A cross-section graphic showing the depth of the ice changing with latitude. Graphic courtesy of NASA
In the thermal and epithermal neutron maps of Mars, the deep blue areas indicate the presence of hydrogen, or water-ice, and the red, carbon dioxide.
The North and South Polar Caps as seen in Thermal and Epithermal Neutrons
Thermal (top) and epithermal (bottom) polar maps show neutrons above the south (left) and north (right) polar caps. Enhancements of the thermal fluxes at both caps show that the central portion of the north cap is covered by dry-ice frost because it is late winter. The residual polar cap at the south cap also is covered by a thick layer of dry ice even though it is late summer. The maps of epithermal neutrons show a high abundance of subsurface water ice near the surface south of about negative 60 degrees latitude and within a ring that almost completely surrounds the central portion of the north polar cap that is covered by a thick layer of dry-ice frost. A meridional lane extending southward from the mouth of Chasma Borealis is probably covered by a thin layer of dry ice frost at this time of year (late northern winter).
Global Map of Epithermal Neutrons
Epithermal neutrons provide the most sensitive measure of hydrogen in surface soils. Inspection of the global epithermal map shows high hydrogen content in surface soils south of about negative 60 degree latitude and in a ring that almost surrounds the north polar cap. The maximum intensity in the northern ring coincides with a region of high albedo and low thermal inertia, which are both required for near-surface water ice to be stable at latitudes equatorward of about positive/negative 60 degree latitude. Also seen are large regions near the equator that contain enhanced near-surface hydrogen, which is most likely in the form of chemically and/or physically bound water and/or hydroxyl radicals because water ice is not stable near the equator.
Global Map of Thermal Neutrons
Thermal neutrons provide a sensitive identification of subsurface hydrogen deposits and surface deposits of carbon dioxide frost. The strongest signatures of subsurface hydrogen are seen south of negative 60 degrees latitude and within a ring that almost completely surrounds the north polar cap. The central portion of the north polar cap has a thick layer of carbon dioxide frost. A less prominent enhancement is seen over the residual south polar cap, indicating that it also is covered by a thick layer of carbon dioxide.
An artist's rendering of ice below Mars' surface with Odyssey hovering above. Graphic courtesy of NASA