DISTRIBUTION OF PERCHLORATE SALTS AT THE MARS PHOENIX LANDING SITE: INITIAL RESULTS FROM SPECTRAL MAPPING

The Mars Phoenix mission, which landed on the northern plains of Mars in 2008, analyzed several soil samples using its Wet Chemistry Laboratory (WCL) and returned evidence of the perchlorate anion. Because perchlorate is a soluble salt, its distribution through the soil column can illustrate the extent to which the soil has interacted with liquid water. The WCL experiments reported a similar concentration of perchlorate in surface and subsurface samples; although, only three soil samples were run. A more detailed mapping of the perchlorate distribution is needed if we are to understand the processes at work at the site.

Here, we use spectra from the Surface Stereo Imager (SSI) aboard Phoenix to map the distribution of perchlorate salts at the Phoenix landing site. We find that perchlorate is concentrated into small subsurface patches with distinct spectral signatures. These patches are morphologically similar to salt patches that form from aqueous redistribution on Earth, for example, in the Antarctic Dry Valleys. We propose that thin films of liquid water, arising from minor melting associated with seasonal ices or long-term obliquity cycles, are responsible for dissolving and concentrating perchlorate into these subsurface patches.