Subsurface ices at the Mars Phoenix Landing Site: Assessing emplacement mechanisms

Several mechanisms have been proposed to explain the emplacement of subsurface ices on Mars: vapor diffusion from the atmosphere, freezing of bodies of surface water (e.g., lakes or oceans), buried glaciers, or accumulation and burial of packed snow. These formation mechanisms predict different physical properties for the subsurface ices: vapor diffusion should produce pore ice, whereas other mechanisms should produce massive, relatively pure ice. NASA's Phoenix Lander uncovered two types of ice at its 2008 landing site on the northern plains of Mars: a light-toned ice (Dodo-Goldilocks) that broke into pieces during backhoe operations; and a hard, darker icy surface that had to be scraped to provide particulate materials for sampling (Snow White). Here, we use spectra from Phoenix's Surface Stereo Imager (SSI) and a non-linear mixing model with ice and soil components to determine the ice to soil ratio of the ices exposed at the Phoenix landing site. We find Dodo-Goldilocks consists of almost pure water ice. The darker icy material contains ~30 wt% ice (~55 vol%), indicating that it probably formed as pore ice between grains of soil. We conclude that these two types of ice represent two different emplacement mechanisms and periods of deposition. Snow White ice was probably deposited via vapor diffusion from the atmosphere. Dodo-Goldilocks ice was probably deposited through an ice-lens or needle ice mechanism. Buried snow or glacial ice is unlikely for Dodo-Goldilocks, given its restricted spatial extent and the fact that the site is covered by large rocks.