Water ice permafrost on Mars: Layering structure and subsurface distribution according to HEND/Odyssey and MOLA/MGS data

To elucidate the nature of permafrost in the shallow subsurface of Mars, we analyze jointly neutron albedo from the High Energy Neutron Detector (HEND) on the Mars Odyssey spacecraft and near-IR (1064-nm) surface reflectance from the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor. The first dataset measures the content of hydrogen (in the form of water or hydroxyl) in the soil, and the second yields the flux of absorbed solar energy by the surface. We identify a statistically - significant negative cross-correlation between these data at latitudes poleward of 40° latitude in the northern hemisphere and in the latitude band 40°-60° in the southern hemisphere, which we interpret as evidence for the presence of stable water ice under a dry equilibrium top layer (ETL). We deduce an empirical relation between near-IR reflectance and thickness of this ETL, which allows the burial depth of the water ice table to be estimated with km-scale spatial resolution. We observe no correlation between neutron and near-IR albedo within the southern hemisphere poleward of 60° latitude. While it is known from previously analyzed neutron and gamma-ray data that subsurface water ice is present within this region and is covered by a layer of dry regolith, the absence of a correlation indicates that the thickness of this layer is not controlled by an equilibrium condition between the ice table and atmosphere.