Sign of life? Sediments suggest that there was plenty of water--and therefore possibly life--on Mars.

A Dripping Wet Early Mars?

Mars, water, life. That tantalizing mixture has come together again, in a paper scheduled to appear in the 8 December issue of Science. Based on a set of new, detailed images unveiled today at a NASA press conference, researchers conclude that Mars may have had broad lakes and even shallow seas at a relatively clement time in its history. Such places would likely be prime landing sites for NASA missions seeking signs of past life on Mars.

Water on Mars has been one of planetary exploration's hottest topics for the past 30 years. Researchers have presented evidence for rain-fed rivers, an ocean, and huge crater lakes, but all remain controversial. Now, planetary geologists Michael Malin and Kenneth Edgett of Malin Space Science Systems Inc. in San Diego have compiled hundreds of images from the Mars Global Surveyor spacecraft, which is currently orbiting Mars. The compilation suggests that the planet once was a very wet place indeed.

Malin and Edgett report that many martian craters--formed by cosmic collisions early in the history of the solar system--are filled with layered sediments consisting of such weak material that the wind has eroded it away in places. The sediment seems to have been deposited as early as 3.5 billion to 4 billion years ago, during a time called the Noachian--roughly the era that life got started on Earth. The most likely explanation, Malin and Edgett say, is that a thicker, warmer atmosphere than today allowed water to flow and erode the martian highlands. The same currents would have carried clay, silt, and sand into crater lakes and perhaps some shallow seas between craters. Such seas and lakes could have been excellent places for life to have developed.

Because of the field's long history, "a lot of the pieces [in the paper] are not new," says Mars geologist Michael Carr of the U.S. Geological Survey. But the study "brings all these bits and pieces together with much better support for layered deposits and makes a good story out of it," he says. However, it will be a controversial story, predicts James Head of Brown University in Providence, Rhode Island. That's because there are many processes that produce sedimentary deposits, ranging from water and wind erosion to volcanic eruptions and meteorite impacts. "Sediments don't necessarily mean water," Head says.

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