Nasa’s Curiosity rover has found the largest organic compounds ever seen on Mars, raising tantalising questions about whether life emerged on the red planet billions of years ago.
The compounds were detected in a 3.7bn-year-old rock sample collected in Yellowknife Bay, an ancient Martian lakebed that harboured all the necessary ingredients for life in the planet’s warmer, wetter past.
Tests onboard the rover found that the rock contained long-chain alkanes, organic molecules thought to be remnants of fatty acids. The compounds can be made by lifeless chemical reactions, but are crucial constituents of cell membranes in all living organisms on Earth.
The researchers do not claim to have found a biosignature – a “smoking gun” indicating life was once present – but one expert said the material represented the best chance that scientists had ever had for identifying remains of life on Mars.
“These molecules can be made by chemistry or biology,” said Dr Caroline Freissinet, an analytical chemist who led the research at the Atmospheres and Space Observations Laboratory in Guyancourt, near Paris. “If we have long-chain fatty acids on Mars, those could come – and it’s only one hypothesis – from membrane degradation of cells present 3.7bn years ago.”
The Curiosity rover has trundled more than 20 miles (32km) across the Gale crater since landing on Mars in 2012. Six years into the mission, it detected traces of organics in the ancient mudstone, but all were relatively short carbon-chain molecules.
For the latest study, Freissinet and her colleagues developed a new procedure to test more of the sample drilled from the mudstone. This time, Curiosity detected much larger organics, namely decane, undecane and dodecane.
Work on Earth showed that the Martian rock sample, known as Cumberland, probably contained carboxylic acids, or fatty acids, that converted to alkanes in the heating process. “Although abiotic processes can form these acids, they are considered universal products of biochemistry, terrestrial, and perhaps Martian,” the scientists wrote in Proceedings of the National Academy of Sciences.
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Further analysis of the organics only deepened the intrigue. When organisms on Earth make fatty acids, the compounds tend to contain more even numbers than odd numbers of carbon atoms. This is because some enzymes build fatty acids by adding two carbon atoms at a time. The scientists saw hints of this in the Martian organics, too. “Cumberland is teasing us,” Freissinet said. “The one in the middle with 12 carbons is more abundant than the other two. We have the same trend on Mars, but a trend drawn from three molecules is not a real trend. Still, it’s very intriguing.”
The finding suggests, at the very least, that organic signatures of life can be preserved in Martian rock for billions of years, bolstering hopes that should life ever have emerged on the planet, its remnants might still be found.
The pressing question is what to do next. Curiosity is carrying a second sample of the rock that scientists want to analyse for even larger organics. This might boost evidence for more fatty acids containing even numbers of carbons. But that would still not be conclusive.
John Eiler, a professor of geology and geochemistry at the California Institute of Technology, said analysing the different isotopes of carbon and hydrogen in the organics could reveal their origins. However, the tests require equipment found in only a handful of labs on Earth. “At present, there is no plausible path to making such measurements using an in-situ instrument on Mars,” he said. That will have to wait for a Mars sample-return mission.
“The findings reported in this paper present the best chance we have seen for identifying the remains of life on Mars,” said Eiler. “But sealing the deal absolutely requires return of such samples to Earth.”
