A new global map of Mars shows likely locations of water ice based on observations of hydrogen made by NASA's Odyssey spacecraft.
The presence of hydrogen is a strong indicator that water -- most of it almost surely frozen -- exists near the surface of Mars, embedded in the soil. Liquid water might exist on the red planet, but no data so far has provided firm indications.
The new map is based on more than a year's worth of Odyssey data, much of which has already been announced. The purpose is to show the extent of frozen water on Mars in a visual format.
Bill Feldman, a Los Alamos National Laboratory researcher who led the observations, called the map "breathtaking."
"There's a lane of hydrogen-rich material following the western slopes of the biggest volcanoes in the solar system, a maximum reading sits right on Elysium Mons, and another maximum is in the deepest canyon in the solar system," Feldmen said in a statement.
Feldmen is scheduled to present the map today at the Sixth International Conference on Mars at the California Institute of Technology, in Pasadena.
The hydrogen has been mapped by a neutron spectrometer, designed at Los Alamos. It measures changes in neutrons given off by soil, an indicator of hydrogen likely in the form of water ice, scientists say.
From about 55 degrees latitude to the poles, Mars has extensive deposits of soils that appear to be rich in water ice, bearing an average of 50 percent water by mass, studies show. A typical pound of soil scooped up in the polar regions would yield an average of half a pound of water if it were heated in an oven, Feldmen explained.
Similar traces of hydrogen also found in lower concentrations closer to Mars' equator, ranging from 2 to 10 percent water by mass.
Surprisingly, two large areas, one within Arabia Terra, the 1,900-mile-wide Martian desert, and another on the opposite side of the planet, show indications of relatively large concentrations of sub-surface hydrogen.
The vast water icecaps at the poles may be the source of the subsurface water nearer the equatorial regions, Feldmen and his colleagues say. The thickness of the icecaps themselves may be enough to bottle up geothermal heat from below, increasing the temperature at the bottom and melting the bottom layer of the icecaps, which then could feed a global water table.
Another idea is being pondered. There is evidence that about a million years or so ago, Mars' axis was tilted about 35 degrees, which might have caused the polar ice caps to evaporate and briefly create enough water in the atmosphere to make ice stable planetwide, the researchers say.
The resultant thick layer of frost may then have combined chemically with hydrogen-hungry soils and rocks.
"We're not ready yet to precisely describe the abundance and stratigraphy of these deposits, but the neutron spectrometer shows water ice close to the surface in many locations, and buried elsewhere beneath several inches of dry soils," Feldman said. "Some theories predict these deposits may extend a half mile or more beneath the surface; if so, their total water content may be sufficient to account for the missing water budget of Mars."
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