Asteroid fallout. Tremendous impacts on early Mars might have liquified frozen water, leading to surface-carving floods.

Early Mars Water Shortage Solved?

The Red Planet is bitterly cold and bone dry--at least at the surface--and seems to have been that way for at least several billion years. Some evidence suggests that the very earliest Mars was watery, but modelers can't explain how its climate could have sustained liquid water. Now, a group of planetary scientists proposes that giant asteroid impacts might solve this conundrum.

The mystery cropped up 30 years ago when planetary geologists saw signs that water had run free on the surface of early Mars. Branching channels on the ancient, heavily cratered highlands look like river valleys. Large crater rims are worn down as if swept by persistent rains. And, more recently, geologists have recognized that great craters appear to have been overflowing on early Mars. But climate modelers can't stuff enough greenhouse gases into a theoretical martian atmosphere to offset the chilly faintness of the sun 4 billion years ago. Their calculations have Mars frozen solid.

Now scientists think that the great impacts that pockmarked early Mars might have thawed the frozen planet's subsurface water and led to brief bouts of scalding rains followed by flash floods. Atmospheric scientists Teresa Segura and Brian Toon of the University of Colorado, Boulder, and their colleagues point to the 25 largest martian impact craters to make their point. The craters, which range from 600 to 4000 kilometers in diameter, were formed roughly 4 billion years ago. Even the small asteroids considered in this study--a 100-kilometer object--would have delivered about 4 x 1026 joules of energy to the planet. Based on the group's calculations, this is enough energy to have triggered 2 meters of rain over the planet within a few years and melted 3 meters' worth of ice. All that thawed water would have made for plenty of erosion, the group reports in the 6 December issue of Science.

Many planetary geologists are skeptical that this theory would explain the extent of erosion observed on Mars. "It's a valiant effort," says planetary geologist Michael Carr of the U.S. Geological Survey in Menlo Park, California, but "I'm skeptical it really does much to explain what we're seeing." Others are more optimistic. "The idea is well founded," says planetary scientist Stephen Clifford of the Lunar and Planetary Institute in Houston, Texas, who specializes in martian hydrology.