Coordinating CRISM Observations of Sulfates near Valles Marineris with the Subsurface Bright Salty Soils Exposed in Gusev Crater via Lab Experiments

CRISM has identified unique spectral signatures in inverted channels near Juventae Chasma and other chasmata in the greater Valles Marineris region [1] that are composed of light-toned layered sediments interpreted to be fluvial in origin [2]. These include a weak, broad feature near 1.45 μm, a strong, broad band centered near 1.94 μm, a sharp band at 2.23 μm, and a shoulder or band near 2.4 μm [3]. This signature is not characteristic of any single mineral; however, it is consistent with partially dehydrated ferricopiapite [original formula of Fe0.33Fe2(SO4)3(OH)·10H2O]. Our lab experiments show that incrementally heating this hydrated ferric sulfate to 300 °C at 1 atm changes the color, XRD pattern, and spectral properties of this mineral. The resulting spectral signature of our ferricopiapite heated to 300 °C matches well with the CRISM spectra of the material observed in inverted channels near Juventae Chasma. This result is of particular interest as ferricopiapite is the mineral thought to be present in the bright salty soils exposed by the Spirit rover at Paso Robles and other sites in combined analyses of the Pancam, Mössbauer, Mini-TES and APXS data [4, 5, 6]. Other sulfate minerals possibly present in lower abundance include butlerite, (para)coquimbite, fibroferrite, and metahohmanite [4]. Continued lab experiments are underway to characterize the spectral properties of partially dehydrated ferricopiapite and other OH and H2O-bearing sulfates. Opaline silica is also found in these inverted channels near Juventae Chasma [1] and both monohydrated (szomolnokite and kieserite) and polyhydrated (e.g. ferricopiapite) sulfates are observed in the bright mounds inside the chasma [7]. Identification of partially dehydrated ferricopiapite in these inverted channels provides a link between the aqueous processes occurring in the plains outside the chasma and those processes that formed the light-toned layered mounds inside the chasma. Sulfate deposits in the greater Valles Marineris region are consistent with lithification and diagenetic modification of eolian sediments by evaporation of near-surface groundwater [8], processes similar to those inferred at Meridiani [9,10]. [1] Milliken R. E. et al. (2008) Geology, in press. [2] Weitz C. M. et al. (2008) GRL, doi:10.1029/2008GL035317, in press. [3] Bishop J. L. et al. (2008) JGR, to be submitted. [4] Lane M. D. et al. (2008) Am. Miner., 93, 728-739. [5] Parente M. et al. (2008) Icarus, in review. [6] Johnson J. R. et al. (2007) GRL, 34, L13202, doi:10.1029/2007GL029894. [7] Bishop J. L. et al. (2008) LPSC, abs. 2334. [8] Murchie S. L. et al. (2008) Nature, in revision. [9] Squyres S. W. et al. (2006) Science, 313, 1403- 1407. [10] Andrews-Hanna J. C. et al. (2007) Nature, 446, 163-166 doi:10.1038/nature05594.