moisture changes or partial surface water via the rise of an aquifer to shallow depths, followed dehydration of some minerals (Chan et al., 2008). Also Knoll et al. 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). As the outcrop was infiltrated by surface liquid H2O, that water likely became hypersaline, with soluble sulfates and Surface chlorides, or enriched in Na and Cl (Knoll et al., 2008). Brushed post-RAT 3. Amazonian water-surface interactions in the Gusev crater rocks and soils

101 Decades of Mars exploration and derived photogeological Escher / Virginia interpretations have illuminated an evolutionary sequence for Gusev crater, which includes Ma’adim Vallis-related flooding and associated ponding and sedimentation on at least more than one occasion during the Noachian and Hesperian (Scott et al., 1993; Grin and Cabrol, 1997; Cabrol et al., 1998; Kuzmin et al., 2004). 3 2 5 3 2 3 2 3 O O

Ni The multi-levelled terraces in the crater interior were interpreted CI Br Zn 2 O O 2 O O 2 2 2 2 SO K TiO CaO SiO MgO

P to mark episodic flood-related erosional events, while the smooth Na Al MnO Cr Fe 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) into Endurance crater. Escher’s levels of chlorine relative to Virginia’s proposed paleohydrologic history, revealed through geologic and went up, and sulfur down, implying that the surface of Escher has been chemically geomorphic mapping, was the basis for selecting Gusev as the first altered to a greater extent than the surface of Virginia. Data obtained by the Mars Exploration Rover Opportunity’s alpha particle X-ray spectrometer. Image courtesy of the two MER targets, with the twofold objective of (1) collecting NASA/JPL/Cornell/Max Planck Institute. in situ evidence to test the hypothesis of the existence of an ARTICLE IN PRESS

282 A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287 ancient paleolake, and (2) determining the characteristics of this to the previously hypothesized wetter impact crater paleoenvir- putative ancient depositional environment. onment, noted particularly for the West Spur area of the Columbia Hills, where the record preserved in the ancient rocks points to 3.1. Early MER investigations 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 ranges from moderate to extensive (Cabrol et al., 2006; Ming et al., plains-forming materials (encountered at the landing site and 2006). along the traverse to the Columbia Hills, see Fig. 5) within the The interaction between the volcanic rocks and minor amounts Gusev impact crater did not support the purported flood- of water has been highlighted by Spirit’s in situ local reconnais- lacustrine environment, but rather a mostly volcanic surface sance. Aqueous-related activity within Gusev impact crater is mainly composed of dark, fine-grained, vesicular rocks interpreted accentuated in the Columbia Hills through the identification as basaltic lava flows (Squyres et al., 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- recorded in the plains-forming materials. The composition of the 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 soluble salts) in rocks and soils (Farrand et al., 2006; Gellert et al., encountered at the Viking and Pathfinder landing sites, inter- 2006). In addition, Haskin et al. (2005) reported high concentra- preted to be the possible result of global mixing and distribution tion of S, Cl, and Br in both multi-layered coatings and interiors of 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, and rocks are angular and dispersed across the surface, associated with alteration zones or veins, suggesting that water suggesting that they are by-products of nearby impacts (McSween played a major role in Cl/Br fractionation within the crater (Gellert et al., 2004). This led to the initial interpretation that the et al., 2004). ‘‘Mazatzal’’ shows high concentrations of sulfur, evolution of the crater surface was dominated by physical rather 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 et al., 2004; Morris et al., 2004) over a largely volcanic surface. water and subsequent evaporation. ‘‘Humphrey’’ shows bright material in interior pores, crevices, and cracks that are similar to minerals crystallized out of water flowing through volcanic rocks 3.2. Evidence for brines on the surface of Gusev on Earth. The identification of goethite in rocks in the Columbia Hills, such as significant concentrations in the rock ‘‘Clovis’’, Further along the traverse, Spirit encountered a major geologic confirms aqueous processes in the West Spur (Morris et al., 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 important part of its structure (10% H O by weight). Hematite is assemblages observed in the Columbia Hills (Crumpler et al., 2 also abundant in both rock outcrops and mantles of the Columbia 2005; Ming et al., 2006). Here, after some 160 Sols following touch 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 ?)

3

Intensity 2

1

0

0 Velocity Fig. 5. Spirit’s traverse through Columbia Hills. To find the exact location of Spirit, see /http://marsrover.nasa.gov/mission/traverse_maps.htmlS. Image courtesy Fig. 6. Mo¨ssbauer spectrum (200–220 K) showing the presence of hematite and NASA/JPL/Cornell/MRO-HiRISE/New Mexico Museum of Natural History and goethite in a rock called ‘‘Clovis’’ in the Columbia Hills. This is definitive evidence Science. for water activity in Gusev. Image courtesy NASA/JPL/University of Mainz. ARTICLE IN PRESS

A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287 283

Optically thick coatings of fine-grained ferric iron-rich dust associated ponding in topographic depressions during the Middle dominate most bright soil and rock surfaces at Gusev. Noachian Period, prior to the formation of Ma’adim Vallis. Subsurface materials, exposed by trenching and wheel move- Geochemical conditions during this more ancient period of Mars ments of the rover, show additional evidence for aqueous point to moderately to highly acidic waters, due to the CO2- processes. Spirit measured higher Mg, S, and Br concentrations dominated atmosphere and the steady supply of sulfate and iron in rock materials exposed in ‘‘Boroughs’’ trench compared to to the water accumulations (Faire´n et al., 2004). Subsequently, surface materials. Another example are rock materials exposed by episodic flooding ensued during the Late Noachian and into the the movement of Spirit’s wheels referred to as ‘‘Paso Robles’’, Hesperian, resulting in the formation of Ma’adim Vallis, as well as where Fe is increased, SO3 represents more than 30% wt, and the multi-tiered terraces cut into the crater floor and rim materials; sulfate and phosphate contents are the highest measured so far on water input into the crater included relatively small flows that Mars (Gellert et al., 2006; Ming et al., 2006; Morris et al., 2006). dissected Hesperian terrain to form a large number of small valley Interestingly, Mg-sulfates in trenches appear to be in a higher networks. Impact cratering and wind activity dominate the post- hydration state than is potentially stable at the surface under flood record. current Mars atmospheric conditions (as speculated by Wang Minor aqueous activity extended into the Amazonian, followed et al., 2006), suggesting that they may have formed very recently, by wind-related activity prevalent at present-day, which involves perhaps during the most recent period of high obliquity (Head mantling and etching of flood-related deposits and landforms. et al., 2006), an epoch during the Late Amazonian that may have One of the more important processes that may suggest provided elevated humidity and somewhat warmer temperatures. 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 separated according to solubility. Bromine is also observed to be equivalent of the smectite montmorillonite (Clark et al., 2007) elevated in materials that fill the vugs and veins of plains-forming could argue for a higher pH; but as it seems to be an amorphous basaltic rocks where chemical mobility and separation are form of pre- or post-montmorillonite, we favor the interpretation 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. completion in the low-pH environment that characterized water Positive correlations between salt components have also been occurrences at Gusev. identified in the trenches (Haskin et al., 2005). In addition, the Traces of water based on data from instruments onboard Spirit soluble elements S and Cl are exposed at the surface, 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 MgSO4 phase have been mobilized and geologic units and resulting in the formation of surficial thin concentrated in near-surface intercrater plains-forming materials crusts through recent evaporation of brines (Herkenhoff et al., exposed by rover activities (Crumpler et al., 2005). Wang et al. 2006). The redistribution of Cl, S, Br, and oxidized iron may have (2006) concluded that the geochemical trends observed in the been mobilized by thin films of liquid water; these mobile phases trenches are best explained by ion transportation via fluids, 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 aqueous activity could have occurred during the Late Amazonian (Herkenhoff et al., 2006) observed on the top layer of some soils epoch. and rocks may include atmosphere/surface interactions, as the atmospheric pressure at Gusev is above the triple-point pressure of water and the melting point of surficial water could be 3.3. The history of water at Gusev substantially depressed by the high ionic content of the surface layers (Cabrol et al., 2006). This is consistent with Spirit data Taken together, the geological and geochemical data collected showing generally enhanced concentrations of S, Mg, Br, Cl, and 3+ in the field at Gusev indicate an evolutionary sequence consistent Fe /Fetotal in the trenched soil materials when compared to the 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 suggesting initial aqueous alteration of volcanic rocks in an acidic and paleohydrologic activities recorded in the Gusev impact crater environment followed by mostly physical weathering. Elevated and surroundings include significant widespread flooding and and highly variable Br concentrations in stratigraphically young soil materials of Gusev crater argue for episodic brine deposition and evaporation (Haskin et al., 2005), even during the Amazonian Period, with the water and soil interactions occurring at unknown 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 exposed by the actions of the rover wheels (Yen et al., 2008). The minimum exposure time of surface rocks to acidic liquid water in Gusev is 22 ky (Hausrath et al., 2008), likely due to episodic aqueous alteration; but the age of the rocks and the total time they have been placed at their current locations is still unknown, so it is possible that some of these alteration episodes took place recently. The coating, which characterizes most rock surfaces explored by Spirit, may have been produced by varying amounts of water, though only small quantities of transient daily to seasonal acidic liquid water are necessary to explain the above Fig. 7. Stratigraphic diagram showing the ages of five different geologic units in mentioned geochemical signatures. Consequently, the long-term the Ma’adim/Gusev complex hydrologic system. Duration of the Amazonian time is well in accordance with all published Martian cratering geochronologies. After net change of the surface at Gusev at global scales has been very Cabrol et al. (1998). small (Golombek et al., 2006). ARTICLE IN PRESS

284 A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287

4. Setting MER results in the context of other datasets: a model remixed the fragile sediments into the planetary homogeneous for geologically recent acidic brines on Mars soil. GRS has also shown variations of potassium relative to thorium (Taylor et al., 2006), typical indicators of water transfer Much of the exposed Martian rock surfaces generally consist of and weathering. Also the CRISM instrument onboard the Mars volcanic materials, over which chemically different waters have Reconnaissance Orbiter has revealed the presence of H2O- and weathered the late-stage basaltic surfaces through time. Aside SiOH-bearing phases and hydrated Fe-sulfates, including jarosite, from the Meridiani plains, hydrated sulfates have been detected in on late Hesperian to Amazonian deposits on and surrounding extensive dune fields in the north polar regions, as well as within Valles Marineris (Milliken et al., 2008), implying extensive recent Valles Marineris and in Aram Chaos (Gendrin et al., 2005). This water alteration on the surface of Mars. suggests that in these places acidic and oxidizing waters, provided SNC meteorite information is also consistent with liquid water by volcanogenic SO3 and HCl and atmospheric CO2 and H2O, have on Mars during the Amazonian: Baker et al. (2000) conclude that reacted with the mafic–ultramafic surface, dissolving rocks and meteorite-sourcing Martian rocks were intermittently exposed to producing hydrated sulfates and iron oxides, while no carbonates water, Swindle et al. (2000) argue that the Lafayette meteorite are evident as a result of the acidity of the early water masses was weathered by liquid water during the past 650 million years, (Faire´n et al., 2004). In fact, Meridiani-like ground water systems Borg and Drake (2005) suggest episodic flooding at the surface seem to have been widespread and representative of an extensive during almost the entire geological history of Mars, and Rao et al. acid-sulfate aqueous system (Bibring et al., 2007). Also, the (2008) found that Cl-rich fluids seem to have infiltrated into the discovery of phyllosilicates (Poulet et al., 2005) indicates the nakhlite source region 600 Ma ago, whereas SO3-rich fluids interaction of slightly acidic to neutral liquid water with igneous likely percolated into the shergottite source region at 180 Ma (or minerals over geological timescales (Chevrier et al., 2007). less). Taken together, these studies on meteorites provide After an active loss of atmosphere (Melosh and Vickery, 1989; evidence of subsurface water at very low water/rock ratios during 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 provide a detailed examination of where and for how long liquid during the latter part of the wetter Noachian Period, the solutions water may be present in contemporary times, highlighting that became more and more evapoconcentrated. In a later stage, a the regions where liquid water may remain stable for periods up highly saline cryolithosphere derived from the disappearance of to 70 Martian days cover nearly a third of the planet’s surface area. the ionic supersaturated hydrosphere, especially in the northern This correlates remarkably well with reported Amazonian paleo- lowlands and in the bottom of craters and depressions. Particu- lakes (Scott et al., 1995; Cabrol and Grin, 2001), and include larly, Hesperian and Amazonian plains-forming units (e.g., Tanaka Elysium (the region that straddles the rise to near Gusev) and the et al., 2005) cover the Noachian basement of the Martian northern plains of Arabia Terra (which includes Meridiani). lowlands (Frey et al., 2002) with relatively recent secondary and Realizing that atmospheric changes may have been regional water-related minerals, indicating extensive sediment deposition and even global, brine evolution and subsequent evaporite occurring well into the Amazonian era. Conditions on Mars must assemblages in different Martian locations (i.e., Meridiani and have also been wetter than today during transient episodes into Gusev) would have been somewhat different due to varying rock/ 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 different pH levels. Minor amounts of recent (Amazonian) liquid individual rocks and unconsolidated rock materials, as well as acidic water, transiently weathering 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. Some thermokarst processes, including the episodic loss of outcrops detected by Opportunity. Similarly, minor amounts of ponded water by evaporation or drainage, are noted even for the acidic water (from precipitation, condensation, or melting of Late Amazonian (Soare et al., 2008). Also described are quite ground ice; e.g., Haskin et al., 2005) on the mostly basaltic Gusev young dry lake beds and sediments, displaying features such as crater surface may have produced the sulfate coating of rocks and deltas, shorelines and terraces, which require the action of liquid soil materials as highlighted by Spirit. water over extended periods of time (Cabrol and Grin, 2001). The fact that some of these geochemical signatures (jarosite on 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 Stockstill et al., 2005, respectively) and varying surface composi- iron, thereby increasing the density and acidity of the water tions down to the meter scale, have gone undetected in the orbital 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 cannot offer a broad geomorphological perspective, both Thus, the Amazonian Period can be viewed as persistently cold reinforce the importance of a multi-tiered reconnaissance mission and dry, recording small impact events and pervasive wind architecture for future science-driven and distributed planetary activity, with probable continued slow digestion of basaltic rock/ surface and subsurface exploration, which includes simultaneous 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 (Fink et al., 2005). Sample return will contribute to assumed as the sole alteration process in the Amazonian. a first approximation of real age dates for surface features or The above scenario for the Amazonian Period is supported by alteration, the current lack of which leads to speculation about gamma ray spectrometer (GRS) and neutron spectrometer potential ages of modification events, with error bars of a billion investigations showing the presence of water in equatorial regions years or more. of Mars (Boynton et al., 2002; Jakosky et al., 2005), eventually related to some hydrated sulfates (Feldman et al., 2004). In addition, the GRS identified a large anomaly in chlorine in Gusev 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 The scenario of Amazonian water alterations proposed in this hypothesis about recent brine activity in this region, since further paper has profound implications for the ongoing debate about the 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´n et al. / Planetary and Space Science 57 (2009) 276–287 285

2004a, b; Christensen et al., 2004) and Gusev (Haskin et al., 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. Windy Mars: a dynamic planet as seen by the HiRISE camera. Geophys. Res. Lett. 34, L23205. Meridiani Planum and the acid alterations on rock surfaces in Burns, R.G., Fisher, D.S., 1990. Iron–sulfur mineralogy of Mars: magmatic evolution Gusev crater are better explained by the combination of early and chemical weathering products. J. Geophys. Res. 95, 14415–14421. (Noachian–Hesperian) aqueous alteration and later (Amazonian) Burns, R.G., Fisher, D.S., 1993. Rates of oxidative weathering on the surface of Mars. episodic occurrence of small quantities of liquid water on the J. Geophys. Res. 98, 3365–3372. Cabrol, N.A., Grin, E.A., 2001. The evolution of lacustrine environments on Mars: is surface of Mars, rather than by early diagenesis only. Therefore, Mars only hydrologically dormant. Icarus 149, 291–328. we suggest that part of the mineralogical history of Meridiani and Cabrol, N.A., Grin, E.A., Landheim, R., Kuzmin, R.O., Greeley, R., 1998. Duration of Gusev, as derived from data provided by the MER rovers, reflects the Ma’adim Vallis/Gusev crater hydrogeologic system, Mars. Icarus 133, 98–108. recent supercold episodes of local acidic brines, likely extending Cabrol, N.A., Farmer, J.D., Grin, E.A., Richter, L., Soderblom, L., Li, R., Herkenhoff, K., into the Amazonian period. As episodic aqueous processes also Landis, G.A., Arvidson, R.E., 2006. Aqueous processes at Gusev crater inferred occurred several billions of years ago at Meridiani (Faire´n et al., from physical properties of rocks and soils along the Spirit traverse. J. Geophys. Res. 111. 2004; Squyres et al., 2004a, b) and Gusev (Cabrol et al., 1998; Chan, M.A., Yonkee, W.A., Netoff, D.I., Seiler, W.M., Ford, R.L., 2008. Polygonal cracks Haskin et al., 2005), diverse amounts of acidic shallow surface in bedrock on Earth and Mars: implications for weathering. Icarus 194, 65–71. water may have been recurrently present in Meridiani, Gusev, and Chavdarian, G.V., Sumner, D.Y., 2006. Cracks and fins in sulfate sand: evidence for recent mineral-atmospheric water cycling in Meridiani Planum outcrops? other Martian locations during a major part of the history of Mars. Geology 34, 229–232. Chevrier, V., Rochette, P., Mathe´, P.-E., Grauby, O., 2004. Weathering of iron rich phases in simulated Martian atmospheres. Geology 32, 1033–1036. Acknowledgements Chevrier, V., Mathe, P.E., Rochette, P., Grauby, O., Bourrie, G., Trolard, F., 2006. Iron weathering products in a CO2+(H2OorH2O2) atmosphere: implications for weathering processes on the surface of Mars. Geochim. Cosmochim. Acta. 70, The work of A. G. F. and A. F. D. has been supported by NPP- 4295–4317. ORAU. Valuable comments and suggestions were provided by V. Chevrier, V., Poulet, F., Bibring, J.P., 2007. Early geochemical environment of Mars as determined from thermodynamics of phyllosilicates. Nature 448, 60–63. Chevrier and one anonymous reviewer. Christensen, P.R., Morris, R.V., Lane, M.D., Banfield, J.L., Malin, M.C., 2001. Global mapping of Martian hematite mineral deposits: remnants of water-driven References processes on early Mars. J. Geophys. Res. 106, 23873–23885. Christensen, P.R., Wyatt, M.B., Glotch, T.D., Rogers, A.D., Anwar, S., Arvidson, R.E., Bandfield, J.L., Blaney, D.L., Budney, C., Calvin, W.M., Fallacaro, A., Fergason, R.L., Akai, J., Akai, K., Ito, M., Nakano, S., Maki, Y., Sasagawa, I., 1999. Biologically induced Gorelick, N., Graff, T.G., Hamilton, V.E., Hayes, A.G., Johnson, J.R., Knudson, A.T., iron ore at Gunma iron mine, Japan. Am. Mineral. 84, 171–182. McSween Jr., H.Y., Mehall, G.L., Mehall, L.K., Moersch, J.E., Morris, R.V., Smith, Amils, R., Gonza´lez-Toril, E., Ferna´ndez Remolar, D., Go´ mez, F., Aguilera, A., M.D., Squyres, S.W., Ruff, S.W., Wolff, M.J., 2004. Mineralogy at Meridiani Rodrı´guez, N., Malki, M., Garcı´a-Moyano, A., Faire´n, A.G., de la Fuente, V., Sanz, Planum from the Mini-TES experiment on the Opportunity Rover. Science 306, J.L., 2007. Extreme environments as Mars terrestrial analogs: the Rio Tinto case. 1733–1739. Planet. Space Sci. 55, 370–381. Clark, B.C., Morris, R.V., McLennan, S.M., Gellert, R., Jolliff, B., Knoll, A.H., Squyres, Andrews-Hanna, J.C., Phillips, R.J., Zuber, M.T., 2007. Meridiani Planum and the S.W., Lowenstein, T.K., Ming, D.W., Tosca, N.J., Yen, A., Christensen, P.R., global hydrology of Mars. Nature 446, 163–166. Gorevan, S., Bru¨ ckner, J., Calvin, W., Dreibus, G., Farrand, W., Klingelhoefer, G., Arvidson, R.E., Poulet, F., Bibring, J.-P., Wolff, M., Gendrin, A., Morris, R.V., Freeman, Waenke, H., Zipfel, J., 2005. Chemistry and mineralogy of outcrops at Meridiani J.J., Langevin, Y., Mangold, N., Bellucci, G., 2005. Spectral reflectance and Planum. Earth Planet. Sci. Lett. 240, 73–94. morphologic correlations in Eastern Terra Meridiani, Mars. Science 307, Clark, B.C., Arvidson, R.E., Gellert, R., Morris, R.V., Ming, D.W., Richter, L., Ruff, S.W., 1591–1594. Michalski, J.R., Farrand, W.H., Yen, A., 2007. Evidence for montmorillonite or its Arvidson, R.E., Poulet, F., Morris, R., Bibring, J.-P., Bell III, J., Squyres, S., Christensen, compositional equivalent in Columbia Hills, Mars. J. Geophys. Res. 112. P., Bellucci, G., Gondet, B., Ehlmann, B., Farrand, W., Fergason, R., Golombek, M., Clifford, S.M., Parker, T.J., 2001. The evolution of the Martian hydrosphere: Griffes, J., Grotzinger, J., Guinness, E., Herkenhoff, K., Johnson, J., Klingelhofer, implications for the fate of a primordial ocean and the current state of the G., Langevin, Y., Ming, D., Seelos, K., Sullivan, R., Ward, J., Wiseman, S., Wolff, M., northern plains. Icarus 154, 40–79. 2006. Nature and origin of the hematite-bearing plains of Terra Meridiani Crumpler, L.S., Squyres, S.W., Arvidson, R.E., Bell III, J.F., Blaney, D., Cabrol, N.A., based on analyses of orbital and Mars Exploration rover data sets. J. Geophys. Christensen, P.R., DesMarais, D.J., Farmer, J.D., Fergason, R., Golombek, M.P., Res. 111. Grant, F.D., Grant, J.A., Greeley, R., Hahn, B., Herkenhoff, K.E., Hurowitz, J.A., Baker, V.R., 2001. Water and the Martian landscape. Nature 412, 228–236. Knudson, A.T., Landis, G.A., Li, R., Maki, J., McSween, H.Y., Ming, D.W., Moersch, Baker, L.L., Agenbroad, D.J., Wood, S.A., 2000. Experimental hydrothermal J.E., Payne, M.C., Rice, J.W., Richter, L., Ruff, S.W., Sims, M., Thompson, S.D., alteration of a Martian analog basalt: implications for Martian meteorites. Tosca, N., Wang, A., Whelley, P., Wright, S.P., Wyatt, M.B., 2005. Mars Meteor. Planet. Sci. 35, 31–38. Exploration Rover Geologic traverse by the Spirit rover in the plains of Gusev Barro´ n, V., Torrent, J., Greenwood, J.P., 2006. Transformation of jarosite to hematite Crater, Mars. Geology 33, 809–812. in simulated Martian brines. Earth Planet. Sci. Lett. 251, 380–385. Dohm, J.M., Ferris, J.C., Baker, V.R., Anderson, R.C., Hare, T.M., Strom, R.G., Barlow, Bibring, J.P., Langevin, Y., Mustard, J.F., Poulet, F., Arvidson, R., Gendrin, A., Gondet, N.G., Tanaka, K.L., Klemaszewski, J.E., Scott, D.H., 2001. Ancient drainage basin B., Mangold, N., Pinet, P., Forget, F., the OMEGA team, 2006. Global of the Tharsis region, Mars: potential source for outflow channel systems and mineralogical and aqueous Mars history derived from OMEGA/Mars Express putative oceans or paleolakes. J. Geophys. Res. 106, 32943–32958. data. Science 312, 400–404. Elwood Madden, M.E., Bodnar, R.J., Rimstidt, J.D., 2004. Jarosite as an indicator of Bibring, J.P., Arvidson, R.E., Gendrin, A., Gondet, B., Langevin, Y., Le Mouelic, S., water-limited chemical weathering on Mars. Nature 431, 821–823. Mangold, N., Morris, R.V., Mustard, J.F., Poulet, F., Quantin, C., Sotin, C., 2007. Faire´n, A.G., Dohm, J.M., Baker, V.R., de Pablo, M.A., Ruiz, J., Ferris, J., Anderson, R., Coupled ferric oxides and sulfates on the Martian surface. Science 317, 2003. Episodic flood inundations of the northern plains of Mars. Icarus 165, 1206–1210. 53–67. Borg, L., Drake, M.J.A., 2005. Review of meteorite evidence for the timing of Faire´n, A.G., Ferna´ndez-Remolar, D., Dohm, J.M., Baker, V.R., Amils, R., 2004. magmatism and of surface or near-surface liquid water on Mars. J. Geophys. Inhibition of carbonate synthesis in acidic oceans on early Mars. Nature 431, Res. 110. 423–426. Boynton, W.V., Feldman, W.C., Squyres, S.W., Prettyman, T.H., Bru¨ ckner, J., Evans, Faire´n, A.G., Davila, A., Lim, D., Uceda, E.R., Zavaleta, J., Amils, R., McKay, C.P., 2008. L.G., Reedy, R.C., Starr, R., Arnold, J.R., Drake, D.M., Englert, P.A.J., Metzger, A.E., The case for a cold and wet Mars. In: Planet Mars Research Focus. NOVA Mitrofanov, I., Trombka, J.I., d’Uston, C., Wa¨nke, H., Gasnault, O., Hamara, D.K., Publishers. Janes, D.M., Marcialis, R.L., Maurice, S., Mikheeva, I., Taylor, G.J., Tokar, R., Farrand, W.H., Bell III, J.F., Johnson, J.R., Squyres, S.W., Soderblom, J., Ming, D.W., Shinohara, C., 2002. Distribution of hydrogen in the near surface of Mars: 2006. Spectral variability among rocks in visible and near-infrared multi- evidence for subsurface ice deposits. Science 297, 81–85. spectral Pancam data collected at Gusev crater: examinations using spectral Boynton, W.V., Taylor, G.J., Evans, L.G., Reedy, R.C., Starr, R., Janes, D.M., Kerry, K.E., mixture analysis and related techniques. J. Geophys. Res. 111. Drake, D.M., Kim, K.J., Williams, R.M.S., Crombie, M.K., Dohm, J.M., Baker, V., Farrand, W.H., Bell, J.F., Johnson, J.R., Jolliff, B.L., Knoll, A.H., McLennan, S.M., Metzger, A.E., Karunatillake, S., Keller, J.M., Newsom, H.E., Arnold, J.R., Bruckner, Squyres, S.W., Calvin, W.M., Grotzinger, J.P., Morris, R.V., Soderblom, J., J., Englert, P.A.J., Gasnault, O., Sprague, A.L., Mitrofanov, I., Squyres, S.W., Thompson, S.D., Watters, W.A., Yen, A.S., 2007. Visible and near-infrared Trombka, J.I., d’Uston, C., Wanke, H., Hamara, D.K., 2007. Concentration of H, Si, multispectral analysis of rocks at Meridiani Planum, Mars, by the Mars Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars. J. Geophys. Res. Exploration Rover Opportunity. J. Geophys. Res. 112. 112. Feldman, W.C., Mellon, M.T., Maurice, S., Prettyman, T.H., Carey, J.W., Vaniman, D.T., Brain, D.A., Jakosky, B.M., 1998. Atmospheric loss since the onset of the Martian Bish, D.L., Fialips, C.I., Chipera, S.J., Kargel, J.S., Elphic, R.C., Funsten, H.O.,

geologic record: combined role of impact erosion and sputtering. J. Geophys. Lawrence, D.J., Tokar, R.L., 2004. Hydrated states of MgSO4 at equatorial Res. 103, 22689–22694. latitudes on Mars. Geophys. Res. Lett. 31. ARTICLE IN PRESS

286 A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287

Fink, W., Dohm, J.M., Tarbell, M.A., Hare, T.M., Baker, V.R., 2005. Next-generation Johnson, J.R., Bell III, J.F., Cloutis, E., Staid, M., Farrand, W.H., McCoy, T., Rice, M., robotic planetary reconnaissance missions: a paradigm shift. Planet. Space Sci. Wang, A., Yen, A., 2007. Mineralogic constraints on sulfur-rich soils from 53, 1419–1426. Pancam spectra at Gusev crater, Mars. Geophys. Res. Lett. 34. Frey, H.V., Roark, J.H., Shockey, K.M., Frey, E.L., Sakimoto, S.E.H., 2002. Ancient Keller, J.M., Boynton, W.V., Karunatillake, S., Baker, V.R., Dohm, J.M., Evans, L.G., lowlands on Mars. Geophys. Res. Lett. 29. Finch, M.J., Hahn, B.C., Hamara, D.K., Janes, D.M., Kerry, K.E., Newsom, H.E., Geissler, P.E., 2005. Three decades of Martian surface changes. J. Geophys. Res. 110. Reedy, R.C., Sprague, A.L., Squyres, S.W., Starr, R.D., Taylor, G.J., Williams, R.M.S., Gellert, R., Rieder, R., Anderson, R.C., Bru¨ ckner, J., Clark, B.C., Dreibus, G., Economou, 2006. Equatorial and midlatitude distribution of chlorine measured by Mars T., Klingelhoo¨fer, G., Lugmair, G.W., Ming, D.W., Squyres, S.W., d’Uston, C., Odyssey GRS. J. Geophys. Res. 111. Wa¨nke, H., Yen, A., Zipfe, J., 2004. Chemistry of rocks and soils in Gusev Crater Klingelho¨fer, G., Morris, R.V., Bernhardt, B., Schro¨der, C., Rodionov, D.S., de Souza Jr., from the Alpha Particle X-ray Spectrometer. Science 305, 829–832. P.A., Yen, A., Gellert, R., Evlanov, E.N., Zubkov, B., Foh, J., Bonnes, U., Kankeleit, E., Gellert, R., Rieder, R., Bru¨ ckner, J., Clark, B.C., Dreibus, G., Klingelho¨fer, G., Lugmair, Gu¨ tlich, P., Ming, D.W., Renz, F., Wdowiak, T., Squyres, S.W., Arvidson, R.E., G., Ming, D.W., Wa¨nke, H., Yen, A., Zipfel, J., Squyres, S.W., 2006. Alpha particle 2004. Jarosite and hematite at Meridiani Planum from Opportunity’s X-ray spectrometer (APXS): results from Gusev crater and calibration report. Moessbauer spectrometer. Science 306, 1740–1745. J. Geophys. Res. 111. Knauth, L.P., Burt, D.M., Wohletz, K.H., 2005. Impact origin of sediments at the Gendrin, A., Mangold, N., Bibring, J., Langevin, Y., Gondet, B., Poulet, F., Bonello, G., Opportunity landing site on Mars. Nature 438, 1123–1128. Quantin, C., Mustard, J., Arvidson, R., LeMoue´lic, S., 2005. Sulfates in Martian Knoll, A.H., Jolliff, B.L., Farrand, W.H., Bell III, J.F., Clark, B.C., Gellert, R., Golombek, layered terrains: the OMEGA/Mars Express view. Science 307, 1587–1591. M.P., Grotzinger, J.P., Herkenhoff, K.E., Johnson, J.R., McLennan, S.M., Morris, R., Golden, D.C., Ming, D.W., Morris, R.V., Mertzman, S.A., 2005. Laboratory-simulated Squyres, S.W., Sullivan, R., Tosca, N.J., Yen, A., Learner, Z., 2008. Veneers, rinds, acid-sulfate weathering of basaltic materials: implications for formation of and fracture fills: relatively late alteration of sedimentary rocks at Meridiani sulfates at Meridiani Planum and Gusev crater, Mars. J. Geophys. Res. 110. Planum, Mars. J. Geophys. Res. 113, E06S16. Golombek, M.P., Arvidson, R.E., Bell III, J.F., Christensen, P.R., Crisp, J.A., Crumpler, Kuzmin, R.O., Zabalueva, E.V., 1998. On salt solutions in the Martian criolitho- L.S., Ehlmann, B.L., Fergason, R.L., Grant, J.A., Greeley, R., Haldemann, A.F.C., sphere. Solar Sys. Res. 32, 187–197. Kass, D.M., Parker, T.J., Schofield, J.T., Squyres, S.W., Zurek, R.W., 2005. Kuzmin, R.O., Greeley, R., Landheim, R., Cabrol, N.A., Farmer, J.D., 2004. Geologic Assessment of Mars Exploration Rover landing site predictions. Nature 436, map of the MTM–15182 and MTM–15187 quadrangles, Gusev crater—Ma’adim 44–48. Vallis region, Mars. USGS Map I-2666. Golombek, M.P., Crumpler, L.S., Grant, J.A., Greeley, R., Cabrol, N.A., Parker, T.J., Rice Lammer, H., Lichtenegger, H.I.M., Kolb, C., Ribas, I., Guinan, E.F., Abart, R., Bauer, S.J., Jr., J.W., Ward, J.G., Arvidson, R.E., Moersch, J.E., Fergason, R.L., Christensen, P.R., 2003. Loss of water from Mars: implications for the oxidation of the soil. Icarus Castan˜o, A., Castan˜o, R., Haldemann, A.F.C., Li, R., Bell III, J.F., Squyres, S.W., 165, 9–25. 2006. Geology of the Gusev cratered plains from the Spirit rover transverse. Lane, M.D., Christensen, P.R., Hartmann, W.K., 2003. Utilization of the THEMIS J. Geophys. Res. 111. visible and infrared imaging data for crater population studies of the Meridiani Grant, J.A., Arvidson, R., Bell III, J.F., Cabrol, N.A., Carr, M.H., Christensen, P., Planum landing site. Geophys. Res. Lett. 30. Crumpler, L., Des Marais, D.J., Ehlmann, B.L., Farmer, J., Golombek, M., Grant, Lane, M.D., Bishop, J.L., Darby Dyar, M., King, P., Parente, M., Hyde, B.C., 2008. F.D., Greeley, R., Herkenhoff, K., Li, R., McSween, H.Y., Ming, D.W., Moersch, J., Mineralogy of the Paso Robles soils on Mars. Am. Min. 93, 728–739. Rice Jr., J.W., Ruff, S., Richter, L., Squyres, S., Sullivan, R., Weitz, C., 2004. Lorand, J.P., Chevrier, V., Sautter, V., 2005. Sulfide mineralogy and redox conditions Surficial deposits at Gusev crater along Spirit rover traverses. Science 305, in some Shergottites. Meteor. Planet. Sci. 40, 1257–1272. 807–810. Lundin, R., Barabash, S., Andersson, H., Holmstro¨m, M., Grigoriev, A., Yamauchi, M., Grant, J.A., Arvidson, R.E., Crumpler, L.S., Golombek, M.P., Hahn, B., Haldemann, Sauvaud, J.A., Fedorov, A., Budnik, E., Thocaven, J.J., Winningham, D., Frahm, R., A.F.C., Li, R., Soderblom, L.A., Squyres, S.W., Wright, S.P., Watters, W.A., 2006. Scherrer, J., Sharber, J., Asamura, K., Hayakawa, H., Coates, A., Linder, D.R., Crater gradation in Gusev crater and Meridiani Planum, Mars. J. Geophys. Res. Curtis, C., Hsieh, K.C., Sandel, B.R., Grande, M., Carter, M., Reading, D.H., 111. Koskinen, H., Kallio, E., Riihela, P., Schmidt, W., Sa¨les, T., Kozyra, J., Krupp, N., Grin, E.A., Cabrol, N.A., 1997. Limnologic analysis of Gusev crater paleolake, Mars. Woch, J., Luhmann, J., McKenna-Lawler, S., Cerulli-Irelli, R., Orsini, S., Maggi, M., Icarus 130, 461–474. Mura, A., Milillo, A., Roelof, E., Williams, D., Livi, S., Brandt, P., Wurz, P., Haberle, R.M., McKay, C.P., Schaeffer, J., Cabrol, N.A., Grin, E.A., Zent, A.P., Quinn, R., Bochsler, P., 2004. Solar wind-induced atmospheric erosion at Mars: first 2001. On the possibility of liquid water on present day Mars. J. Geophys. Res. results from ASPERA-3 on Mars Express. Science 305, 1933–1937. 106, 23317–23326. Malin, M.C., Edgett, K.S., 2000. Sedimentary rocks of early Mars. Science 290, Hartmann, W.K., Neukum, G., 2001. Cratering chronology and the evolution of 1927–1937. Mars. Space Sci. Rev. 96, 165–194. Malin, M.C., Edgett, K.S., 2003. Evidence for persistent flow and aqueous Hartmann, W.K., Anguita, J., de la Casa, M.A., Berman, D.C., Ryan, E.V., 2001. sedimentation on early Mars. Science 302, 1931–1934. Martian cratering, 7, the role of impact gardening. Icarus 149, 37–53. Mangold, N., Quantin, C., Ansan, V., Delacourt, C., Allemand, P., 2004. Evidence for Haskin, L.A., Wang, A., Jolliff, B.L., McSween, H.Y., Clark, B.C., Des Marais, D.J., precipitation on Mars from dendritic valleys in the Valles Marineris area. McLennan, S.M., Tosca, N.J., Hurowitz, J.A., Farmer, J.D., Yen, A., Squyres, S.W., Science 305, 78–81. Arvidson, R.E., Klingelho¨fer, G., Schro¨der, C., de Souza Jr., P.A., Ming, D.W., Martı´nez-Frı´as, J., Lunar, R., Rodrı´guez-Losada, J.A., Delgado, A., Rull, A., 2004. The Gellert, R., Zipfel, J., Bru¨ ckner, J., Bell III, J.F., Herkenhoff, K., Christensen, P.R., volcanism-related multistage hydrothermal system of El Jaroso (SE Spain): Ruff, S., Blaney, D., Gorevan, S., Cabrol, N.A., Crumpler, L., Grant, J., Soderblom, implications for the exploration of Mars. Earth Planets Space 56, v–viii. L., 2005. Water alteration of rocks and soils on Mars at the Spirit rover site in Martı´nez-Frı´as, J., Delgado-Huertas, A., Garcı´a-Moreno, F., Reyes, E., Lunar, R., Rull, Gusev crater. Nature 436, 66–69. F., 2007. Isotopic signatures of extinct low-temperature hydrothermal Hausrath, E.M., Navarre-Sitchler, A.K., Sak, P.B., Steefel, C.I., Brantley, S.L., 2008. chimneys in the Jaroso Mars analog. Planet. Space Sci. 55, 441–448. Basalt weathering rates on Earth and the duration of liquid water on the plains McCollom, T.M., Hynek, B.M., 2005. A volcanic environment for bedrock diagenesis of Gusev Crater, Mars. Geology 36, 67–70. at Meridiani Planum on Mars. Nature 438, 1129–1131. Head, J.W., Marchant, D.R., Agnew, M.C., Fassett, C.I., Kreslavsky, M.A., 2006. McLennan, S.M., Bell III, J.F., Calvin, W.M., Christensen, P.R., Clark, B.C., de Souza, Extensive valley glacier deposits in the northern mid-latitudes of Mars: P.A., Farmer, J., Farrand, W.H., Fike, D.A., Gellert, R., Ghosh, A., Glotch, T.D., evidence for Late Amazonian obliquity-driven climate change. Earth Planet. Grotzinger, J.P., Hahn, B., Herkenhoff, K.E., Hurowitz, J.A., Johnson, J.R., Johnson, Sci. Lett. 241, 663–671. S.S., Jolliff, B., Klingelhofer, G., Knoll, A.H., Learner, Z., Malin, M.C., McSween Jr., Herkenhoff, K.E., Squyres, S.W., Anderson, R., Archinal, B.A., Arvidson, R.E., Barrett, H.Y., Pocock, J., Ruff, S.W., Soderblom, L.A., Squyres, S.W., Tosca, N.J., Watters, J.M., Becker, K.J., Bell III, J.F., Budney, C., Cabrol, N.A., Chapman, M.G., Cook, D., W.A., Wyatt, M.B., Yen, A., 2005. Provenance and diagenesis of the evaporite- Ehlmann, B.L., Farmer, J., Franklin, B., Gaddis, L.R., Galuszka, D.M., Garcı´a, P.A., bearing Burns Formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, Hare, T.M., Howington-Kraus, E., Johnson, J.R., Johnson, S., Kinch, K., Kirk, R.L., 95–121. Lee, E.M., Leff, C., Lemmon, M., Madsen, M.B., Maki, J.N., Mullins, K.F., Redding, McSween, H.Y., Arvidson, R.E., Bell III, J.F., Blaney, D., Cabrol, N.A., Christensen, P.R., B.L., Richter, L., Rosiek, M.R., Sims, M.H., Soderblom, L.A., Spanovich, N., Clark, B.C., Crisp, J.A., Crumpler, L.S., Des Marais, D.J., Farmer, J.D., Gellert, R., Springer, R., Sucharski, R.M., Sucharski, T., Sullivan, R., Torson, J.M., Yen, A., Ghosh, A., Gorevan, S., Graff, T., Grant, J., Haskin, L.A., Herkenhoff, K.E., Johnson, 2006. Overview of the Microscopic Imager Investigation during Spirit’s first J.R., Jolliff, B.L., Klingelhoefer, G., Knudson, A.T., McLennan, S., Milam, K.A., 450 sols in Gusev crater. J. Geophys. Res. 111, E02S04. Moersch, J.E., Morris, R.V., Rieder, R., Ruff, S.W., de Souza, P.A., Squyres, S.W., Hurowitz, J.A., McLennan, S.M., Tosca, N.J., Arvidson, R.E., Michalski, J.R., Ming, Wa¨nke, H., Wang, A., Wyatt, M.B., Yen, A., Zipfel, J., 2004. Basaltic rocks D.W., Schro¨der, C., Squyres, S.W., 2006. In situ and experimental evidence for analyzed by the Spirit rover in Gusev crater. Science 305, 842–845. acidic weathering of rocks and soils on Mars. J. Geophys. Res. 111, E02S19. Melosh, H.J., Vickery, A.M., 1989. Impact erosion of the primordial atmosphere of Hynek, B.M., 2004. Implications for hydrologic processes on Mars from extensive Mars. Nature 338, 487–489. bedrock outcrops throughout Terra Meridiani. Nature 431, 156–159. Milliken, R.E., Swayze, G.A., Arvidson, R.E., Bishop, J.L., Clark, R.N., Ehlmann, B.L., Hynek, B.M., Arvidson, R.E., Phillips, R.J., 2002. Geologic setting and origin of Terra Green, R.O., Grotzinger, J.P., Morris, R.V., Murchie, S.L., Mustard, J.F., Weitz, C., Meridiani hematite deposit on Mars. J. Geophys. Res. 107. 2008. Opaline silica in young deposits on Mars. Geology 36, 847–850. Irwin III, R.P., Maxwell, T.A., Howard, A.D., Craddock, R.A., Leverington, D.W., 2002. Ming, D.W., Mittlefehldt, D.W., Morris, R.V., Golden, D.C., Gellert, R., Yen, A., Clark, A large paleolake basin at the head of Ma’Adim Vallis, Mars. Science 296, B.C., Squyres, S.W., Farrand, W.H., Ruff, S.W., Arvidson, R.E., Klingelho¨fer, G., 2209–2212. McSween, H.Y., Rodionov, D.S., Schro¨der, C., de Souza Jr., P.A., Wang, A., 2006. Jakosky, B.M., Mellon, M.T., Varnes, E.S., Feldman, W.C., Boynton, W.V., Haberle, Geochemical and mineralogical indicators for aqueous processes in the R.M., 2005. Mars low-latitude neutron distribution: possible remnant near- Columbia Hills of Gusev crater, Mars. J. Geophys. Res. 111. surface water ice and a mechanism for its recent emplacement. Icarus 175, Morris, R.V., Klingelho¨fer, G., Bernhardt, B., Schro¨der, C., Rodionov, D.S., de Souza, 58–67. P.A., Yen, A., Gellert, R., Evlanov, E.N., Foh, J., Kankeleit, E., Gu¨ tlich, P., Ming, ARTICLE IN PRESS

A.G. Faire´n et al. / Planetary and Space Science 57 (2009) 276–287 287

D.W., Renz, F., Wdowiak, T., Squyres, S.W., Arvidson, R.E., 2004. Mineralogy at Squyres, S.W., Grotzinger, J.P., Arvidson, R.E., Bell III, J.F., Calvin, W., Christensen, Gusev crater from the Mo¨ssbauer spectrometer on the Spirit rover. Science 305, P.R., Clark, B.C., Crisp, J.A., Farrand, W.H., Herkenhoff, K.E., Johnson, J.R., 833–836. Klingelho¨fer, G., Knoll, A.H., McLennan, S.M., McSween Jr., H.Y., Morris, R.V., Morris, R.V., Klingelho¨fer, G., Schro¨der, C., Rodionov, D.S., Yen, A., Ming, D.W., de Rice Jr., J.W., Rieder, R., Soderblom, L.A., 2004b. In situ evidence for an ancient Souza Jr., P.A., Fleischer, I., Wdowiak, T., Gellert, R., Bernhardt, B., Evlanov, E.N., aqueous environment at Meridiani Planum, Mars. Science 306, 1709–1714. Zubkov, B., Foh, J., Bonnes, U., Kankeleit, E., Gu¨ tlich, P., Renz, F., Squyres, S.W., Squyres, S.W., Arvidson, R.E., Blaney, D.L., Clark, B.C., Crumpler, L., Farrand, W.H., Arvidson, R.E., 2006. Mo¨ssbauer mineralogy of rock, soil, and dust at Gusev Gorevan, S., Herkenhoff, K.E., Hurowitz, J., Kusack, A., McSween, H.Y., Ming, crater, Mars: Spirit’s journey through weakly altered olivine basalt on the plains D.W., Morris, R.V., Ruff, S.W., Wang, A., Yen, A., 2006a. Rocks of the Columbia and pervasively altered basalt in the Columbia Hills. J. Geophys. Res. 111, E02S13. Hills. J. Geophys. Res. 111. Neukum, G., Jaumann, R., Hoffmann, H., Hauber, E., Head, J.W., Basilevsky, A.T., Squyres, S.W., Knoll, A.H., Arvidson, R.E., Clark, B.C., Grotzinger, J.P., Jolliff, B.L., Ivanov, B.A., Werner, S.C., van Gasselt, S., Murray, J.B., McCord, T., the HRSC Co- McLennan, S.M., Tosca, N., Bell III, J.F., Calvin, W.M., Farrand, W.H., Glotch, T.D., Investigator Team, 2004. Recent and episodic volcanic and glacial activity on Golombek, M.P., Herkenhoff, K.E., Johnson, J.R., Klingelho¨fer, G., McSween, H.Y., Mars revealed by the high resolution stereo camera. Nature 432, 971–979. Yen, A.S., 2006b. Two years at Meridiani Planum: results from the Opportunity Newsom, H.E., Crumpler, L.S., Reedy, R.C., Petersen, M.T., Newsom, G.C., Evans, L.G., Rover. Science 313, 1403–1407. Taylor, G.J., Keller, J.M., Janes, D.M., Boynton, W.V., Kerry, K.E., Karunatillake, S., Stockstill, K.R., Moersch, J.E., Ruff, S.W., Baldridge, A., Farmer, J., 2005. Thermal 2007. Geochemistry of Martian soil and bedrock in mantled and less mantled Emission Spectrometer hyperspectral analyses of proposed paleolake basins on terrains with gamma ray data from Mars Odyssey. J. Geophys. Res. 112. Mars: no evidence for in-place carbonates. J. Geophys. Res. 110. Parker, T.J., Gorsline, D.S., Saunders, R.S., Pieri, D.C., Schneeberger, D.M., 1993. Sullivan, R., Banfield, D., Bell III, J.F., Calvin, W., Fike, D., Golombek, M., Greeley, R., Coastal geomorphology of the Martian northern plains. J. Geophys. Res. 98, Grotzinger, J., Herkenhoff, K., Jerolmack, D., Malin, M., Ming, D., Soderblom, 11061–11078. L.A., Squyres, S.W., Thompson, S., Watters, W.A., Weitz, C.M., Yen, A., 2005. Poulet, F., Bibring, J.P., Mustard, J.F., Gendrin, A., Mangold, N., Langevin, Y., Arvidson, Aeolian processes at the Mars Exploration rover Meridiani Planum landing site. R.E., Gondet, B., Go´ mez, C., the Omega Team, 2005. Phyllosilicates on Mars and Nature 436, 58–61. implications for early Martian climate. Nature 438, 623–627. Swindle, T.D., Treiman, A.H., Lindstrom, D.J., Burkland, M.K., Cohen, B.A., Grier, J.A., Rao, M.N., Nyquist, L.E., Wentworth, S.J., Sutton, S.R., Garrison, D.H., 2008. The Li, B., Olson, E.K., 2000. Noble gases in iddingsite from the Lafayette meteorite: nature of Martian fluids based on mobile element studies in salt-assemblages evidence for liquid water on Mars in the last few hundred million years. from Martian meteorites. J. Geophys. Res. 113. Meteor. Planet. Sci. 35, 107–115. Schulze-Makuch, D., Dohm, J.M., Fan, C., Faire´n, A.G., Rodriguez, J.A.P., Baker, V.R., Fink, Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. Geologic map of the northern plains of W., 2007. Exploration of hydrothermal targets on Mars. Icarus 189, 308–324. Mars. USGS Misc. Inv. Ser. Map SIM-2888 (scale 1:1,50,00,000). Scott, D.H., Dohm, J.M., Applebee, D.J., 1993. Geologic map of science study area 8, Taylor, G.J., Stopar, J.D., Boynton, W.V., Karunatillake, S., Keller, J.M., Bruckner, J., Apollinaris Patera region of Mars. USGS Misc. Inv. Ser. Map I-2351, (1:5,00,000). Wanke, H., Dreibus, G., Kerry, K.E., Reedy, R.C., Evans, L.G., Starr, R.D., Martel, Scott, D.H., Dohm, J.M., Rice, J.W., 1995. Map of Mars showing channels and L.M.V., Squyres, S.W., Gasnault, O., Maurice, S., d’Uston, C., Englert, P., Dohm, possible paleolake basins. USGS Misc. Inv. Ser. Map I-2461, (1:3,00,00,000). J.M., Baker, V.R., Hamara, D., Janes, D., Sprague, A.L., Kim, K.J., Drake, D.M., Segura, T., Toon, O.B., Colaprete, A., Zahnle, K., 2002. Environmental effects of large McLennan, S.M., Hahn, B.C., 2006. Variations in K/Th on Mars. J. Geophys. Res. impacts on Mars. Science 298, 1977–1980. 111. Soare, R.J., Osinski, G.R., Roehm, C.L., 2008. Thermokarst lakes and ponds on Mars Tosca, N.J., McLennan, S.M., Clark, B.C., Grotzinger, J.P., Hurowitz, J.A., Knoll, A.H., in the very recent (late Amazonian) past. Earth Planet. Sci. Lett. 272, 382–393. Schro¨der, C., Squyres, S.W., 2005. Geochemical modeling of evaporation Soderblom, L.A., Anderson, R.C., Arvidson, R.E., Bell III, J.F., Cabrol, N.A., Calvin, W., processes on Mars: insight from the sedimentary record at Meridiani Planum. Christensen, P.R., Clark, B.C., Economou, T., Ehlmann, B.L., Farrand, W.H., Fike, Earth Planet. Sci. Lett. 240, 122–148. D., Gellert, R., Glotch, T.D., Golombek, M.P., Greeley, R., Grotzinger, J.P., Touma, J., Wisdom, J., 1993. The chaotic obliquity of Mars. Science 259, 1294–1296. Herkenhoff, K.E., Jerolmack, D.J., Johnson, J.R., Jolliff, B., Klingelho¨fer, G., Knoll, Wang, A., Haskin, L.A., Squyres, S.W., Jolliff, B.L., Crumpler, L., Gellert, R., Schro¨der, A.H., Learner, Z.A., Li, R., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, C., Herkenhoff, K., Hurowitz, J., Tosca, N.J., Farrand, W.H., Anderson, R., D.W., Morris, R.V., Rice, J.W., Richter, L., Rieder, R., Rodionov, D., Schro¨der, C., Knudson, A.T., 2006. Sulfate deposition in subsurface regolith in Gusev crater, Seelos, F.P., Soderblom, J.M., Squyres, S.W., Sullivan, R., Watters, W.A., Weitz, Mars. J. Geophys. Res. 111. C.M., Wyatt, M.B., Yen, A., Zipfel, J., 2004. Soils of Eagle crater and Meridiani Yen, A.S., Gellert, R., Schro¨der, C., Morris, R.V., Bell III, J.F., Knudson, A.T., Clark, B.C., Planum at the Opportunity rover landing site. Science 306, 1723–1726. Ming, D.W., Crisp, J.A., Arvidson, R.E., Blaney, D., Bru¨ ckner, J., Christensen, P.R., Squyres, S.W., Knoll, A.H., 2005. Sedimentary rocks at Meridiani Planum: origin, DesMarais, D.J., de Souza, P.A., Economou, T.E., Ghosh, A., Hahn, B.C., diagenesis and implications for life on Mars. Earth Planet. Sci. Lett. 240, 1–10. Herkenhoff, K.E., Haskin, L.A., Hurowitz, J.A., Joliff, B.L., Johnson, J.R., Squyres, S.W., Arvidson, R.E., Bell III, J.F., Bru¨ ckner, J., Cabrol, N.A., Calvin, W., Carr, Klingelho¨fer, G., Madsen, M.B., McLennan, S.M., McSween, H.Y., Richter, L., M.H., Christensen, P.R., Clark, B.C., Crumpler, L., Des Marais, D.J., d’Uston, C., Rieder, R., Rodionov, D., Soderblom, L., Squyres, S.W., Tosca, N.J., Wang, A., Economou, T., Farmer, J., Farrand, W., Folkner, W., Golombek, M., Gorevan, S., Wyatt, M., Zipfe, J., 2005. An integrated view of the chemistry and mineralogy Grant, J.A., Greeley, R., Grotzinger, J., Haskin, L., Herkenhoff, K.E., Hviid, S., of Martian soils. Nature 436, 49–54. Johnson, J., Klingelho¨fer, G., Knoll, A., Landis, G., Lemmon, M., Li, R., Madsen, Yen, A.S., Morris, R.V., Clark, B.C., Gellert, R., Knudson, A.T., Squyres, S., Mittlefehldt, M.B., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, D.W., Moersch, J., D.W., Ming, D.W., Arvidson, R., McCoy, T., Schmidt, M., Hurowitz, J., Li, R., Morris, R.V., Parker, T., Rice Jr., J.W., Richter, L., Rieder, R., Sims, M., Smith, M., Johnson, J.R., 2008. Hydrothermal processes at Gusev Crater: an evaluation of Smith, P., Soderblom, L.A., Sullivan, R., Wa¨nke, H., Wdowiak, T., Wolff, M., Yen, Paso Robles class soils. J. Geophys. Res. 113. A., 2004a. The Spirit rover’s Athena science investigation at Gusev crater, Mars. Zolotov, M.Y., Shock, E.L., 2005. Formation of jarosite-bearing deposits through Science 305, 794–799. aqueous oxidation of pyrite at Meridiani Planum, Mars. Geophys. Res. Lett. 32.