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NASA explains this map:
"Coloring of the map is coded to concentrations of shallow subsurface water ice found by the Gamma Ray Spectrometer - Neutron Spectrometer on NASA's Mars Odyssey orbiter. Blue, at high latitudes north and south, indicates higher concentrations of water ice (deduced from detection of hydrogen); orange designates lowest concentrations."
"The white squares in the northern hemisphere mark locations of small fresh impact craters that exposed water ice close to the surface and validated the neutron spectrometer data."
Image Credit: NASA/JPL-Caltech/ASU/UA/LANL/MSSS