Equatorial and midlatitude distribution of chlorine measured by Mars Odyssey GRS

[ 1] The 2001 Mars Odyssey Gamma Ray Spectrometer (GRS) has made the first measurement of the equatorial and midlatitude distribution of Cl at the near-surface of Mars. A mean concentration value of 0.49 wt% Cl has been determined from a grand sum of GRS spectra collected over the planet excluding high-latitude regions. Cl is significantly enriched within the upper few tens of centimeters of the surface relative to the Martian meteorites and estimates for the bulk composition of the planet. However, Cl is not homogeneously distributed and varies by a factor of similar to 4 even after smoothing of data with a 10 degrees-arc-radius filter. Several contiguous, geographically large (> 20 degrees) regions of high and low Cl concentrations are present. In particular, a region centered over the Medusae Fossae Formation west of Tharsis shows significantly elevated Cl. A large region north of Syrtis Major extending into Utopia Planitia in the northern hemisphere shows the lowest Cl concentrations. On the basis of hierarchical multivariate correlations, Cl is positively associated with H while negatively associated with Si and thermal inertia. We discuss four possible geologic mechanisms ( aeolian, volcanic, aqueous, and hydrothermal) that may have affected the Cl distribution seen by GRS. While some of the distribution may be due to Cl-rich dust deposits transported by aeolian processes, this mechanism does not appear to account for all of the observed variability. We propose that reactions with volcanic exhalations may have been important for enriching Cl in Medusae Fossae Formation material.