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Physical Chemistry

Curiosity Finds A Hospitable Mars

Planetary Science: Rock dust shows watery minerals, neutral pH, and an unusual energy source for microbes

by Elizabeth K. Wilson
March 14, 2013 | APPEARED IN VOLUME 91, ISSUE 11

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Credit: NASA/JPL-Caltech/MSSS
Curiosity collected rock samples on Mars near its landing site in an ancient lake bed.
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Credit: NASA/JPL-Caltech/MSSS
Curiosity collected rock samples on Mars near its landing site in an ancient lake bed.

The planet Mars was hospitable to microbial life at some point in its history. That’s the conclusion from analyses of martian rock samples pulverized by instruments aboard the NASA rover Curiosity seven months after it landed on the Red Planet.

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Credit: NASA/JPL-Caltech/MSSS
Analyses of rock sample drilled by Curiosity show a once-habitable environment.
09111-notw1-marsdustcxd.jpg
Credit: NASA/JPL-Caltech/MSSS
Analyses of rock sample drilled by Curiosity show a once-habitable environment.

Curiosity’s analyses of the first sample of rock ever drilled on another planet have yielded strong evidence that microbes could have thrived on Mars several billion years ago. NASA scientists announced the results at a March 12 press conference.

Scientists have known that Mars has a watery past—many spacecraft have discovered water-laden minerals and geographic features altered by water. But never before has a chemical examination provided so many lines of evidence for habitability.

The rover’s suite of analytical instruments revealed that 20 to 30% of the gray-green rock powder it analyzed is composed of the clay mineral smectite, which forms in water. The rock also contains calcium sulfate, which has a neutral—and therefore organism-friendly—pH. This contrasts with the more acidic martian environments containing magnesium and iron sulfates detected elsewhere on the planet by the long-lived NASA rovers Spirit and Opportunity.

“We have found a habitable environment that is so benign and supportive of life that if you were around, you would have been able to drink” the water, John P. Grotzinger, Curiosity project scientist, said at the press conference.

Portions of the rock sample, when heated in Curiosity’s ovens, gave off CO2. Mission scientists say the CO2 was produced during either a reaction with carbon or by the decomposition of a carbonate in the sample. Finally, the analysis also detected a possible microbial food source in the form of sulfur dioxide and hydrogen sulfide. These compounds can serve as a chemical battery, facilitating a flow of electrons for microbes known as chemolithoautotrophs. Such organisms, which get energy by oxidizing inorganic compounds and use CO2 as a carbon source, live on Earth in exotic habitats such as deep-sea vents.

The significance of the simple organics found in the martian sample remains complicated, the team members noted. Curiosity’s gas chromatograph/mass spectrometer detected dichloromethane and chloromethane in the recently analyzed rock sample. Both compounds were also detected in earlier test samples from Curiosity’s wanderings (C&EN, Dec. 10, 2012, page 44). But in each case, it’s possible that the compounds were generated by reactions with contaminants in Curiosity’s instruments. The team plans to repeat the experiment, with greater controls, with another rock sample.

Despite its initial success, Curiosity still has more work to do. In April, Mars will be on the opposite side of the sun from Earth, and so the rover will be incommunicado. But in May, it will drill another rock sample. It will then begin wending its way toward an intriguing location known as Mount Sharp, which has more clay and sulfate minerals.

Samuel Kounaves, a chemistry professor at Tufts University who was lead scientist for the Wet Chemistry Lab on NASA’s Mars Phoenix Lander mission, tells C&EN the discovery of organic chemicals could make the case for habitability even stronger. “Even though organisms such as chemolithoautotrophs could survive without an organic carbon source, finding organics would bolster the habitability for more complex microbial life, especially photosynthetic organisms,” he says.

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