DISTRIBUTION AND NATURE OF MID-LATITUDE GROUND ICE ON MARS

The last several years have seen a growing appreciation that Mars is an ice rich planet. Hydrogen abundances inferred from gamma-ray and neutron spectrometer data show that the upper-most decimeters of the martian surface contain large amounts of water ice poleward of latitudes 60 degrees, which can exceed 75% by volume. Geomorphological indicators such as viscous flow features, putative snowpacks and pervasive polygons all contribute to the growing body of evidence that the deposition and removal of water ice and the associated glacial and periglacial processes have had a dominant effect on shaping the high-latitude surfaces of the planet. Interest generated by these discoveries recently culminated in the Phoenix mission, which focused on the investigation of high-latitude ground ice.

Here we report on a new method to probe subsurface ice on Mars. New, meter-scale, impact craters that formed within the period covered by spacecraft datasets have been observed. Using data from the Context Camera on the Mars Reconnaissance Orbiter spacecraft, we have identified new mid-latitude craters at five sites that excavated material with a brightness and color in High-Resolution Imaging Science Experiment (HiRISE) images that is strongly indicative of water ice. Although small in area, a water ice composition for this bright material can be confirmed in one case with lower-resolution hyperspectral data from the Compact Reconnaissance Imaging Spectrometer for Mars.

Monitoring of these sites with HiRISE showed the ice sublimated enough to generate an optically thick lag deposit over a few months. Modeling of this process indicates that this ice must be extremely clean and not the pore-filling ground ice expected. We discuss several formation scenarios for this unexpectedly pure ice.

Theoretical models predict that buried water ice is stable in the high-latitudes of Mars beneath a desiccated soil layer with an extent and depth that depend on temperature and humidity (which vary with changing orbital elements). A key parameter setting the extent of stable ground ice is the global average atmospheric water vapor. The presence of ground ice at the latitudes of these impacts is consistent with an atmospheric water vapor content significantly higher than present, indicating current ice-table retreat in the martian mid-latitudes.