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This Feb 3, 2013 image provided by NASA shows a self portrait of the Mars rover, Curiosity. NASA's Curiosity rover has uncovered signs of an ancient freshwater lake on Mars that may have teemed with microbes for tens of millions of years, far longer than scientists had imagined, new research suggests.(AP Photo/NASA)

On Mars, an ancient lake and perhaps even life

About 3.5 billion years ago — around the time life is thought to have first arisen on Earth - Mars had a large freshwater lake that might well have been hospitable to life, scientists reported Monday.

The lake lay in the same crater where NASA’s Mars rover Curiosity landed last year and has been exploring ever since. It lasted for hundreds or thousands of years, and possibly much longer.

Whether any life ever appeared on Mars is not yet known, and Curiosity was not designed to answer that question. But the data coming back from the planet indicate that the possibility of life, at least in the ancient past, is at least plausible.

John P. Grotzinger, a professor of geology at the California Institute of Technology who is the project scientist for the Curiosity mission, said that if certain microbes like those on present-day Earth had plopped into that ancient Martian lake, they would most likely have found a pleasant place to call home.

“The environment would have existed long enough that they could have been sustained, prospered, grown, multiplied,” he said. “All the essential ingredients for life were present.

“Potentially the aqueous stream, lake, groundwater system could have existed for millions to tens of millions of years,” he added. “You could easily get a lake with the area of the Finger Lakes in upstate New York.”

The interpretation comes from detailed analysis of two mudstones drilled by Curiosity earlier this year. The structure, chemistry and mineralogy of the sedimentary rocks were not alien.

“The whole thing just seems extremely Earthlike,” Grotzinger said.

The scientists presented their latest findings at a meeting of the American Geophysical Union in San Francisco and in a set of six articles published in the journal Science.

The surface of Mars today is frigid and arid, bombarded by sterilizing radiation, but after it formed and cooled with the rest of the solar system about 4.5 billion years ago, it was initially a warmer and wetter place during its first billion years. Over the past decade, scientists have identified several sites on Mars that they think were once habitable.

In 2004, after NASA’s rover Opportunity discovered evidence that the Martian places it was traversing had once been soaking wet, Steven W. Squyres, the mission’s principal investigator, declared, “This is the kind of place that would have been suitable for life.”

But that location would have been an extremely challenging environment for life to take hold - very salty and highly acidic. Later, the scientists said the soils had been soaked not so much by water as by sulfuric acid.

NASA chose the 96-mile-wide Gale Crater as Curiosity’s landing site because readings from orbit identified the presence of clay minerals, which appear to have formed at the lake bottom, not swept down from the walls of Gale Crater, strengthening the case that the lake water was not acidic.

“If there were microbial organisms around, I think they would have liked that environment,” said David T. Vaniman, a researcher at the Planetary Science Institute in Tucson and the lead author of a Science paper examining the mineralogy.

An impact, probably by an asteroid, excavated Gale Crater 3.6 billion to 3.8 billion years ago, and the John Klein and Cumberland mudstones formed out of sediments that subsequently accumulated in the crater. That is roughly the same age as rocks on Earth with the earliest signs of life.

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