Watery clues from Martian valleys ... and channels.

Did it ever rain on Mars? Recent climate models have suggested that the ancient Martian atmosphere couldn't have held enough carbon dioxide to warm the planet and permit rainfall. A new analysis supports that view, although it doesn't rule out the possibility that snow once fell on the Red Planet.

Virginia C. Gulick and Victor R. Baker of the University of Arizona in Tucson compared the nature of valleys - features carved by water erosion over hundreds of thousands of years on Earth and Mars. The geologists considered two types of valleys: sapping valleys and runoff valleys. Sapping valleys typically form when groundwater rises to the surface, eating away support rock at the base of a cliff and eventually carving a trough. Runoff valleys are sculpted by the flow of surface water, possibly rainwater.

On Earth, the researchers note, sapping valleys and runoff valleys occur near one another. On Mars, however, runoff valleys don't seem to accompany the sapping valleys in the ancient terrain of the southern highlands. Moreover, the Martian sapping valleys tend to cluster together, with vast expanses of uneroded surface between them.

Gulick and Baker say the clustering of sapping valleys and the apparent lack of runoff features suggest that Mars produced its valley networks by localized release of heat from its interior. The heat release would not have been uniform across the Martian surface, they add, since not all areas on the planet have valleys. Global rainfall would have produced a broader, less concentrated pattern of valleys and would have led to runoff valleys large enough to have been detected by the Viking craft, says Gulick.

The new findings, she notes, don't exclude the possibility that snow once fell on Mars. If snow did exist, it might have served to replenish the vast supplies of underground water.

Gulick and Baker presented their findings at the Lunar and Planetary Science Conference, held in Houston in mid-March.

... and channels

While an internal heat source may have helped create Martian valleys, external forces may have triggered the formation of the planer's long, sinuous channels, suggested other researchers at last month's conference.

According to this model, when asteroids slammed into Mars billions of years ago, they may have created intense shock waves that spread out around the planet. These waves would have exerted tremendous pressure on underlying, porous rock and the water held by these rocks. That water pressure would have eventually cracked the planet's frozen crust. Once the underground water emerged, the combination of gravity and erosion would have acted to cut the channels.

The scientists proposing this scenario -- Ivett A. Leyva of the California Institute of Technology in Pasadena and Stephen M. Clifford of the Lunar and Planetary Institute in Houston - note that the number of craters points to numerous "Marsquakes" triggered by impacts. In addition, geologic evidence indicates that the ancient Martian crust contained lots of water.

Leyva and Clifford say they took their cue from the effects of a 1964 earthquake in Alaska, which created geysers and a rush of other watery uprisings as distant as 400 kilometers from its epicenter. The quake also significantly altered levels of well water as far away as Perry, Fla., 5,500 kilometers from the site

On Mars, a shock wave powerful enough to globally compress submerged water could be generated by an asteroid big enough to form a crater at least 1,000 kilometers in diameter, Leyva and Clifford calculate. Such pressure might crack frozen ground several kilometers thick, resulting in a catastrophic discharge of underground water deposits, they say..