On Mars, not just water, but a sea

New photos from Opportunity show evidence of rocks once covered by surface water, a sign conducive to life.

The same string of rocks that recently confirmed the past presence of water on Mars is now telling an even more intriguing story. It suggests that this craggy outcrop on the dark and desolate desert of the Meridiani plain was once the shoreline of an ancient sea.

Researchers announced Tuesday that the Opportunity rover has found minute ripples and textures on the face of a Martian rock, pointing conclusively to the fact indicating that it was formed beneath a standing body of salt water.

Though scientists cannot determine how long ago the sea covered this corner of the Martian surface, the finding - more than any other this mission - begins to reveal the Mars of imagination. The existence of a briny sea offers the strongest evidence yet that the Red Planet might have once been a very different place - more reminiscent of Earth than the current cosmic locker of rock and ice.

Moreover, a broad variety of life could have developed in surface seas, and though Opportunity does not have the tools to find biological traces, the rocks suggest this is just the sort of place to look for them.

"This is a piece of evidence that had been lacking," says Bruce Jakosky, a astrophysicist at the University of Colorado in Boulder. "It would suggest there was an early warm and wet Mars."

Exactly what this sea might have looked like or how long it lasted remain a mystery. But the new data paint a picture of what could have been a broad salt flat, alternately covered by water and then dry as liquid evaporated.

The ripples and patterns in the rock, catalogued in 152 photos by Opportunity's microscopic imager, indicate that the water must have been at least 2 inches deep - and possibly much deeper. And they look similar to formations in desert basins or along ocean coasts on Earth. The presence of bromine and chlorine in the rocks also point to immersion in a gradual ebb and flow of salty water.

The discovery is a significant step forward from what the rover team announced just three weeks ago, when scientists said they found evidence that ground water once coursed beneath the planet's skin.

Surface water opens the possibility of more diverse forms of life, such as microbes that use sunlight to create energy. It also increases the chances of finding fossils of organic chemicals during future missions. "If you look at what kinds of rocks on Earth best preserve evidence of microbial life, [they] are the rocks that precipitated from liquid water," says Steve Squyres, the mission's chief scientist.

Yet the discovery of an ancient surface sea clearly hints at something more, as well. It is a clue into the mystery of Mars's past. Surface water, after all, would instantly evaporate on today's Mars. The atmosphere is simply too thin to support it. The presence of ancient lakes, then, suggests that - at some point perhaps billions of years ago - Mars might have undergone a radical transformation.

Scientists have suspected this for decades. But the quest to understand how and why a warm, wet Mars with a thick atmosphere could have changed into today's icebox has yielded only contradictions. Computer models can't seem to replicate it. In addition, scientists point out that the surface sea could have been covered by ice - allowing it to exist in a "cold, dry" environment.

The new data, however, will give fresh momentum to the idea that Mars might have been different once - and to the search for missing pieces of the puzzle. Says Dr. Jakosky: "It will force us to look for physical scenarios to explain it."