Neutron Mars

Looking for water on Mars, researchers using detectors on board the orbiting Mars Odyssey
spacecraft have created this false-color global map of energetic neutrons from the otherwise
Red Planet. What do neutrons have to do with water? As cosmic rays from interplanetary space
penetrate the thin martian atmosphere and reach the surface they interact with elements in the
upper layer of soil, scattering neutrons back into space. But if the martian soil contains hydrogen,
it seriously absorbs energetic scattered neutrons. Tracking variations in absorption, neutron
detectors can map changes in surface hydrogen content from orbit. Hydrogen content is taken as
a surrogate measure of frozen water (H20), the most likely form of hydrogen close to the martian
surface. Thus, bluer shades in the above map correspond to larger presumed concentrations of
near-surface water ice. Water ice at the martian poles came as no surprise, but significant
concentrations also seem to be present at lower latitudes. The melting of such near-surface ice
could be responsible for the formation of martian gullies.