Martian History: Weathering a new notion

A frigid desert transformed, now and again, into hell. That’s the view of ancient Mars just proposed by a team of planetary scientists, who suspect that intermittent impacts by huge asteroids and comets some 3.5 billion years ago profoundly influenced the planet’s personality.

Those massive bodies may have melted surface and underground deposits of ice, vaporized debris that then fell out of the sky as a global shower of molten rock, and generated a torrent of scalding rain lasting for decades or possibly centuries. The hot rain would have carved and filled the channels and tributary-like structures seen on Mars today.

Although the impactors could have brought water to the planet, or melted the ice already there, they were so rare–arriving every 10 to 20 million years–that Mars has generally been locked in winter.

“We believe that throughout most of its history, Mars has been a cold, dry planet,” says Teresa L. Segura of NASA’s Ames Research Center in Mountain View, Calif. She and her colleagues, including Owen B. Toon of NASA Ames, reported their findings last week in San Francisco at a meeting of the American Geophysical Union. They also describe the study in the Dec. 6 Science.

This portrait clashes with the prevailing model, in which ancient Mars had a thick atmosphere of greenhouse gases that kept it warm and sustained bodies of liquid water for millions of years. Such conditions might have provided a haven for life.

Toon says that he and his collaborators were motivated to consider the role of impactors in creating ancient Martian rivers because recent studies have revealed problems with the greenhouse-gas model.

The researchers seized on an observation that most other scientists had viewed as a mere coincidence: The drainage networks were carved at about the same time that large asteroids and comets routinely roamed the inner solar system and gouged the largest craters on Mars. In the new study, Segura’s team shows that the comets and asteroids that produced these craters generated enormous amounts of liquid water.

Jonathan I. Lunine of the University of Arizona in Tucson comments that the new model provides a ready explanation for why the drainage systems on Mars aren’t nearly as large as those on Earth. If Martian surface water was liquid for only short periods, it couldn’t have carved extensive tributary systems, he says.

Although the prospect that Mars was almost always cold and dry might make the existence of Martian life less likely, it “by no means closes the book on that possibility,” asserts David J. DesMarais of NASA Ames. During its first billion years, Mars experienced extensive surface upheavals, and its interior retained a fair amount of the heat left over from its formation. Those conditions, as well as the liquid water they may have produced, could have given life a foothold. They also could have created underground niches suitable for protecting organisms from the scalding rains triggered by large impactors, DesMarais says.

Toon says that his team’s model suggests that the usually dried-up drainage systems aren’t the best places to seek life. Instead, researchers should take a closer look at areas such as Martian gullies where buried reservoirs of water seem to have recently seeped to the surface (SN: 7/1/00, p. 5: Martian leaks: Hints of present-day water). Such locations may be tied to extensive underground sources of water where organisms might thrive.

“I’m sure there will be an organized effort over the next year to disprove this theory,” says Toon. “And I think people will learn a lot” in trying to do so. “This is a new idea, and there hasn’t been a new idea in this field for quite some time now.”

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