Icarus 209 (2010) 416–421

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Icarus

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Identification of the Ca-sulfate bassanite in Mawrth Vallis, Mars

James J. Wray a,*, Steven W. Squyres a, Leah H. Roach b, Janice L. Bishop c, John F. Mustard d,Eldar Z. Noe Dobrea e

a Department of Astronomy, Cornell University, Ithaca, NY 14853, USAb Frontier Technology, Inc., 100 Cummings Center, Suite 450G, Beverly, MA 01915, USAc Carl Sagan Center, SETI Institute, Mountain View, CA 94043, USAd Department of Geological Sciences, Brown University, Providence, RI 02912, USAe Planetary Science Institute, 1700 E. Fort Lowell Rd., Suite 106, Tucson, AZ 85719, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 February 2010Revised 29 May 2010Accepted 1 June 2010Available online 9 June 2010

Keywords:Mars, SurfaceMineralogySpectroscopy

0019-1035/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.icarus.2010.06.001

* Corresponding author. Address: 425 Space ScienceIthaca, NY 14853, USA.

E-mail address: jwray@astro.cornell.edu (J.J. Wray

The region surrounding the Mawrth Vallis outflow channel on Mars hosts thick layered deposits contain-ing diverse phyllosilicate minerals. Here we report detection of the Ca-sulfate bassanite on the outflowchannel floor, requiring a more complex aqueous chemistry than previously inferred for this region.The sulfate-bearing materials underlie phyllosilicate-bearing strata, and provide an opportunity for test-ing proposed models of martian geochemical evolution with a future landed mission.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Hydrated minerals on the surface of Mars including sulfates andphyllosilicates attest to a rich planetary history of water–rockinteractions (e.g., Squyres et al., 2004; Bibring et al., 2006). Thereis increasing evidence for diverse conditions of aqueous alteration,which likely varied over time and space (e.g., Mustard et al., 2008;Murchie et al., 2009a; Ehlmann et al., 2009). From the perspectiveof planetary habitability, perhaps the most intriguing are the Noa-chian-aged exposures of phyllosilicates, which are inferred to haveformed through alteration at weakly acidic to alkaline pH (Chevrieret al., 2007).

The region surrounding Mawrth Vallis contains one of the larg-est exposures of phyllosilicates on the martian surface (Poulet et al.,2005; Bibring et al., 2006). The phyllosilicates occur in light-tonedlayered deposits of possible sedimentary or pyroclastic origin(Loizeau et al., 2007, 2010; Michalski and Noe Dobrea, 2007), andexhibit a ubiquitous stratigraphic pattern of Al-rich phyllosilicatesincluding kaolinite and montmorillonite overlying Fe-rich nontro-nite (Wray et al., 2008; Bishop et al., 2008; McKeown et al.,2009). Exposures are scattered across a region �1000 � 1000 kmin size (Noe Dobrea et al., 2010). Formation of these phyllosilicatesmust have produced excess cations that were accommodated in

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complementary salts or oxides (Milliken et al., 2009), yet to dateonly ferric oxides/hydroxides (Wray et al., 2008; Poulet et al.,2008b; McKeown et al., 2009) and highly localized jarosite (KFe3-[SO4]2[OH]6; Farrand et al., 2009) have been identified in the region.Weak absorptions at �2.4–2.5 lm in spectra dominated by phyllo-silicates may be consistent with other secondary minerals as a les-ser constituent of these rocks (Bishop et al., 2009b), but thesespectral features are not yet assigned to specific minerals.

Here we report on a search for locations in the Mawrth Vallis re-gion in which hydrated salts are the spectrally dominant compo-nent, for which we used data from the Compact ReconnaissanceImaging Spectrometer for Mars (CRISM; Murchie et al., 2007) sup-plemented by images from the High Resolution Imaging ScienceExperiment (HiRISE; McEwen et al., 2007), both on NASA’s MarsReconnaissance Orbiter.

2. Methods

We examined browse products (Seelos et al., 2008) for CRISMtargeted observations distributed across the region surroundingMawrth Vallis (Fig. 1). In particular, we searched for spatially con-tiguous areas highlighted by the BD1900 parameter, which trackshydrated minerals, but not highlighted by the BD2210 or D2300parameters that track Al- or Fe/Mg-phyllosilicates, respectively(Pelkey et al., 2007). We took special note of locations highlightedby the SINDEX parameter used to map hydrated sulfates (Roachet al., 2009).

Fig. 1. The region surrounding Mawrth Vallis (24�N, 341�E). CRISM targeted imagesacquired to date are outlined in red. Background is THEMIS daytime IR mosaiccolorized with MOLA digital elevation model. Candidate Mars Science Laboratorylanding ellipse (yellow, near center) is 25 by 20 km. The three locations at which wedetect probable hydrated sulfate are numbered, while the location at which jarositewas identified by Farrand et al. (2009) is marked with a ‘‘J”. (For interpretation ofthe references to color in this figure legend, the reader is referred to the web versionof this article.)

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CRISM I/F observations of interest were subsequently processedas described by Murchie et al. (2009b), including division by thecosine of the solar incidence angle and atmospheric separationvia division by a scaled transmission spectrum derived from obser-vations over Olympus Mons (McGuire et al., 2009). We also appliedthe spatial and spectral noise filtering algorithm of Parente (2008).Large numbers of pixels were averaged to improve signal-to-noiseratio, and the result was divided by an average of spectrally neutralpixels in the same CRISM scene. This spectral ratio method sup-presses residual artifacts of instrument calibration and atmo-spheric separation while accentuating unique spectral signaturesin the numerator spectrum. Known artifacts that remain in someratio spectra include a discontinuity near 1.65 lm due to a detectorfilter boundary and small features near 2.0 lm resulting fromimperfect removal of atmospheric CO2 bands (Murchie et al.,2009b); we masked these artifacts as well as ‘‘bad bands” near1.0 lm in the spectra plotted here.

Spectral summary parameters (Pelkey et al., 2007) enable map-ping of CRISM pixels having spectra with absorptions characteristicof various minerals. In particular, we used the D2300 parameter ofPelkey et al. (2007) to map Fe/Mg-phyllosilicates, and the BD2200and BD2500 parameters of Ehlmann et al. (2009) to map, respec-tively, Al-phyllosilicates and the hydrated phase(s) with a�2.5 lm absorption presented below. These maps—which we havevalidated by plotting spectra from key locations—were overlain onHiRISE images to elucidate compositional stratigraphy. HiRISE IRBcolor images (i.e., those with the near-IR, red, and blue–greenimages displayed in red, green, and blue channels, respectively)and stereo anaglyphs (McEwen et al., 2010) were used to gain addi-tional insights into composition and stratigraphy.

3. Results

We identify three locations with spectra dominated by a non-phyllosilicate hydrated phase, all of which occur on the floor ofMawrth Vallis (Fig. 1). Spectra from these sites share strong bandsat 1.91 and 2.48 lm (Fig. 2a), and they do not have the distinct

absorptions at 2.2 or 2.3 lm characteristic of phyllosilicates (e.g.,nontronite). The overall slope of these spectra from 1 to 2.5 lmis influenced by the choice of spectral denominator, but the posi-tions of the 1.91 and 2.48 lm bands are independent of this choice.The same band positions are also observed in multiple CRISMobservations at each site (Full-Resolution Targets 9326, 10424,and 13E49 for sites 1 and 2; FRTs C872, 12D3C, and 12E72 for site3).

Several different hydrated salts and zeolites have absorptionsnear 1.9 and 2.4–2.5 lm (Fig. 2a); however, Ca-sulfates providethe best match to our CRISM spectra. The spectrum from site 1exhibits the strongest absorptions at 1.91 and 2.48 lm and hasadditional features including a doublet at 1.43 and 1.47 lm andweaker features at 1.77, 2.10, and 2.26 um. Together these spectralfeatures are most consistent with bassanite (CaSO4�½H2O). Forcomparison, gypsum (CaSO4�2H2O) has a similar suite of absorp-tions at different wavelengths: 1.74, 1.94, 2.21, and 2.41 lm, witha broader triplet at 1.4–1.5 lm and a 1.2 lm band not observed inour spectrum from site 1. Some hydrated chlorides such as carnall-ite also have similar spectral features, but again their exact wave-lengths are distinct from what we observe here (Fig. 2a).Polyhydrated Mg-sulfates lack absorptions at 2.5 or 1.75–1.8 lmand are an even poorer match. Indeed, to our knowledge no mea-sured hydrated salts share the narrow 1.91 lm and strong2.48 lm absorptions of bassanite (e.g., Crowley, 1991).

Zeolites are also characterized by strong absorptions at �1.4,1.9, and 2.4–2.5 lm (Cloutis et al., 2002), and their presence inthe Mawrth Vallis region has been suggested on the basis of ther-mal infrared spectral analysis (Michalski and Fergason, 2009).However, near-IR spectra of most zeolites have no distinct mini-mum longward of 2.4 lm nor a band in the 1.75–1.8 lm range(Cloutis et al., 2002). A counterexample is analcime, which hasthese two characteristics and has been identified elsewhere onMars (Ehlmann et al., 2009; Wray et al., 2009), but its band centersat 1.79 lm and 2.52 lm are distinct from those in our site 1 spec-trum (Fig. 2a). No other mineral or combination of minerals in ourspectral libraries has all the features in the observed spectrum;therefore, we conclude that the spectrum of site 1 indicates bassa-nite as the spectrally dominant component, although additionalminerals that are featureless in this spectral range (e.g., anhydroussulfates) could also be present. Spectra from site 2 and the noisierobservations of site 3 are also consistent with—though less diag-nostic of—bassanite, possibly in mixtures with Fe-bearing phasesthat could be responsible for the spectral curvature from 1 to1.5 lm and the obscuration of the 1.4 lm absorption (Fig. 2a).

Comparing the mapped distribution of the probable bassaniteto imaging and topographic data reveals that it is found in topo-graphic depressions (e.g., Fig. 2d) that are surrounded by exposuresof Fe/Mg-smectite (Fig. 2b and c). Although some of the bassanitecould be erosional debris accumulating in these depressions, HiR-ISE images show that it also occurs in distinct layers (Fig. 3a) thatstratigraphically underlie the adjacent Fe/Mg-smectites (Fig. 3band c). Stratigraphic boundaries between the smectite and sulfaterange from gradual (Fig. 3a) to sharp (Fig. 3c) relative to CRISM res-olution. Color images show that the areas with the strongest sul-fate signatures are bluer than adjacent Fe/Mg-smectite-bearingmaterials (e.g., Fig. 3b), consistent with a lower Fe3+ content inthe spectrally dominant mineral (Delamere et al., 2010). The sul-fate-bearing materials exhibit varying fracture patterns at scalesof a few meters (Fig. 3a–c), and appear texturally distinct fromthe overlying smectite-bearing materials, suggesting differentphysical or chemical properties and/or a different modification his-tory. The base of the sulfate-bearing strata is not observed, butexposures such as that in Fig. 3a suggest a thickness of at least afew meters, based on the scarp geometry and assuming the bedsare approximately horizontal.

Fig. 2. Spectra and locations of newly identified hydrated phase in Mawrth Vallis. (a) CRISM spectra (median-filtered) from Mawrth Vallis (top) and lab spectra of hydratedminerals (bottom). CRISM spectra correspond to numbered locations in (b and c). Spectrum 1 (FRT00009326) is most consistent with bassanite, while 2 (FRT00009326) and 3(FRT00012E72) are less definitive and may be consistent with other hydrated salts or zeolites. Bassanite (CaSO4�½H2O) is sample GDS145, gypsum (CaSO4�2H2O) is HS333.3B,carnallite (KMgCl3�6H2O) is NMNH98011, analcime (Na-zeolite) is GDS1, and stilbite (Na/Ca-zeolite) is GDS8 from Clark et al. (2007); hexahydrite (MgSO4�6H2O) is LASF57Aand nontronite (Fe-smectite) is NBJB26 from CRISM spectral library. (b) CRISM FRT00009326 parameter maps overlain on HiRISE ESP_014007_2030; red is Fe/Mg-phyllosilicate, green Al-phyllosilicate, and blue a hydrated mineral with strong 2.5 lm band. See Fig. 1 for location. (c) CRISM FRT00012D3C on HiRISE ESP_013229_2050,colors same as in (b). (d) Stereo anaglyph covering same area as (c), from HiRISE ESP_013229_2050 and ESP_013295_2050.

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4. Discussion

On the basis of their distribution across terrains of differentages, Bibring et al. (2006) proposed that martian sulfates generallyformed later than phyllosilicates. This hypothesis can be tested byobservations of places where these two minerals are found in con-tact. Here we have shown that sulfate-bearing beds in Mawrth Val-lis underlie Fe/Mg-smectite-bearing beds, which had previouslybeen identified as the lowest stratigraphic unit exposed in the re-gion (Wray et al., 2008; McKeown et al., 2009). Although the tim-ing and location of smectite formation are unknown, the principleof superposition implies that the beds hosting the sulfates predate

those hosting the phyllosilicates. Taken together with stratigraphicanalyses of several other regions where phyllosilicates and sulfatesare interbedded or intimately mixed (Poulet et al., 2008a; Wrayet al., 2009; Milliken et al., 2010; Weitz et al., 2010), our resultssuggest that the alteration history of at least several regions onMars may have been different or more complex than the modelof Bibring et al. (2006) suggests.

However, an alternative explanation at Mawrth Vallis could bethat sulfates formed in the subsurface coevally with (or after)phyllosilicates. Specifically, if at least some of the region’s phyllo-silicates formed via top-down weathering that leached mono-and divalent cations (Na, Ca, Mg) from the upper horizons (e.g.,

Fig. 3. Meter-scale textures of probable sulfate-bearing layers and overlying smectite-bearing layers. (a) and (b) are from site 3 (see Fig. 2c for context); (c) is from site 2(context in Fig. 2b). All panels have the same scale and have been rotated �180� so that the downslope direction is toward the bottom. Red lines mark approximatemineralogic boundaries inferred from CRISM maps in Fig. 2b and c. (a and b) are from HiRISE ESP_013229_2050, (c) from ESP_014007_2030.

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McKeown et al., 2009; Noe Dobrea et al., 2010), then these shouldhave precipitated elsewhere in salts or oxides (Milliken et al.,2009). Especially on Mars, Ca-sulfate is the least soluble of the ex-pected salts (Tosca et al., 2008) and therefore could have precipi-tated in the lower horizons at Mawrth Vallis while more solublesalts—including the Mg/Fe sulfates that appear to dominate manyequatorial layered deposits (e.g., Gendrin et al., 2005; Roach et al.,2009; Bishop et al., 2009a)—were transported elsewhere either ini-tially or after subsequent dissolution (Milliken et al., 2009). Even ifbassanite in Mawrth Vallis formed only from the Ca liberated fromsmectite interlayer sites, leaching of the uppermost �50 m (Wrayet al., 2008; McKeown et al., 2009) of layered deposits �50%nontronite by volume (Poulet et al., 2008b) could have formed a�meter-thick layer of pure bassanite. If additional Ca was leachedfrom primary silicates and/or other minerals are present in the bas-sanite-bearing unit (both likely), then its thickness could be greater.This hypothesis could be tested by future in situ observations of theAl-clay, Fe/Mg-clay, and sulfate-bearing layers; in particular, majorelement chemistry and microscopic textures would constrainwhether the Al-clay horizons could have been the source of cationsthat combined with sulfate in solution to form the observed salts.

The detection of bassanite (CaSO4�½H2O) in Mawrth Vallis(23�N) is of interest to global studies of Ca-sulfates on Mars. Previ-ously, gypsum (CaSO4�2H2O) has been found in circumpolar dunesat �80�N (Langevin et al., 2005) and in Columbus crater at 29�S(Wray et al., 2009); preliminary reports of gypsum in equatoriallayered deposits (e.g., Gendrin et al., 2005) have been refuted bysubsequent analyses (Noe Dobrea et al., 2008; Kuzmin et al.,2009; Bishop et al., 2009a), although Ca-sulfates may be presentin one portion of Noctis Labyrinthus (Mangold et al., 2010). BothMars Exploration Rovers have also found evidence for Ca-sulfates,for which the orbital and surface remote sensing data are mostconsistent with anhydrite (CaSO4) in both the rocks of MeridianiPlanum at 2�S (Clark et al., 2005; Glotch et al., 2006) and the brightsoils of the Columbia Hills at 15�S (Johnson et al., 2007; Lane et al.,2008). Collectively, these observations suggest a general trend ofincreasing Ca-sulfate hydration with latitude, as might be expectedif relative humidity and/or surface temperature are controlling thehydration state. However, the slow kinetics of Ca-sulfate hydrationstate changes under Mars-like conditions may prevent equilibra-tion with the atmosphere (e.g., Vaniman et al., 2008, 2009), as ap-pears to be the case for martian Mg-sulfates (Roach et al., 2009).

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Bassanite could instead have formed via dehydration from gyp-sum under a warmer paleoclimate (Vaniman et al., 2009). Otheralternatives proposed by Vaniman et al. (2008) include bassaniteformation under hydrothermal conditions, via burial diagenesisof gypsum, or from acid-sulfate alteration of preexisting calciumcarbonates. The range of hydration states observed across the pla-net may therefore reflect differences in precursor mineralogy and/or conditions of sulfate formation or diagenetic history. For exam-ple, Ca-sulfates were likely buried to hundreds of meters depth atMeridiani Planum (Hynek et al., 2002) and Mawrth Vallis (Loizeauet al., 2007, 2010; Noe Dobrea et al., 2010), which may not have oc-curred in Columbus crater or the polar sand sea. In situ study of theminerals accompanying bassanite at Mawrth Vallis would help todistinguish between the diagenetic, hydrothermal, and acid alter-ation hypotheses for its origin. Once formed, bassanite may persistlonger at the surface of Mars than it does on Earth because of thecolder, more arid martian conditions (Vaniman et al., 2009).

Together with the jarosite identified by Farrand et al. (2009),our detection of bassanite adds to the already considerable miner-alogic diversity of the Mawrth Vallis region (Loizeau et al., 2007;Wray et al., 2008; Bishop et al., 2008; McKeown et al., 2009; NoeDobrea et al., 2010). Mawrth Vallis is one of the final candidatelanding sites for the Mars Science Laboratory mission, for whichit has been argued that an ideal site would provide access to bothphyllosilicates and sulfates (Grotzinger, 2009). Relative to the cur-rently favored landing ellipse, Farrand et al.’s (2009) jarosite lies�95 km from its northwest edge, whereas our site 3 discussedabove is �30 km to the northeast (Fig. 1). Additional CRISM cover-age of the channel floor should be acquired to search for other sul-fate exposures even nearer the ellipse. If the way is trafficable,driving eastward and down into Mawrth Vallis would also enablea direct test of whether some or all of the region’s phyllosilicatesand layered deposits predate the outflow channel (Poulet et al.,2005; Loizeau et al., 2007, 2010; Michalski and Noe Dobrea,2007) or constitute a younger drape deposit (Howard and Moore,2007; Wray et al., 2008), with significant implications for whenand how these deposits formed. Mawrth Vallis remains an intrigu-ing and geologically diverse region for future surface exploration.

Acknowledgments

J.J.W. thanks the Fannie and John Hertz Foundation and the NSFGraduate Research Fellowship Program for support. We thankBethany Ehlmann and Ralph Milliken for helpful discussions. Com-ments from Dave Vaniman and an anonymous reviewer improvedthe paper.

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