Data recorded by the Mars Odyssey Neutron Spectrometer has been used to create maps showing an abundant layer of buried water equivalent hydrogen, or WEH

The water/WEH is situated just below the surface of Mars possibly existing in the form of water ice or H₂O/OH molecules contained in hydrous materials just below the surface.

The maps, using data collected since February 2002, show near-surface hydrogen content of this planet.  The units shown in the maps are in weight percent water equivalent hydrogen (WEH).

These maps were generated assuming that there is a layer of enhanced hydrogen-containing molecules with thickness D, and abundance of WEH equal to 1 percent overlaying a buried layer having infinite depth and WEH equal to “WEH Down,” as labeled in the upper left-hand corner of the figure. These analyses suggest that there is a buried layer of abundant WEH centered on the Martian equator and longitude +30 degrees.  

There are also deposits that are just south of the intersection between the southern highlands and the northern lowlands near the equator centered at longitude of -160 degrees.  All deposits could be in the form of water ice or H₂O/OH molecules contained in hydrous minerals, just below the surface.  We note that part of the nearby Vallis Marineres is in a large region of relatively modest amounts of buried WEH.  

Correlation between these maps with those of topographic features of Mars are helping scientists to understand the history of water on Mars and even its climate history.

Source: Planetary Science Institute