An Early, Muddy Mars Just Right for Life

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Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1898-1899
DOI: 10.1126/science.310.5756.1898b

SAN FRANCISCO, CALIFORNIA— When the Opportunity rover found the salty sedimentary remains of standing water on Mars, the prospects for early life on another planet brightened considerably. Although acid-laden, those early waters were nothing that martian life couldn't have adapted to. It's harder to imagine life originating under such conditions, however. Now, by analyzing the infrared “colors” of the martian surface, planetary scientists have identified clayey rocks that mark an even earlier warm and wet era, one more persistently wet and blessedly less acidic. The origin of martian life now looks brighter too.

Key to refining the water history of Mars was the powerful OMEGA spectrometer aboard the orbiting Mars Express spacecraft. OMEGA can probe the ground in enough detail and in the right range of infrared wavelengths to identify the distinctive absorption peaks of clays and sulfate salts (Science, 6 August 2004, p. 770).

Since announcing the first firm detection of martian clay last March, OMEGA team members have formed a clearer picture of how clays fit in the geologic history of Mars, reported OMEGA team leader Jean-Pierre Bibring of the University of Paris South in Orsay. Clays and the sulfate salts that mark the Opportunity deposits do not generally occur together, they found. And clays seem to have formed in a time before martian acid was corroding rock to produce the sulfate salts. In the Nili Fossae region, for example, clays appear beneath— and therefore were deposited earlier than—fresh, unweathered rock rich in olivine. They may even have formed before the early giant Isidis impact of some 4 billion years ago. All in all, clays appear to have formed within hundreds of millions of years after the planet did, and before the sulfates formed, said Bibring, about the time life could have been appearing on Earth.

That timing—first clay, then sulfate—boosts the prospects for life on Mars by providing it with a possible birthplace other than the later acid bath. “The kind of chemical reactions we think were important to giving rise to life on Earth simply could not have happened” under the conditions Opportunity found, says paleontologist Andrew Knoll of Harvard University, a rover team member. The pH 1 sulfuric acid that leached rock to produce Opportunity's sulfates would have worked against the evolution of increasingly complex organic compounds that could lead to life. And it wasn't even always wet. The Opportunity landing site wasn't a “shallow sea,” as initially assumed, but a salty sand sea with intermittent puddles between the dunes, team members write in a set of papers in the 30 November issue of Earth and Planetary Science Letters.

Clay, on the other hand, connotes a more hospitable environment, Bibring noted. The earlier clay era was “probably most favorable to have hosted the emergence of life,” he said, “and could still host biorelics.” On Earth, the smectite clays identified by OMEGA form under the mild, more continuously wet, and far less acidic conditions of the midlatitudes. OMEGA data are “pretty good evidence” of “more Earthlike conditions” on earliest Mars, agrees planetary geologist James Head of Brown University. Now planetary scientists must decide where to send their next, far more capable rover: to the well-characterized and safe Opportunity site, one of the newly enticing but poorly understood clay sites, or somewhere else found by the upcoming Mars Reconnaissance Orbiter?

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