Phyllosilicates and sulfates at Endeavour Crater, Meridiani Planum,

Mars

J. J. Wray,1 E. Z. Noe Dobrea,2 R. E. Arvidson,3 S. M. Wiseman,3 S. W. Squyres,1

A. S. McEwen,4 J. F. Mustard,5 and S. L. Murchie6

Received 31 August 2009; revised 20 September 2009; accepted 21 September 2009; published 4 November 2009.

[1] Phyllosilicates have been identified on the Martiansurface from orbit, and are hypothesized to have formedunder wet, non-acidic conditions early in the planet’shistory. Exposures of these minerals have not yet beenexamined by a landed mission. Using Mars ReconnaissanceOrbiter data, we report the detection of phyllosilicate-bearing outcrops that may be accessible by the MarsExploration Rover Opportunity currently exploringMeridiani Planum. The phyllosilicates are associated withlayered, polygonally fractured rocks exposed in the rim ofthe 20 km diameter crater Endeavour. These rocks may haveformed via regional or global-scale processes of aqueousalteration that predated the period of acid sulfate formationrecorded in the rocks that Opportunity has studied to date.Detailed characterization by Opportunity could betterconstrain the conditions under which these phyllosilicatesformed. Hydrated sulfates are also detected from orbit in theplains adjacent to Endeavour’s rim, providing the firstopportunity for ground truth of these detections.Citation: Wray, J. J., E. Z. Noe Dobrea, R. E. Arvidson, S. M.

Wiseman, S. W. Squyres, A. S. McEwen, J. F. Mustard, and S. L.

Murchie (2009), Phyllosilicates and sulfates at Endeavour Crater,

Meridiani Planum, Mars, Geophys. Res. Lett., 36, L21201,

doi:10.1029/2009GL040734.

1. Introduction

[2] Orbital detection of an alteration mineral (crystallinegray hematite) guided the Mars Exploration Rover (MER)Opportunity to its landing site in Meridiani Planum[Christensen et al., 2000; Golombek et al., 2003]. Withinweeks of landing there, rover measurements revealed thatboth the hematite and abundant sulfates had formed insedimentary rocks that were diagenetically altered—and insome cases deposited—in an aqueous environment [Squyreset al., 2004]. Orbital detection of hydrated sulfates innorthern Meridiani—although not in the hematite-bearing

plains sampled by Opportunity—suggested this aqueousenvironment was regionally extensive [Arvidson et al.,2005]. But this environment was characterized by low pHand low water activity, suggested to be significant chal-lenges for habitability on Mars [Tosca et al., 2008]. Allbedrock examined by the rover to date likely formed undera similar range of conditions, spanning a small fraction ofMartian history [Squyres et al., 2006, 2009].[3] Elsewhere on Mars, primarily in Noachian terrains,

phyllosilicates have been detected in orbital near-infraredspectra [e.g., Poulet et al., 2005; Mustard et al., 2008;Bishop et al., 2008]. The observed phyllosilicates typicallyform under neutral to alkaline conditions [Chevrier et al.,2007], and on Mars they are hypothesized to reflect an earlyperiod of more abundant water and possibly more habitableconditions than existed during the period when sulfatesformed [Bibring et al., 2006]. However, much remainsunknown about the context of phyllosilicate formation:whether wet conditions were persistent or transient, at thesurface or beneath it, and whether the observed phyllosili-cates formed in situ or have been redistributed by sedimen-tary and impact processes. Mineral abundances are difficultto constrain from near-infrared spectra, and additionalphases in the phyllosilicate-bearing rocks remain largelyunidentified [Milliken et al., 2009]. Ground truth wouldsignificantly advance our understanding of Martian phyllo-silicates and is a major goal of future landed missions [e.g.,Grotzinger, 2009].[4] Here we report the orbital identification of both

phyllosilicates and hydrated sulfates in outcrops that maybe accessible by Opportunity. The rover is driving south-ward from Victoria crater toward the much larger (20 kmdiameter) Endeavour crater (Figure 1a). Endeavour craterwas largely buried by the Late Noachian to Early Hesperiansulfate-rich sediments studied by the rover to date, and thusits formation predates these sediments [Arvidson et al.,2006]. Segments of its eroded rim protrude above thesediments, exposing ancient strata. We use Mars Recon-naissance Orbiter (MRO) data to investigate the mineralogyand morphology of the exposed Endeavour rim segmentsand the adjacent plains sediments.

2. Spectral Analysis

[5] We analyzed near-infrared hyperspectral images fromthe MRO Compact Reconnaissance Imaging Spectrometerfor Mars (CRISM) [Murchie et al., 2007]. CRISM I/F datawere processed as described by Murchie et al. [2009],including division by the cosine of the solar incidence angleand atmospheric removal via division by a scaled transmis-sion spectrum derived from observations over Olympus

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1Department of Astronomy, Cornell University, Ithaca, New York,USA.

2Jet Propulsion Laboratory, California Institute of Technology,Pasadena, California, USA.

3Department of Earth and Planetary Sciences, Washington University,Saint Louis, Missouri, USA.

4Lunar and Planetary Laboratory, University of Arizona, Tucson,Arizona, USA.

5Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

6Johns Hopkins University Applied Physics Laboratory, Laurel,Maryland, USA.

Copyright 2009 by the American Geophysical Union.0094-8276/09/2009GL040734$05.00

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Mons [McGuire et al., 2009]. Spatial and spectral noise-filtering [Parente, 2008] were also applied.[6] Pixels having spectra with absorptions characteristic

of hydrated minerals are mapped (Figures 1b and 1c) usingspectral summary parameters [Pelkey et al., 2007]. Largenumbers of these pixels are averaged to improve signal-to-noise, and the result is divided by an average of spectrallyneutral pixels in the same CRISM scene. This spectral ratiomethod suppresses residual systematic artifacts of instru-ment calibration and atmospheric removal [Mustard et al.,2008] while accentuating unique spectral signatures in thenumerator spectrum.

2.1. Phyllosilicates

[7] Spectra of portions of the Endeavour crater rimcontain absorption bands diagnostic of phyllosilicates(Figure 2a). In particular, bands at �1.9, 2.3, 2.4 mm anda weaker feature at 1.4 mm are characteristic of Fe/Mg-smectite clays [e.g., Bishop et al., 2002]. Higher Mg/Fecontent shifts the wavelength of the 2.3 mm band, from�2.28 mm in Fe3+-rich dioctahedral nontronite to �2.31–2.32 mm in Mg-rich trioctahedral saponite [Swayze et al.,2002]. The positions of the 2.3 and 2.4 mm bands inEndeavour rim spectra are intermediate between those ofnontronite and saponite, suggesting that both Fe and Mg are

Figure 1. (a) MRO Context Camera mosaic of Endeavour crater (2.3�S, 5.2�W) and surroundings, with boxes outliningsubsequent figures. (b, c) Distribution of Fe/Mg-phyllosilicates (red) and polyhydrated sulfates (cyan) in CRISM spectralparameter maps (b = FRT00008541, c = FRT0000CE1D) overlain on HiRISE PSP_010486_1775. Red = D2300, green =SINDEX [Pelkey et al., 2007], blue = BD1900H [Ehlmann et al., 2009]. Mapped band depths are typically 0.4–1.1% forD2300 and BD1900H, 0.6–1.8% for SINDEX. (d) Median-filtered elevation profile (transect shown by the blue line inFigure 1a); from HRSC h1183_0000 digital elevation model, relative to spheroid and hypothesized stratigraphy, with sub-horizontal hematitic plains layers (gray, short dashes) overlying layers with hydration signature (cyan, long dashes), whichonlap tilted phyllosilicate-bearing layers (red lines) exposed in Endeavour rim.

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present, similar to other smectite occurrences on Mars[e.g., Bishop et al., 2008]. Consistent results are obtainedfrom multiple observations covering the rim segments inFigure 1b, including FRT00008541, FRT0000CE1D,FRT00011A38, and HRL0000D09A.[8] In comparison to laboratory spectra of terrestrial

smectite samples, the Endeavour rim spectrum has a weak1.9 mm band relative to its 2.3 mm band (Figure 2a),possibly due to dehydration under Martian atmosphericconditions [Bishop and Pieters, 1995;Milliken and Mustard,2005]. Mixing with other components and/or rock coatingscould also weaken the bands at 1.9 and especially 1.4 mmrelative to pure lab samples, as Fe-minerals in coatings candominate spectra shortward of �2 mm, but become moretransparent at longer wavelengths [Swayze, 2004]. Addi-

tional hydrated phase(s) with bands at 1.92–1.98 mm [e.g.,Crowley, 1991; Cloutis et al., 2006] could account for the1.9 mm band minimum occurring at a slightly longerwavelength than that observed in lab spectra of purephyllosilicates.

2.2. Hydrated Sulfates

[9] Spectra of the plains adjacent to the western Endeav-our rim segments (Figures 1b and 1c) contain absorptionbands at �1.94 and �2.4 mm characteristic of polyhydratedsulfates (Figure 2b). In particular, laboratory spectra ofMg-sulfates such as hexahydrite (MgSO4�6H2O) are con-sistent with the Endeavour plains spectra, although othersulfate cations such as Fe2+ or Fe3+ cannot be ruled out. Anadditional weak feature at �2.2 mm could reflect a contri-bution from another hydroxyl-bearing mineral. Althoughthe �2.4 mm band has been attributed at least in part to anS–O overtone absorption [Cloutis et al., 2006], somehydrated minerals other than sulfates do have similarfeatures in this spectral region [e.g., Crowley, 1991]. Never-theless, Opportunity results indicate that abundant Mg-sulfate is present in nearby Meridiani plains outcrops[Squyres et al., 2006, and references therein], supportingits identification in the CRISM spectrum in Figure 2b.

3. Morphology and Stratigraphy

[10] Images of the phyllosilicate-bearing crater rim seg-ments from the MRO High Resolution Imaging ScienceExperiment (HiRISE) [McEwen et al., 2007] reveal layeringand a range of polygonal textures (Figure 3a), similarin appearance to other phyllosilicate-bearing outcrops inMeridiani [Wiseman et al., 2008; Marzo et al., 2009] andmany other locations on Mars [e.g., Wray et al., 2008;Bishop et al., 2008; Ehlmann et al., 2009]. Stereo views(Figure 3b) show that layers within the western rim dipaway from the crater interior, as expected if the beds predateEndeavour crater and were back-tilted by the impact. Incontrast, bright layers bounding many Endeavour rim seg-ments [e.g., McEwen et al., 2009, Figure 29] dip downtoward the crater interior; we cannot clearly determinewhether these layers predate or postdate the impact basedon orbital images.[11] Polygonal patterns are observed on bedrock expo-

sures in all segments of the rim imaged to date (e.g.,Figures 3c–3e). The presence of these patterns virtuallyeverywhere that rim bedrock is exposed suggests that thebedrock is pervasively altered. The diversity of texturesexhibited in Figure 3 likely reflects diversity in compositionand/or physical properties of the Endeavour rim materials.[12] As with the traverse to Victoria crater [Squyres et al.,

2009], the traverse to Endeavour involves an elevationchange, in this case �100 m downward and thus possiblydown-section (Figure 1d). The detection of hydrated sul-fates in the lower-elevation plains adjacent to Endeavourmay be stratigraphically consistent with their detection inthe Meridiani ‘‘etched terrains’’ underlying the hematiticplains [Arvidson et al., 2005]. Orbital images (e.g.,Figure 3b) suggest that these sulfate-bearing plains onlapthe Endeavour rim segments [Arvidson et al., 2006],leading to the hypothesized, simplified stratigraphy shown

Figure 2. Ratio spectra from (top) CRISM FRT00008541(bold lines median-filtered) and (bottom) lab spectra(vertically offset for clarity). Small features near 1.65 and2.0 mm in CRISM spectra result from a filter boundary andfrom imperfect removal of atmospheric CO2 bands,respectively. (a) Endeavour spectrum is 232-pixel averagefrom rim segments in Figure 1b. Fe/Mg smectite (scaled x8)is sample GDS759A from Flagstaff Hill, CA (courtesyG. Swayze). Nontronite is sample NG-1 from the ClayMinerals Society, Source Clays Repository (spectrumNBJB26 in CRISM spectral library). Saponite isSapCa1.AcB from USGS spectral library [Clark et al.,2007]. (b) Spectra were continuum-removed in ENVI toeliminate an artificial slope resulting from the use ofspectral ratios. Endeavour spectrum is 562-pixel averagefrom plains adjacent to rim segments in Figure 1b.Hexahydrite (MgSO4�6H2O) is LASF57A and rozenite(FeSO4�4H2O) is BKR1JB626B from CRISM spectrallibrary.

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in Figure 1d. These interpretations can be tested and refinedby rover observations.

4. Discussion and Conclusions

[13] As with the crystalline gray hematite in MeridianiPlanum, obtaining ground truth on both phyllosilicates andhydrated sulfates identified from orbit would allow a morerefined evaluation of what these mineral detections implyabout environmental conditions on ancient Mars. Definitiveidentification of sulfate cations is difficult using near-infrared spectroscopy (e.g., Figure 2b), but possible withthe Opportunity payload. Furthermore, although detectionsof hydrated minerals are widespread in Meridiani, includinghydrated sulfates at some locations [e.g., Gendrin et al.,2005], hydration has not been observed in near-infraredspectra at the locations sampled by Opportunity to date[Arvidson et al., 2006; Poulet et al., 2008]. Abundanthydroxylated and hydrated sulfates exist at these locations[e.g., Squyres et al., 2006], but are apparently spectrallymasked by effects that may include surface dehydration andcoatings. Rover measurements at Endeavour crater couldprovide insight into the relative importance of such outcropcharacteristics vs. absolute mineral abundances in control-ling the band strengths observed from orbit, both forhydrated sulfates and phyllosilicates.

[14] Fe/Mg-smectites are the most commonly detectedphyllosilicates on Mars [e.g., Bibring et al., 2006; Mustardet al., 2008; Wray et al., 2009]. They have been identifiedelsewhere in Meridiani [Poulet et al., 2008; Wiseman et al.,2008; Marzo et al., 2009] and more broadly in westernArabia Terra, in scattered outcrops stretching northward tothe extensive exposures around Mawrth Vallis [Poulet et al.,2005; Bishop et al., 2008; E. Z. Noe Dobrea et al.,Mineralogy and stratigraphy of phyllosilicate-bearing anddark mantling units in the greater Mawrth Vallis/WestArabia Terra area: Constraints on geologic origin, submittedto Journal of Geophysical Research, 2009]. The phyllosili-cates in the rim of Endeavour crater may thus record aprocess of regional (or larger)-scale aqueous alterationdistinct from any yet sampled by a landed mission. It hasbeen suggested that the correlation of remotely sensedphyllosilicates to Early–Mid Noachian terrains and hydratedsulfates to Late Noachian–Hesperian terrains reflects aglobal change in aqueous conditions, with decreasing pHand water activity, and a corresponding decline in theplanet’s habitability over time [Bibring et al., 2006].Endeavour crater may represent our first opportunity totest this hypothesis in situ.[15] The closest point on the rim of Endeavour (Figure 1c)

is �12 km from the rover’s position on sol 2009(18 September 2009). This nearest rim segment exposesbedrock with polygonal textures (Figure 3c) and a tentativephyllosilicate signature (Figure 1c), and the adjacent plainshave a hydrated sulfate spectral signature. If Opportunityreaches this location with its Athena payload [Squyres et al.,2003] still largely functional, the Mossbauer spectrometermay be able to independently confirm the presence of Fe-bearing phyllosilicates in the rim, and could identify theiroxidation state and associated minerals [e.g., Dyar et al.,2008]. The Miniature Thermal Emission Spectrometer mayalso be able to confirm the presence of phyllosilicates andconstrain their composition [e.g., Michalski et al., 2006].The Alpha Particle X-Ray Spectrometer could determinemajor and minor element chemistry, and the Pancam andMicroscopic Imager could document grain sizes, shapes,and possible sedimentary textures (e.g., cross-bedding orlaminations too fine to resolve from orbit), providing criticalnew constraints on the processes that emplaced thesephyllosilicate-bearing rocks on ancient Mars.

[16] Acknowledgments. We thank S. W. Ruff and G. A. Swayze forhelpful discussions. Comments from J. L. Bishop, R. E. Milliken, B. L.Ehlmann, L. L. Tornabene, and our two anonymous reviewers improved themanuscript. JJW thanks the Fannie & John Hertz Foundation and NSFGraduate Research Fellowship for support. We are indebted to the CRISM,HiRISE, and MER science and engineering teams for their dedication to asynergistic program of Mars exploration that has made our work possible.

ReferencesArvidson, R. E., F. Poulet, J.-P. Bibring, M. Wolff, A. Gendrin, R. V.Morris, J. J. Freeman, Y. Langevin, N. Mangold, and G. Bellucci(2005), Spectral reflectance and morphologic correlations in eastern TerraMeridiani, Mars, Science, 307, 1591 – 1594, doi:10.1126/science.1109509.

Arvidson, R. E., et al. (2006), Nature and origin of the hematite-bearingplains of Terra Meridiani based on analyses of orbital and Mars Explora-tion rover data sets, J. Geophys. Res., 111, E12S08, doi:10.1029/2006JE002728.

Bibring, J.-P., et al. (2006), Global mineralogical and aqueous Mars historyderived from OMEGA/Mars Express data, Science, 312, 400 –404,doi:10.1126/science.1122659.

Figure 3. Fine-scale textures of phyllosilicate-bearing rimmaterials, illuminated from lower left. (a) Western rim;enhanced color from HiRISE ESP_012398_1775, grayscalefrom PSP_010486_1775. (b) Western rim anaglyph;PSP_010341_1775/PSP_010486_1775. Light-toned mate-rials at lower left are sulfate-bearing sediments inferred toonlap the rim. (c) Northwest rim; PSP_010486_1775.(d) Southwest rim; PSP_010341_1775. (e) Northern rim;ESP_013044_1775.

L21201 WRAY ET AL.: PHYLLOSILICATES AT ENDEAVOUR CRATER L21201

4 of 5

Bishop, J. L., and C. M. Pieters (1995), Low-temperature and low atmo-spheric pressure infrared reflectance spectroscopy of Mars soil analogmaterials, J. Geophys. Res., 100, 5369–5379, doi:10.1029/94JE03331.

Bishop, J. L., E. Murad, and M. D. Dyar (2002), The influence of octahe-dral and tetrahedral cation substitution on the structure of smectites andserpentines as observed through infrared spectroscopy, Clay Miner., 37,617–628, doi:10.1180/0009855023740064.

Bishop, J. L., et al. (2008), Phyllosilicate diversity and past aqueous activityrevealed at Mawrth Vallis, Mars, Science, 321, 830–833, doi:10.1126/science.1159699.

Chevrier, V., F. Poulet, and J.-P. Bibring (2007), Early geochemical envir-onment of Mars as determined from thermodynamics of phyllosilicates,Nature, 448, 60–63, doi:10.1038/nature05961.

Christensen, P. R., et al. (2000), Detection of crystalline hematite miner-alization on Mars by the Thermal Emission Spectrometer: Evidence fornear-surface water, J. Geophys. Res., 105, 9632–9642, doi:10.1029/1999JE001093.

Clark, R. N., G. A. Swayze, R. A. Wise, K. E. Livo, T. M. Hoefen, R. F.Kokaly, and S. J. Sutley (2007), USGS digital spectral library splib06a,U.S. Geol. Surv. Digital Data Ser., 231, (Available at http://speclab.cr.usgs.gov/spectral.lib06).

Cloutis, E. A., et al. (2006), Detection and discrimination of sulfate miner-als using reflectance spectroscopy, Icarus, 184, 121–157, doi:10.1016/j.icarus.2006.04.003.

Crowley, J. K. (1991), Visible and near-infrared (0.4–2.5 mm) reflectancespectra of playa evaporite minerals, J. Geophys. Res., 96, 16,231–16,240, doi:10.1029/91JB01714.

Dyar, M. D., M. W. Schaeffer, E. C. Sklute, and J. L. Bishop (2008),Mossbauer spectroscopy of phyllosilicates: Effects of fitting models onrecoil-free fractions and redox ratios, Clay Miner., 43, 3 – 33,doi:10.1180/claymin.2008.043.1.02.

Ehlmann, B. L., et al. (2009), Identification of hydrated silicate minerals onMars using MRO-CRISM: Geologic context near Nili Fossae and impli-cations for aqueous alteration, J. Geophys. Res., 114, E00D08,doi:10.1029/2009JE003339.

Gendrin, A., et al. (2005), Sulfates inMartian layered terrains: The OMEGA/Mars Express view, Science, 307, 1587 –1591, doi:10.1126/science.1109087.

Golombek, M. P., et al. (2003), Selection of the Mars Exploration Roverlanding sites, J. Geophys. Res., 108(E12), 8072, doi:10.1029/2003JE002074.

Grotzinger, J. (2009), Beyond water on Mars, Nat. Geosci., 2, 231–233,doi:10.1038/ngeo480.

Marzo, G. A., T. L. Roush, N. L. Lanza, P. C. McGuire, H. E. Newsom,A. M. Ollila, and S. M. Wiseman (2009), Association of phyllosilicatesand the inverted channel in Miyamoto crater, Mars, Geophys. Res. Lett.,36, L11204, doi:10.1029/2009GL038703.

McEwen, A. S., et al. (2007), Mars Reconnaissance Orbiter’s High Resolu-tion Imaging Science Experiment (HiRISE), J. Geophys. Res., 112,E05S02, doi:10.1029/2005JE002605.

McEwen, A. S., et al. (2009), The High Resolution Imaging ScienceExperiment (HiRISE) during MRO’s primary science phase (PSP),Icarus, doi:10.1016/j.icarus.2009.04.023, in press.

McGuire, P. C., et al. (2009), An improvement to the volcano-scan algo-rithm for atmospheric correction of CRISM and OMEGA spectral data,Planet. Space Sci., 57, 809–815, doi:10.1016/j.pss.2009.03.007.

Michalski, J. R., M. D. Kraft, T. G. Sharp, L. B. Williams, and P. R.Christensen (2006), Emission spectroscopy of clay minerals and evidencefor poorly crystalline aluminosilicates on Mars from Thermal EmissionSpectrometer data, J. Geophys. Res., 111, E03004, doi:10.1029/2005JE002438.

Milliken, R. E., and J. F. Mustard (2005), Quantifying absolute water con-tent of minerals using near-infrared reflectance spectroscopy, J. Geophys.Res., 110, E12001, doi:10.1029/2005JE002534.

Milliken, R. E., W. W. Fischer, and J. A. Hurowitz (2009), Missing saltson early Mars, Geophys. Res. Lett., 36, L11202, doi:10.1029/2009GL038558.

Murchie, S., et al. (2007), Compact Reconnaissance Imaging Spectrometerfor Mars (CRISM) on Mars Reconnaissance Orbiter (MRO), J. Geophys.Res., 112, E05S03, doi:10.1029/2006JE002682.

Murchie, S. L., et al. (2009), Compact Reconnaissance Imaging Spectro-meter for Mars investigation and data set from the Mars ReconnaissanceOrbiter’s primary science phase, J. Geophys. Res., 114, E00D07,doi:10.1029/2009JE003344.

Mustard, J. F., et al. (2008), Hydrated silicate minerals on Mars observed bythe Mars Reconnaissance Orbiter CRISM instrument, Nature, 454, 305–309, doi:10.1038/nature07097.

Parente, M. (2008), A new approach to denoising CRISM images, LunarPlanet. Sci., XXXIX, Abstract 2528.

Pelkey, S. M., et al. (2007), CRISM multispectral summary products: Para-meterizing mineral diversity on Mars from reflectance, J. Geophys. Res.,112, E08S14, doi:10.1029/2006JE002831.

Poulet, F., et al. (2005), Phyllosilicates on Mars and implications for earlyMartian climate, Nature, 438, 623–627, doi:10.1038/nature04274.

Poulet, F., R. E. Arvidson, C. Gomez, R. V. Morris, J.-P. Bibring,Y. Langevin, B. Gondet, and J. Griffes (2008), Mineralogy of TerraMeridiani and western Arabia Terra from OMEGA/MEx and implicationsfor their formation, Icarus, 195, 106–130, doi:10.1016/j.icarus.2007.11.031.

Squyres, S. W., et al. (2003), Athena Mars rover science investigation,J. Geophys. Res., 108(E12), 8062, doi:10.1029/2003JE002121.

Squyres, S. W., et al. (2004), In situ evidence for an ancient aqueousenvironment at Meridiani Planum, Mars, Science, 306, 1709–1714,doi:10.1126/science.1104559.

Squyres, S. W., et al. (2006), Overview of the Opportunity Mars Explora-tion Rover Mission to Meridiani Planum: Eagle Crater to PurgatoryRipple, J. Geophys. Res., 111, E12S12, doi:10.1029/2006JE002771.

Squyres, S. W., et al. (2009), Exploration of Victoria Crater by the MarsRover Opportunity, Science, 324, 1058–1061, doi:10.1126/science.1170355.

Swayze, G. A. (2004), Using reflectance spectroscopy to evaluate mineralsof environmental concern, in Infrared Spectroscopy in Geochemistry,Exploration Geochemistry, and Remote Sensing, edited by P. L. King,M. S. Ramsey, and G. A. Swayze, pp. 181–196, Mineral. Assoc. Can.,London, Ontario, Canada.

Swayze, G. A., R. N. Clark, S. J. Sutley, C. A. Gent, B. W. Rockwell, D. L.Blaney, J. L. Post, and B. P. Farm (2002), Mineral mapping Mauna Keaand Mauna Loa shield volcanoes on Hawaii using AVIRIS data and theUSGS Tetracorder spectral identification system: Lessons applicable tothe search for relict Martian hydrothermal systems, in Proceedings of the11th JPL Airborne Earth Science Workshop, edited by R. O. Green, JPLPubl. 03-4, 373–387.

Tosca, N. J., A. H. Knoll, and S. M. McLennan (2008), Water activity andthe challenge for life on early Mars, Science, 320, 1204 – 1207,doi:10.1126/science.1155432.

Wiseman, S. M., et al. (2008), Phyllosilicate and sulfate-hematite depositswithin Miyamoto crater in southern Sinus Meridiani, Mars, Geophys.Res. Lett., 35, L19204, doi:10.1029/2008GL035363.

Wray, J. J., B. L. Ehlmann, S. W. Squyres, J. F. Mustard, and R. L. Kirk(2008), Compositional stratigraphy of clay-bearing layered deposits atMawrth Vallis, Mars, Geophys. Res. Lett., 35, L12202, doi:10.1029/2008GL034385.

Wray, J. J., S. L. Murchie, S. W. Squyres, F. P. Seelos, and L. L. Tornabene(2009), Diverse aqueous environments on ancient Mars revealed in thesouthern highlands, Geology, 37, 1043–1046, doi:10.1130/G30331A.1.

�����������������������R. E. Arvidson and S. M. Wiseman, Department of Earth and Planetary

Sciences, Washington University, 1 Brookings Dr., Saint Louis, MO 63130-4862, USA.A. S. McEwen, Lunar and Planetary Laboratory, University of Arizona,

1541 E. Univ. Blvd., Tucson, AZ 85721-0092, USA.S. L. Murchie, Johns Hopkins University Applied Physics Laboratory,

11100 Johns Hopkins Rd., Laurel, MD 20723, USA.J. F. Mustard, Department of Geological Sciences, Brown University,

324 Brook Street, Campus Box 1846, Providence, RI 02912, USA.E. Z. Noe Dobrea, Jet Propulsion Laboratory, California Institute of

Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA.S. W. Squyres and J. J. Wray, Department of Astronomy, Cornell

University, 425 Space Sciences, Ithaca, NY 14853, USA. (jwray@astro.cornell.edu)

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