Columbus Crater and Other Possible Paleolakes in Terra Sirenum, Mars

Introduction: Inferred paleolakes on the surface of Mars are key targets for astrobiology. To date, pale-olakes have been identified primarily based on mor-phologic evidence, including inlet and outlet channels, deltas, and horizontal bedding in closed basins [1-3]. Clay minerals have been identified in some of these settings, but these may be detrital [4]. Other alteration products such as evaporite salts have been elusive in these environments [5]. Here we describe a unique set of finely layered in-tracrater deposits in northwest Terra Sirenum, in which several types of sulfates are found interbedded with kaolin group clays. These may be sediments precipitated in lakes under a range of pH conditions. Mineral Identifications: Hydrated minerals are identified and mapped using high resolution CRISM data [6]. Standard atmospheric and photometric corrections were applied to I/F spectra [4]. Spectra shown are ratios between hydrated mineral exposures and areas of low spectral contrast in the same scene. Ratios suppress instrumental artifacts and residual atmospheric bands [4]. We focus on the 100 km diameter crater Columbus (29ºS, 166ºW), which presently has the best CRISM coverage in this region. Hydrated minerals are present in all eight CRISM targeted images of Columbus. The most common phases are polyhydrated sulfates (Fig. 1) and a kaolin group clay (Fig. 2). Two spectrally distinct polyhy-drated salts are observed; the first has absorptions at 1.43, 1.93, and ~2.4 μm, consistent with a Mg-sulfate (e.g., hexahydrite, MgSO 4 ·6H 2 O; Fig. 1a), whereas the other—distinguished by additional absorptions at 1.19 and 1.75 μm—may be either a different sulfate or a hydrated chloride (Fig. 1b). Gypsum (CaSO 4 ·2H 2 O) best matches the positions and shapes of most bands, but has absorptions at ~2.2 μm not seen in the CRISM spectrum. Bassanite (CaSO 4 ·0.5H 2 O) and mirabilite (Na 2 SO 4 ·10H 2 O) are flatter near 2.2 μm. Carnallite (KMgCl 3 ·6H 2 O) is also spectrally similar to gypsum. Of these candidates, only gypsum has previously been detected from orbit, in the northern circumpolar dunes [7]. Opportunity has also identified Ca-sulfates in Me-ridiani Planum [e.g., 8]. One small exposure of layered materials has a distinct spectrum with an absorption at 2.26 μm (Fig. 3); the only library spectra [6,9] with this band are those of jarosite ([K,Na]Fe 3 [SO 4 ] 2 [OH] 6). A second absorp