Frozen ripples. Curving sediment layers mean that flowing water rippled a martian sea floor.

Opportunity Tells a Salty Tale

Dick writes about Earth and planetary science for Science magazine.

It may have come and gone from year to year, might barely have reached your ankles, and would have tasted like acid mine drainage. But a sea of sorts once covered a large region on the equator of ancient Mars.

That's the word from the Mars Opportunity rover, which inspected an outcrop of salt-laden sediment on Meridiani Planum and found thin intersecting layers that must be sand ripples shaped by flowing water. "It is a profound discovery," NASA space science chief Edward Weiler said at a press conference this week in Washington, D.C. "Water is the key to life. As of today, Meridiani is the place we'd want to send our next rover on Mars."

Scientists have long believed that water flowed across the martian surface billions of years ago, if only from melting snows. But did it pool in life-affirming lakes and oceans or just seep into the ground to stay? The rock outcrop that Opportunity had been analyzing is rich in sulfate salts with varying amounts of the element bromine, rover scientists reported early this month. That is just the sort of thing that a shallow, evaporating sea or lake would deposit on its floor (ScienceNOW, 2 March).

The clincher came when Opportunity microscopically imaged large parts of rock outcrops dubbed Last Chance and the Dells. The images show fine layering not in the neat, parallel layers of dust sifting out of the air or sand settling quietly to a lake bottom but in layers curved upward in "smiles" that intersect one another. "We feel quite confident these 'smiles' add up to a story of ripples moving in water rather than wind," says geologist and rover science team member John Grotzinger of the Massachusetts Institute of Technology.

Opportunity's salty, shallow sea was probably a sizable one. From orbit, the bit of light-toned rock analyzed by Opportunity appears to extend over an area at least the size of Oklahoma, Grotzinger notes. And the sea may have been there, at least intermittently, for quite a while. That light-toned layer is about 300 meters thick. Opportunity's next task is to rove to Endeavor crater to inspect what may be outcropping sea floor 100 times thicker than the one it just left.

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