Odyssey’s Homer: Hints of water near both poles of Mars

Sensors on board the Mars Odyssey spacecraft have spied strong signs of ice buried near both poles of the Red Planet, exactly the regions where scientists previously had said that frozen water deposits could exist.

If confirmed, the large permafrost-like reservoirs could contain a large part of Mars’ suspected water resources. Finding substantial amounts of readily accessible water would be the space-faring equivalent of a grand slam home run because the water could be used by people during future Mars explorations. The water also raises scientists’ hopes of finding life on Mars.

Even though the Odyssey probe has been taking data for just a few

months and hasn’t yet deployed its most sensitive instruments, data from three devices–one that measures gamma rays and two that detect neutrons–suggest there are copious amounts of hydrogen buried 1 meter or so beneath the soil in the polar regions of Mars. Gamma rays and neutrons are emitted from hydrogen-rich soil when cosmic rays strike Mars’ surface, says William V. Boynton of the University of Arizona in Tucson.

Hydrogen could exist on Mars in materials, such as hydrated minerals, other than ice, says Boynton. However, none of those substances can account for the large amount of hydrogen Odyssey has detected. From the concentrations of hydrogen measured, scientists estimate that in some spots up to 50 percent of the subsurface material by weight could be ice.

Within those areas for which Boynton’s team measured average concentrations, some sites will be less hydrogen-rich and others will have more than their share, notes William C. Feldman of the Los Alamos (N.M.) National Laboratory.

“There’s got to be places where there’s [pure] ice,” he notes.

Boynton, Feldman, and their colleagues discussed their results at a meeting of the American Geophysical Union in Washington, D.C., last week. Both teams’ findings will also appear in an upcoming issue of Science.

The analyses by Boynton’s group suggest that the hydrogen-rich material lies beneath about 30 to 40 centimeters of dry, hydrogen-poor soil. That soil would insulate the ice-bearing subsurface from daily and seasonal variations in temperature, says Boynton. Because neutrons don’t break though more than about 1.5 m of overlying material, Odyssey’s instruments can’t tell how deep the ice may reach.

Several lines of evidence indicate that Mars may be blanketed with a 1-kilometer-thick layer of rocky debris and dust generated by repeated meteorite impacts. In theory, that entire layer could hold ice. Jim Bell of Cornell University notes that the Odyssey discovery could mark just the tip of a subterranean Martian iceberg.

Boynton’s team first reported the possibility of buried ice around Mars’ south pole 3 months ago (SN: 3/9/02, p. 149: Odyssey’s First Look: Craft spies signs of ice at the Martian south pole). At that time, it was winter in the northern hemisphere, and a neutron-absorbing layer of carbon dioxide frost blanketed the ground there. Since March, says Feldman, the carbon dioxide frost has been subliming, and neutrons from the hydrogen-rich subsurface are now reaching Odyssey’s sensors.

Feldman and Boynton note that the discovery of hydrogen-rich material on Mars needn’t have waited until the 21st century. The lander on each of the Viking missions, which arrived at Mars in the mid-1970s, scraped trenches 10 to 20 cm deep to sample the arid soil. The new data from Odyssey suggest that Viking 1 dropped into an area on Mars with little if any buried ice. Viking 2, however, descended in a region apparently endowed with some buried, hydrogen-rich material. After a trip of more than 100 million kilometers, the lander may have stopped digging just a few centimeters shy of striking ice.