Diverse Aqueous Conditions on Mars from New Orbital Detections of Carbonate and Sulfate

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Scientific paper

Diverse aqueous environments on ancient Mars have been a key inference from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on NASA's Mars Reconnaissance Orbiter, which has identified many alteration minerals in a range of settings [e.g., 1-4]. Here we report two new minerals detected using CRISM.
In the southern highlands northwest of the Hellas basin, a mid-sized crater exposes carbonate in its central uplift. Spectral absorptions at 1, 2.33, and 2.53 microns are most consistent with Fe-carbonate, distinct from the Mg-carbonates identified from orbit by [5]. Fe-carbonate is associated with Mg-phyllosilicate in fractured materials formerly buried kilometers beneath the surface, and--like the Mg/Fe-carbonate found by the Spirit rover [6]--suggests a reducing, neutral-to-alkaline alteration environment.
One of the largest phyllosilicate exposures on Mars occurs in the Mawrth Vallis region [e.g., 7]. We identify bassanite (Ca-sulfate hemihydrate) in layers underlying the phyllosilicate-bearing beds [8], a stratigraphy distinct from that predicted by global models of martian aqueous history [9]. Bassanite could have formed via acid-sulfate alteration of Ca-carbonate, through dehydration of gypsum, or under hydrothermal conditions [10].
These detections expand the known mineralogic diversity of Mars and the range of environments to explore for past habitability.
[1] Mustard, J. F. et al. (2008) Nature 454, 305-309.
[2] Murchie, S. L. et al. (2009) J. Geophys. Res. 114, E00D06.
[3] Ehlmann, B. L. et al. (2009) J. Geophys. Res. 114, E00D08.
[4] Wray, J. J. et al. (2009) Geology 37, 1043-1046.
[5] Ehlmann, B. L. et al. (2008) Science 322, 1828-1832.
[6] Morris, R. V. et al. Science, in press, doi:10.1126/science.1189667.
[7] Poulet, F. et al. (2005) Nature 438, 623-627.
[8] Wray, J. J. et al. Icarus, in press, doi:10.1016/j.icarus.2010.06.001.
[9] Bibring, J.-P. et al. (2006) Science 312, 400-404.
[10] Vaniman, D. T. et al. (2009) LPSC 40, 1654.

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