PROSPECTING FOR MARTIAN ICE FROM ORBIT

Recent data from Gamma Ray Spectrometer on Mars Odyssey has confirmed the presence of hydrogen (Boynton et al., 2002), and thus water ice, tens of centimeters beneath the Martian surface. High-resolution MOC images on Mars Global Surveyor reveal Martian geomorphic indicators of subsurface ice in unprecedented detail. Polygonal terrain is the dominant indicator of subsurface ice on Mars.

Between a latitudinal band of 30^oN-65^oN, 5,280 narrow-angle MOC images (1.55 – 12.39 m/pixel) were analyzed and 283 images revealed the presence of polygonal terrain. Distribution of polygonal terrain is consistent with Seibert and Kargel (2001): areas of polygonal terrain are scattered throughout the regional band at low elevations and nearly all latitudes, with a regional concentration in the Casius quadrangle (western Utopia Panitia) between 278^oW-258^oW and 40^oN-50^oN. In this region, 74% of the total 132 images analyzed showed the presence of polygonal terrain. However, when the locations of polygonal terrain are compared to GRS hydrogen abundance data the presumed correlation between near subsurface ice and surface expression fails. Polygons are present in areas of low concentrations of near subsurface ice as frequently as they are present in areas of high concentrations of near subsurface ice. High concentrations of polygons in western Utopia Panitia are found to be associated with an area of low hydrogen abundance.

Explanations for the anti-correlation can be described by the following models: (1) water ice is absent in the deep subsurface, polygonal terrain is formed from desiccated sediment or unique geologic facies which provide subsurface control of surface expression, (2) there are two distinct ice layers correlating to Martian climate history, a near surface layer correlating to the present Martian climate fluctuations and a deep ground layer correlating to the past Martian climate when the planet was at a higher obliquity, or (3) polygonal terrain exposure is a result of recent erosion and deposition. We favor combinations of these models for the occurrence of polygonal terrain in the 30^oN-65^oN latitude regions as a whole. However, the extreme obliquity model can explain the data indicating low abundance of subsurface ice as indicated by MGS and the high abundance of polygonal terrain in western Utopia Planitia.