Odyssey's first look: craft spies signs of ice at the Martian south pole.

Astronomers for the first time have found evidence of large amounts of frozen water on Mars. The Red Planet's south-polar region may contain an expanse of ice just beneath its surface, according to data gathered by the Mars Odyssey spacecraft, which began mapping the planet late last month.

Water may account by mass for several percent of the topmost meter of material in this part of the planet, William V. Boynton of the University of Arizona in Tucson announced March 1. Emphasizing that his team had analyzed only the first 10 days of data from a several-year mission (SN: 1/19/02, p. 42), Boynton says the ice seems to be distributed uniformly within an area as large as the United States.

The evidence for water is indirect. Boynton's team bases the preliminary results on the amount of hydrogen--believed tied up in water--in the Martian subsurface. All three instruments on Odyssey's gamma-ray spectrometer--two neutron detectors and a gamma-ray sensor--reveal an abundance of hydrogen.

Detecting hydrogen relies on its interaction with high-speed neutrons. When Odyssey passes over Mars' south pole, the craft's neutron detectors record a relative dearth of energetic neutrons and a wealth of slow ones, a pattern that indicates hydrogen. More evidence comes from the gamma-ray sensor, which detected high-energy radiation that hydrogen atoms emit when they absorb a slow-moving neutron, Boynton reported at a press briefing at NASA'S Jet Propulsion Laboratory in Pasadena, Calif.

Although hydrogen can reside in compounds other than water, Boynton says that if hydrogen accounts for more than 2 percent of the mass of the soil, as he suspects, most of it will be in water.

According to models of the temperature and pressure of the south polar area, frozen water could exist there. Moreover, researchers have calculated that the stacks of frozen carbon dioxide observed at the poles would crumble were it not for some other stabilizing material, such as frozen water, between the layers.

The north polar region of Mars also may be rich in ice, but Odyssey's detectors can't find out for another year. That's when spring will arrive in the region and a top layer of carbon dioxide frost will evaporate, opening a path for slow neutrons from underlying material to reach the spacecraft's detectors.

Because Odyssey's neutron and gamma-ray instruments can only search for water over regions wider than 600 kilometers, they can't detect signs of recent flooding over small areas (see p. 157). However, another Odyssey detector, the thermal emission and imaging system, may do so. By examining the composition of the Martian surface at high resolution, this instrument can detect minerals recently altered by the flow of water. At the briefing, researchers unveiled some of the detector's first images and temperature maps.