Another Swing To Wet Mars

For decades, scientists have been debating the role water has played in the history of Mars. The pendulum has swung from a relatively dry Mars, which suggests life there may not be likely, to a wet Mars early on, which leaves the question of life open.

The latest evidence, images from a probe now in orbit, supports the notion of a wet early Mars. Clays that form in the presence of water have been sighted in outcroppings at thousands of spots throughout the highlands of the planet's southern hemisphere. This suggests water in large quantities flowed over much of Mars very early in its history, up to about 3.7 billion years ago.

No such evidence has yet been found in the northern hemisphere by the rovers Spirit and Opportunity, but scientists say that is still a possibility.

On Earth, clays roughly 3.6 billion years old show evidence of early life, so Mars seems to have begun to dry out at an intriguing point. If life developed early on Earth, it might have done so on a wet Mars, as well. Or maybe not. Our robots may come upon the answer, but finding tiny fossils on a huge world-- or determining no such fossils exist-- may ultimately require human explorers on Mars.