Musing on a warmer, wetter Mars.

Mars resembles a cold desert, with temperatures well below freezing and an all but negligible atmosphere. But ancient valleys and long, narrow channels scar its barren terrain, suggesting that water once flowed freely on the Red Planet.

For water to have carved the Martian landscape, conditions during the first half billion years on the planet must have differed dramatically from those that exist today. In the Aug. 5 Science, Steven W. Squyres of Cornell University and James F. Kasting of Pennsylvania State University in University Park review and evaluate several theories about the evolution of a warmer, wetter Mars.

They note that some scientists have proposed that the ancient Martian valleys were formed by runoff from rainfall. This model assumes that Mars once had an atmosphere dense enough and warm enough for precipitation to occur. But more detailed analyses over the past few years indicate that some of the carbon dioxide thought to be prevalent in the ancient Martian atmosphere would have condensed into clouds. The clouds would have cooled the Martian surface, in part by increasing the reflectivity of the planet to sunlight.

Thus, write Squyres and Kasting, 'if conditions early in Martian history really were warmer, climate may be only part of the story"

Heat from within may have played a key role in bringing water to the surface, they and other researchers suggest. Heat left over from the formation of the planet as well as energy generated by debris bombarding the youthful Mars could have caused water to seep from rock and collect above ground.

The composition of some meteorites that almost certainly come from the planet also indicates that "Mars was awfully hot on the inside," Squyres says. "Groundwater didn't fall from the sky, it seeped out from the ground," he points out.

However, note Squyres and Kasting, the mere presence of water trickling out onto the surface can't explain the flow of fluid for several hundred kilometers across the planet. Small volumes of water emerging from below ground would readily freeze before moving very far - unless the ancient Martian climate was somewhat warmer than it is today. (In contrast to a trickle, a flood of water would resist freezing and might well carve a channel even on a frigid surface.)

The researchers calculate that if the early Martian atmosphere contained small amounts of methane and ammonia in addition to carbon dioxide, it could have warmed the planet enough to permit the flow of groundwater.

The Mars Global Surveyor, a replacement craft for the lost Mars Observer, may help verify the models, Squyres notes. A thermal infrared spectrometer aboard the Mars-orbiting craft, now scheduled for launch in late 1996, could search for the characteristic signature of hydrothermal minerals - minerals created when rock is heated to high temperatures in the presence of water. On Earth, geologists find hydrothermal minerals at hot springs, such as those in Yellowstone National Park.

In addition, high-resolution images taken by the craft should reveal the depth of the Martian channels. Researchers in whether the channels have a typical depth of centimeters or meters.