Only a few months into its 2½-year survey of the red planet, NASA's 2001 Mars Odyssey orbiter has discovered that huge tracts of water ice lie buried in the Martian northern and southern hemispheres. In fact, the frozen reservoir is so pervasive at latitudes poleward of about 60°, an area far larger than Mars's permanent polar caps, that its total volume probably exceeds 10,000 cubic kilometers — enough to fill Lake Michigan twice.

Odyssey's measurements suggest that the ice-rich terrain is overlain by a relatively thin cover of dusty debris that may be 1 meter thick at latitude –42° but no more than about 30 centimeters (1 foot) deep at –77°. While it's clear that the ice lies very close to the surface, "the jury is still out on whether this covering is really thick and thin," notes investigator William C. Feldman (Los Alamos National Laboratory).

Months ago, early returns from the spacecraft had alerted mission scientists that ice was widespread at latitudes poleward of 60°, where previous climatic studies had shown that it should be stable. Since then, according to William V. Boynton (University of Arizona), principal investigator for Odyssey's gamma-ray spectrometer, it's become apparent that the planet's icy holdings must be truly vast. In a series of papers appearing in May 30th's Science Express, Boynton, Feldman, and their colleagues conclude that the dry covering caps a layer containing roughly 50 percent water by volume. Because cosmic rays can only penetrate about a meter into the surface, no one knows how far down the ice might extend.

And the proportion of ice might be significantly higher, Boynton points out. If that's the case, he says, "it's a whole new ball game," because the host rock can't be porous enough to hold that much ice.

"It is a glacier," concludes Feldman, a notion that is not at all far-fetched. Geologists have found solid evidence for glacial gouging inside the Hellas basin, which lies in the Martian southern hemisphere adjacent to the ice-charged regions mapped by the spacecraft.

Odyssey’s gamma-ray spectrometer is not detecting water ice directly. Instead, it records neutrons and gamma rays given off by hydrogen in the ground below. Cosmic rays striking the Martian landscape liberate high-speed neutrons from atoms in the rock; some of these zip directly into space, but most of them lose energy through atomic collisions that free other neutrons. Because hydrogen atoms are particularly good at slowing down these ricocheting particles, a falloff in the count of medium-energy neutrons means that hydrogen is amply present within the top meter of the surface. The energized hydrogen atoms also make their presence known by emitting gamma rays at a specific energy, which Odyssey detects as well. Since rock-forming minerals contain little hydrogen, the neutron deficiency must be predominantly due to water ice (H2O).

Notably, the instrument recorded localized regions of hydrogen enhancement at latitudes close to the Martian equator, where temperatures are too warm for ice to exist near the surface. In these broad patches, Boynton and Feldman suggest, the rock contains minerals with chemically bound water (as OH), such as clays.