"The results, even after only a month of mapping observations, are stunning," writes James Bell of Cornell University in an accompanying analysis for the journal.

Odyssey's instruments cannot detecting anything deeper than about 3 feet (1 meters), but other geologic evidence suggests the reservoir may extend much deeper.

"Separate lines of evidence suggest a loose and/or porous regolith [soil] that could exceed a kilometer or more in thickness, implying that the subsurface ice detected by Odyssey may represent only the tip of an iceberg frozen underground," Bell said.

Water ice is not likely to exist near the surface of equatorial regions, according to the data. The highest concentrations appear to be from the edge of the polar ice caps to latitudes of 60 degrees, the scientists say. In March, they had not yet collected data on the planet's northern hemisphere.

Odyssey, which arrived at Mars last fall, detected the hydrogen by measuring neutrons and gamma rays escaping from the Martian surface. Hydrogen atoms change the speed of the particles.

Hydrogen is one component of water but can exist alone and as a part of other substances; even the newest data will be subjected to further verification, the researchers say.

Scientists are very optimistic about the quantities of water ice. The data suggest that 20 to 35 percent of the weight of the ice-bearing layer is water ice.

"It may be better to characterize this layer as dirty ice rather than as dirt containing ice," said University of Arizona researcher William Boynton, who led part of the work.

"This is really amazing," Boynton said. "This is the best direct evidence we have of subsurface water ice on Mars. We were hopeful that we could find evidence of ice, but what we have found is much more ice than we ever expected."

Boynton, lead author on one of three papers that will be published in Science, is the principal investigator for the Gamma Ray Spectrometer suite of instruments, used to make the discovery.

Biologists say liquid water is essential for life as we know it. But frozen water near the surface of Mars may be a remnant of past liquid oceans, which might have existed millions or billions of years ago when researchers believe Mars was warmer and may have harbored a thicker atmosphere.

Researchers have cited canyons and river-like features as evidence.

"We have suspected for some time that Mars once had large amounts of water near the surface," said Jim Garvin, Mars Program Scientist at NASA Headquarters. Garvin added that the new measurements represent an "important piece of this puzzle, but we need to continue searching, perhaps much deeper underground, for what happened to the rest of the water we think Mars once had."

Scientists already knew there was water locked up in the northern ice cap, along with carbon dioxide ice. But finding water ice away from the permanently frozen polar caps greatly boosts the chances that it might melt seasonally or at least periodically, or that pocket of underground water might exist and could therefore potentially support life.

For water to exist in underground pools, some internal heat source would be needed. Though Mars appears volcanically inactive to the eye, researchers say there could be magma inside the planet and it would be a source of heat that could melt ice.

Scientists have also said fossils could be locked up in the Martian soil. Dormant life might remain in the ice -- something researchers have found to occur on Earth.

Many leading researchers believe only by putting humans on Mars will they be able to answer these questions. NASA has been reluctant to commit to any sort of timetable for such a mission. But speculation of such a mission -- either by NASA or by Europeans -- now runs rampant.

A story about the finding on the BBC's web site claimed that "NASA may now commit itself to a manned landing within 20 years." That claim was not attributed to any official and has not been verified. Other published reports carried similar unsubstantiated claims.

A press statement by NASA today included no mention of changes to its Mars program, now a wholly robotic effort. The agency has said repeatedly in the past that not enough is known about the hazards on Mars, including potentially lethal radiation, to make firm plans.

Odyssey also caries an instrument designed to measure the radiation environment, but no results from that device have been announced.

Water at mid-latitudes on Mars, even if frozen, would make a crewed mission more practical by providing a source of drinking water. More important, it could also be broken down into hydrogen, for rocket fuel, and oxygen.

"The amount of water present on Mars is sufficiently large that it can support future human exploration activities," said Los Alamos National Laboratory reearcher William Feldman, who is a lead author on another of the papers stemming from the new observations.

The lead author of the other paper is Igor Mitrofanov of the Space Research Institute in Moscow.

Odyssey's Gamma Ray Spectrometer is not even fully deployed. It will be extended further from the orbiting, $300 million spacecraft in coming weeks, which should provide better measurements of hydrogen and other chemicals on and under the surface of Mars. The instrument measures the speed of neutrons bouncing off the surface and near surface. Neutrons that have hit hydrogen atoms move more slowly.

The results will be "used to guide the selection of landing sites for future rovers and landers, sample returns, and eventual human exploration," Bell said. "In that sense, the most important implications of the detection of subsurface water ice deposits on Mars may not be realized for decades. It is likely to be worth the wait, however."