The First Signs of Ancient Life on Mars?

Dick writes about Earth and planetary science for Science magazine.

SAN FRANCISCO, CALIFORNIA—The first full analysis of martian soil by the Curiosity rover has detected simple carbon compounds that could be the first traces of past martian life ever found, NASA scientists announced here today at a press conference at the annual fall meeting of the American Geophysical Union. The catch is that Curiosity team members can't tell yet whether the organic matter was once alive, was never alive and drifted onto Mars from space, or was simply cooked up in Curiosity's analytical instrument from lifeless bits of soil. Figuring out the ultimate source of the carbon in this organic matter—biological or not—will take time. "Curiosity's middle name is Patience," cautioned Curiosity project scientist John Grotzinger of the California Institute of Technology in Pasadena.

Although Curiosity did find organic matter, the media's frenzied anticipation that preceded the press conference—and drove it into a ballroom to accommodate all the TV cameras—turned out to have been provoked by a misunderstanding. A reporter overheard Grotzinger praising the superb high-quality data being returned by the rover and assumed the remarks referred to an exciting discovery from Curiosity's first thorough soil analysis. The rover had taken in fine soil particles, heated them, and passed the gases that the heating drove off through its mass spectrometer, which can separate and identify the gases by molecular weight. The only exciting result reporters could imagine was organic matter from life, and so the frenzy of anticipation began.

What Curiosity actually detected were trace amounts of three of the simplest possible carbon-containing compounds: a carbon atom with one, two, or three chlorine atoms attached in place of hydrogen atoms. According to Paul Mahaffy of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the principal investigator of Curiosity's Sample Analysis at Mars (SAM) instrument package, these three chloromethanes were most likely generated in SAM. The heating may have decomposed a natural component of martian soil—the strong oxidizing agent perchlorate—which in turn could have broken down some form of carbon in the soil sample and chlorinated its carbon atoms.

The question would then become what form the carbon was in. It might have been big, complex organic molecules like amino acids, the molecular remains of long-dead martian organisms. Or it could have been big, complex organic molecules like polycyclic aromatic hydrocarbons synthesized between the stars, incorporated into the primordial stuff of comets and asteroids, and still raining onto the surface of every body in the solar system. Or it could have been inorganic compounds like carbonates, much like the baking soda in your kitchen cabinet.

The very same conundrum has been frustrating Mars scientists since 1977. The two Viking landers also scooped up martian soil, heated it, and analyzed the gases that came off. And they, too, detected chloromethanes. Viking team members eventually attributed them to lingering contamination from solvents used to clean the lander before launch. But recently, astrobiologist Christopher McKay of NASA's Ames Research Center in Mountain View, California, and colleagues proposed an alternative interpretation of the Viking results. They argue that the most likely carbon source would be complex, biological organic matter that was too heat-resistant to be driven into Viking instrumentation. Mahaffey isn't picking a favored carbon source, but he's clear on one thing: No one used any of the Viking solvents on Curiosity. Sorting out Curiosity's carbon source is doable, he says; it will just take more samples analyzed in more ways, plus additional analyses of some carbon-free blank samples brought from Earth. And that will take more time and more patience.

Posted in Space