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 Ã Corresponding author. Tel.: +1650 604 3842; fax: +1650 604 3992. E-mail address: [email protected] (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