Mars - Subsurface Hydrogen

Mars topography (MOLA, 10x exaggeration) with an overlain map of hydrogen concentration measured by the GRS/HEND instrument (Mars Global Surveyor - https://mars.nasa.gov/odyssey/mission/instruments/grs/) Subsurface hydrogen is often interpreted to be in the form of water (either as ice or trapped in minerals), so this data might represent the water stored in the subsurface of Mars in one of those forms. Purple/blue areas have little/no hydrogen, whle the green, yellow, and red areas have increasing concentrations (in that order). The letters indicate different landing sites - V1/V2 are the Viking landers, PF is Pathfinder, M is the Opportunity rover, and G is the site for the Spirit rover.