Gray Mars at last. The rock scratching at left, made by Opportunity, revealed rock rotted by an acidic, oxidizing brine. More life-friendly is the boring made by the Curiosity rover at right, which yielded gray, unoxidized rock indicative of benign conditions on ancient Mars.

Mars Was Habitable, but No Sign of Inhabitants

Dick writes about Earth and planetary science for Science magazine.

By drilling into Mars for the first time and analyzing a martian rock in unprecedented detail, Curiosity team members have shown that eons ago, microorganisms could have lived on Mars. At a spot on the floor of Gale crater—the site the rover has been studying since landing on Mars last August—there was mud long-wetted by benign waters, a ready supply of the essential elements of life as we know it, and energy enough to power life. The search for the remains of ancient life was not so lucky.

Curiosity found "all the things we were really hoping for in a habitable environment," said Michael Meyer, lead scientist for NASA's Mars Exploration Program at headquarters in Washington, D.C. Curiosity project scientist John Grotzinger of the California Institute of Technology in Pasadena made it a bit more personal: "We have found a habitable environment so benign that if you were on the planet when water was around, you would have been able to drink it."

The news of certain martian habitability came today at a Curiosity press conference at NASA headquarters. Orbiting spacecraft had imaged lots of places where early in Mars history there was plenty of water—river channels and crater lakes for sure, maybe even an ocean—and some of those once-watery locales even had signs of clay. Rock turns into clay only after long exposure to water of more or less neutral pH. In fact, the Opportunity rover is still roaming across a plain of once water-soaked rock. But it turned out to be rotted by highly acidic brine—not a nice place for life.

The sediment that formed the rock outcrop drilled by Curiosity proved to have been far more hospitable, Curiosity team members reported at the press conference. For starters, geologists say that it could have been mud on the floor of a lake nestled on the crater floor 3 billion years or more ago, about the time fossil life first appeared on Earth. Curiosity's Chemistry and Mineralogy x-ray instrument showed that the rock is 20% to 30% clay. And the Sample Analysis at Mars (SAM) instrument package detected no chemical signs of high acidity or brines. Concentrated brines can suck the water out of any microbes.

SAM also found the essential ingredients for life, including the elements carbon, hydrogen, oxygen, phosphorus, nitrogen, and sulfur. Those and other elements were found in a range of compounds, some more oxidized than others. That is key, because some microbial life on Earth can tap that range of chemical oxidation states for energy rather than using sunlight for photosynthesis or consuming organic matter.

The search for traces of early life did not hit pay dirt this time around. SAM Principal Investigator Paul Mahaffy of NASA's Goddard Space Flight Center in Greenbelt, Maryland, reported that heating of the drill sample produced some but not all of the chlorinated, single-carbon compounds produced when SAM heated a sample of wind-blown dust. Mahaffy now believes those compounds were derived from vapors of a chemical reagent that SAM carried for another sort of analysis and that had leaked into the sample. SAM operators had the rock sample flushed before heating to remove the contaminant, but some of the chlorinated compounds still showed up, although in smaller amounts. The team has a long way to go before determining whether even this trace carbon came from Mars, Mahaffy says.

Curiosity will be taking another drill sample in the next couple of months. Barring dramatic new findings, it will then start the trek to the foot of Mount Sharp, the pile of enticing rock that drew scientists to Gale crater. The rover's arrival there could come before the end of the year.

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