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January, 2008: Chlorine MapThis gamma ray spectrometer map of the mid-latitude region of Mars is based on gamma rays from the element chlorine. Chlorine, with the chemical symbol Cl, is much more abundant at the Martian surface than in the rocks and soil at surface of the Earth. An area of very high chlorine content, shown in red, appear in the region west of Tharsis (left side of map). The other region of the planet with high chlorine is centered around Arabia Terra (center of map). Geologic processes that can enhance the chlorine content of the soil include volcanic activity, hydrothermal activity, evaporation of water leaving behind salt deposits, and deposition of chlorine rich dust. A combination of these processes is probably responsible for the chlorine distribution found at the Martian surface. The white sections at the top and bottom of the map represent regions of the planet where obtaining concentration values for chlorine is more difficult due to large amounts of water ice in the soil. The locations of the five successful lander missions are marked: Viking 1 (VL1), Viking 2 (VL2), Pathfinder (PF), Spirit at Gusev (G), and Opportunity at Meridiani (M). More information about this map can be found in the recent article by Boynton, W. V., et al. (2007), Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars . You may obtain full resolution versions of this and other elemental maps at http://grs.lpl.arizona.edu/specials/. 
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