The team of scientists operating NASA’s Curiosity rover has found organic materials on Mars — but isn’t sure whether the carbon-containing compounds are indigenous to the planet.

On 3 December, the team announced it had found several carbon-containing compounds, such as chloromethane (CH3Cl), after heating soil samples scooped from a wind-blown drift (pictured at right) into ovens on board the rover’s Sample Analysis at Mars (SAM) instrument. The compounds were formed after the highly reactive chlorine combined with carbon in the oven — but the team isn’t sure of the source material for those two compounds.

“We have to make sure that the carbon and the chlorine are coming from Mars,” says SAM principal investigator Paul Mahaffy at the Goddard Space Flight Centre in Greenbelt, Maryland.

Though the team has tried to scrub the intakes to the SAM ovens with the gritty martian soil, it is possible that trace amounts of terrestrial carbon remained on the rover after launch. And even if the scientists can convince themselves that the carbon they’re seeing was once a part of an organic compound in the soil, they will have to sort out whether it comes from meteorites that have fallen to the planet — one expected source — or whether it could signify something of greater astrobiological interest.

The source of the chlorine in the compounds is also of interest. The team suspects that it comes from a calcium perchlorate salt. If confirmed, that would be the second discovery of a perchlorate salt on Mars, after the polar mission Phoenix discovered some in 2008. Though perchlorate is an weak oxidant and capable of destroying organics, some astrobiologists have pointed out that it acts as an anti-freeze and could fuel the metabolism of microbes.

The team’s discoveries were announced at a meeting of the American Geophysical Union in San Francisco, California, where a large pack of journalists were waiting for potentially big news, after a National Public Radio story hyped the findings two weeks ago, and quoted project scientist John Grotzinger, of the California Institute of Technology in Pasadena, as saying, “This data is gonna be one for the history books.”

At the press conference on 3 December, Grotzinger said that he was just talking about his excitement at the performance of the SAM instrument. “The enthusiasm I had … was just misunderstood,” he says.