Mars may be a frigid desert planet today but once had enough water to cover its entire surface with an ocean more than 300 feet deep, a panel of scientists said this week.

"The pictures taken by the two Viking spacecraft in orbit around Mars tell us that Mars had as much water in geologic history as Earth did," Dr. Michael Carr of the U.S. Geological Survey told a workshop at NASA's Ames Research Center in Mountain View, Calif. "Viking has shown us that there is 10 times as much water on Mars as was generally accepted by scientists."

Mars is extremely cold now; its average temperature is 60 degrees below zero Fahrenheit at its equator and 150 degrees below zero at its north and south poles.

Its carbon-dioxide atmosphere is so thin that it blankets the planet with 100 times less pressure than the atmosphere that covers Earth, hardly enough force to support rain in its clouds or water on its surface.

Carr said the 20,000 close-up pictures taken by the two Viking spacecraft since 1976 reveal canyons that are deeper, wider and longer than the Grand Canyon and could be made only by rushing rivers. They also show thousands of gullies formed only by water or snow and ice burrowing into the surface or forcing their way to the surface.

"If all the water that existed on Mars to form these channels covered its surface today," Dr. James Pollack of the Ames Research Center said, "it would be enough to form a global Martian ocean tens to hundreds of meters deep."

Carr and Pollack noted that where Mars' water lies is no longer just an academic puzzle. Mars is the only nearby planet marked for exploration and possibly even colonization in the 21st century, in large part because it is the only neighboring body that can support life. Venus is too hot, and the moon is airless and waterless.

"Suppose we want to go to Mars someday, for whatever reason," Carr said. "We have to know where we can get water."

Carr said the Viking photographs reveal patches of water ice under the north polar cap of solid carbon dioxide when the dry ice vaporizes during Mars' peak summer months.

Viking pictures also suggest that water lies below the surface at latitudes near the Martian equator where the planet's interior heat can keep it from freezing just as do underground rivers in most temperate latitudes on Earth.

"There is a softening of the terrain, a rounding off of the edges . . . that is evidence of underground water flow and the creep of ice near the surface," Carr said.

"This terrain is also located where most of the Martian gullies are today, where snow, ice and water burst out of the ground to cause the colossal floods that formed the channels we see today."

Pollack said Mars long ago lost most carbon dioxide keeping the sun's heat near it, in effect causing a permanent Martian Ice Age.