Curiosity's lab carries quartz sample cups (white) and “wet lab” cups with solvent (silver). PHOTO: NASA GODDARD SPACE FLIGHT CENTER

For almost 40 years—ever since NASA's Viking missions—landers and rovers have searched the desiccated soil of Mars for the carbon-bearing organic compounds that would be essential for any Earth-like life. Last week here at the Lunar and Planetary Science Conference, scientists working with NASA's Curiosity rover announced a major milestone in that search. They reported the most definitive detections of organic molecules yet, along with hints of heavier, longer-chain molecules resembling those in cell walls.

The detected molecules do not necessarily indicate past or present life on Mars. They could also have come from asteroid impacts or rocks that burbled up from Mars's mantle. But they at least show that fragile organic materials can survive in the inhospitable environment—which bodes well for searches for clearer indicators of past life.

“We are all excited about it,” says Paul Mahaffy, a scientist at the Goddard Space Flight Center in Greenbelt, Maryland, and principal investigator for the Sample Analysis at Mars (SAM) instrument on Curiosity, which made the detections. “The fact that there are any organic compounds in the near surface at all is very promising.” It suggests, he says, that “biosignatures,” if any exist, might survive billions of years to be detected today in spite of the harsh surface conditions.

Mars's soil is known to be filled with harsh oxidizers like perchlorate, which break down organic molecules. Ultraviolet (UV) light at the surface can also destroy organics, as can cosmic rays. The Viking landers may have detected organic molecules in the form of chloromethanes, but in such tiny quantities—about 15 parts per billion—that the team was never confident about making a claim.

The Curiosity team is now making a claim with gusto, having detected chlorobenzene—a six-carbon molecule in an aromatic ring structure—at levels of up to 300 parts per billion. They have also detected smaller two-, three-, and four-carbon chains in the alkane family at lesser abundances. The results were posted online on 4 March in advance of publication in the Journal of Geophysical Research: Planets.

The organics come from a sample that Curiosity collected nearly 2 years ago by drilling 6.5 centimeters into a mudstone—hardened sediment from an ancient lake—at a site called Cumberland. The SAM team took so long to announce their finding in part because they wanted to be sure they had not been fooled by contamination from leakage in their “wet labs,” which hold thimblefuls of a solvent called MTBSTFA, used in isolating organic molecules. The SAM team has now controlled for this background contamination, and they have not seen the chlorobenzene at subsequent sampling sites—evidence that the Cumberland detection was real.

Curiosity scientists suspect that the chlorobenzene arose when organic molecules reacted with perchlorate in the soil as the sample was baked in SAM's ovens. To pin down the precursor molecules, the SAM team decided not to use any of their precious thimbles; instead they took advantage of the leaked MTBSTFA. They baked a sample from Cumberland to drive off any perchlorate and then exposed it to ambient MTBSTFA inside the onboard lab for 2 days. The solvent makes organic molecules much more volatile and thus more likely to be detected by SAM's mass spectrometer before they react with anything else (like perchlorate). “It gives them ‘wings’ so they can fly through the instrument unimpeded,” Mahaffy says. After the exposure, the team slowly started heating the sample again. “That's when this beautiful set of rich organics showed up.”

At the meeting, Danny Glavin, a SAM scientist at Goddard, reported compounds that, in a preliminary analysis, were consistent with an eight-carbon molecule akin to a benzoic acid, an 11-carbon alcohol-like molecule, and, most interesting of all, a 10-carbon molecule that could be a fatty acid–like carboxylic acid. Glavin is excited that long-chain organics can survive in spite of the oxidizing compounds and UV-rich sunlight. “The fact that we see something long means this could be a good location for preservation,” he says.

Drill hole from the Cumberland site, in rock that was an ancient lakebed. PHOTO: NASA/JPL-CALTECH/MSSS

Other scientists wonder if the compounds could signify something more than just good odds for preservation. In the case of the 10-carbon fatty acid–like compound, “You're talking about cellular-wall material,” says Marc Fries, a curation scientist at NASA's Astromaterials Acquisition and Curation Office at Johnson Space Center in Houston, Texas. “That's a potential biogenic molecule.” But it could also be a contaminant, he cautions. Fries notes that a 2014 study by scientists at the Jet Propulsion Laboratory in Pasadena, California, where the rover was assembled, warned that Curiosity could harbor traces of carboxylic acids, which are found in plant and animal oils as well as synthetic lubricants.

But Glavin points out that six batches of martian soil went through the SAM instrument before the Cumberland sample and ought to have scrubbed the instrument of residual contamination. And George Cody, a geochemist at the Carnegie Institution for Science in Washington, D.C., says that the compound is unlikely to have come from Earth. Fatty acids from biological sources, like technicians' fingerprints, Cody says, tend to have 14, 16, or 18 carbon atoms, not 10. Also, if the contamination were something like residual machine oil, smaller chain organic compounds would have been detected in the background–molecules that Curiosity does not see.

The Curiosity team has yet to use any of the seven MTBSTFA thimbles, or two with another type of solvent. Glavin says the team is saving them for promising rock formations farther up the mountain that Curiosity is now climbing: clay mineral–bearing deposits that, like the Cumberland mudstone, probably formed in water and could be a good location for preserving organics. Mahaffy hopes to get a chance to test one of his thimbles soon—and hopes to find even more-tantalizing organic molecules. “There's a lot of interesting sites coming along,” he says.