TOPOGRAPHIC ANALYSIS OF CARBONATE-BEARING DEPOSITS IN NILI FOSSAE ON MARS

Small exposures of Mg-carbonate outcrop over an extensive area in eastern Nili Fossae [Ehlmann et al., 2008, 2009]. Significant relief occurs at small scales (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. The best exposures occur near the bases of mesas. Surface ambient and shallow subsurface / low grade hydrothermal carbonate formation mechanisms have been proposed [Ehlmann et al., 2008].

We are analyzing the elevation and topographic expression of carbonate exposures as a means to constrain their formation environment. Carbonates have distinctive spectral signatures in the near infrared [Gafey, 1987] that allow identification with orbital CRISM [Murchie et al., 2007] hyperspectral images. CRISM parameter maps (20-40m/pixel) were used to identify carbonate exposures (75 to 80E, 15 to 25N) and overlain on HRSC [Neukum et al., 2004] digital terrain maps (50 - 100m/pixel). Elevations of carbonate exposures are consistent with the regional slope with southern deposits occurring at elevations as low as -2500m and northern exposures up to -500m below datum. At local scales, carbonate deposits sometimes occur in both topographic lows and along the bases of higher standing terrain with a range of up to 200m in elevation. This may be a result of mobilization and redisposition of carbonate, similar to what is observed within Jezero crater [Ehlmann et al., 2008b, Goudge et al., 2013].

In order to assess carbonate formation mechanisms, we will focus on mapping deposits that occur at the bases of mesas to minimize analysis of redistributed carbonates. Elevations consistent with an equipotential surface could indicate surface deposition in a shallow body of water whereas disjointed exposures may suggest localized hydrothermal activity. Preferential alteration of a preexisting unit to carbonate (e.g., alteration of the olivine-bearing unit as suggested by Ehlmann et al., 2008]) via episodic surface water or as a result of ground water will also be considered to explain observed elevation distributions of carbonate-bearing exposures.