An America spacecraft orbiting Mars has detected enormous quantities of ice just beneath the red planet's frigid, dusty surface. The amount has surprised the American and Russian scientists who have read the data and may help answer the question of where all the water once believed to have covered the planet went.

The evidence that water flowed on a warmer, lusher Mars eons ago and may have supported life stands out clearly in land formations like valleys, gullies and channels that water must have carved. But where did it go?

The U.S. Mars Odyssey spacecraft, which arrived at the Red Planet last October, has come up with at least part of the explanation. Its instruments have discovered strong signs of hydrogen, a major element of water, to a depth of one meter below Mars' barren surface.

"The signal is very clear and very unequivocal," said University of Arizona scientist William Boynton, who is the the lead author of one of three papers on the discovery by U.S. and Russian scientists appearing in the journal Science. "There is really no question whatsoever, but there are very high concentrations of water ice just beneath the surface," he said.

One of Mr. Boynton's co-researchers, William Feldman of the U.S. Department of Energy's Los Alamos National Laboratory, says the amount of water detected is at least twice the size of Lake Michigan, a huge body of water in the northern United States, and well within reach of future robotic or human explorers.

"The water is enormous," said Mr. Feldman. "There is enough to support an awful lot of human activity. It is easily accessible. It is like in Alaska; if you were to take a shovel and push it into the soil, you would hit permafrost."

The Mars Odyssey maps shows that the ice is buried in the high latitudes from the plant's poles to about halfway toward the equator. Large expanses of low and middle latitude hydrogen are also thought to be in ice.

The clues to this abundance came within one month of the start of the spacecraft's mapping mission. One sign was the intense emissions of gamma rays from the hydrogen. Another was the intensity of neutrons, an atomic particle pried loose when cosmic rays from space bombard the atoms in the soil. In this case, the neutrons moved slowly, meaning that large quantities of hydrogen interfered with their movement.

The spacecraft instruments are sensitive to a depth of only one meter, but the scientists may have found only the tip of the iceberg.

"If there is ice, water has penetrated into that topmost layer, and it is likely that it extends down farther than what we can just detect," suggests Cornell University astronomer Jim Bell. "It is speculated that layer of ice may go down perhaps several meters, perhaps maybe tens of meters, maybe even more."

The researchers say that if the depth is a significant part of a kilometer, it could account for most of the water that once flowed on Mars. But they point out that to determine the exact depth would require drilling or remote sensing by radar. The U.S. space agency NASA has such a Mars radar mapping mission planned for later in this decade.

But William Feldman of the Los Alamos National Laboratory says knowing how much water is buried within Mars soil does not explain how it got there and why it no longer rests on the surface.

"There are a lot of ways Mars could have lost its original juvenile inventory of water. Now the question is how much did it start with and what is left, and how quickly was it released to the atmosphere? There are just an awful lot of geophysical questions about the development of Mars that is locked up into the inventory of the water," said Mr. Feldman.

The search for water is only part of the Mars Odyssey mission. It also carries instruments to measure the distribution of minerals and chemical elements in the red planet's surface.