RELATIVE CARBONATE ABUNDANCES THROUGHOUT NILI FOSSAE, MARS

Carbonates are exposed in Nili Fossae and occur in association with olivine, smectite, serpentine [Ehlmann et al., 2008, 2010], and possibly talc [Brown et al., 2010; Viviano et al., 2013]. The Nili Fossae carbonates are hypothesized to have formed via alteration of olivine and/or serpentine [Ehlmann et al., 2008; 2010] at surface or shallow subsurface low temperature hydrothermal conditions [Ehlmann et al., 2008; 2009]. Laboratory spectra of Mg carbonates (magnesite/hydromagnesite) [Ehlmann et al., 2008] are the closest spectral match to the Nili Fossae carbonates. However, the Nili Fossae carbonate-bearing exposures occur as mixed CRISM pixels with olivine and phyllosilicate spectral features in addition to carbonate absorptions.

Small exposures of Mg-carbonate outcrop over an extensive area in eastern Nili Fossae. The carbonate-bearing rocks are exposed as a result of differential erosion. Significant relief occurs at small scales in the Nili Fossae region (e.g., mesas and troughs) and the regional slope is to the SE toward Isidis basin. The carbonate-bearing exposures are associated with Noachian-aged Fe/Mg smectite-bearing basement. Olivine-rich sand occurs in topographic lows and presumably obscures underlying carbonates resulting in discontinuous exposures in some flat-lying areas.

We use Hapke mixture modeling [Hapke, 1993] of CRISM [Muchie et al., 2007] visible/near-infrared spectra to map relative abundances of carbonates across Nili Fossae. In order to better constrain carbonate formation environment, we combine relative carbonate abundance maps with geologic context and mineral assemblages derived from CRISM images.