The Longest Organic Molecules Found on Mars So Far Hint at Signs of Ancient Life

NASA’s Curiosity rover has found evidence of surprisingly long organic molecules in a rock on Mars dating back to the early days of the Solar System that may be the product of biological activity. Although these molecules can be generated by other, non-biological means, the discovery suggests that these compounds may be fairly common beneath the planet’s surface, and adds yet another piece of evidence for the existence of ancient Martian life.

Drilled by Curiosity from a 3.7 billion-year-old rock in a dry Martian lakebed called Yellowknife Bay, the compounds in question are hydrocarbon molecules built on a backbone of 10, 11 and 12 carbon atoms—called decane, undecane, and dodecane, respectively. Hydrocarbons such as these can originate from fatty acids, an important part of biological structures here on Earth, such as the oils and fats produced in the bodies of plants and animals.

“The fact that fragile linear molecules are still present at Mars’ surface 3.7 billion years after their formation allows us to make a new statement: If life ever appeared on Mars billions of years ago, at the time life appeared on the Earth, chemical traces of this ancient life could still be present today for us to detect,” according to study co-author Caroline Freissinet, an analytical chemist at the French National Centre for Scientific Research in the Laboratory for Atmospheres and Space Observations.

The presence of these hydrocarbons went overlooked for more than a decade: the sampled rock, nicknamed “Cumberland”, was drilled by Curiosity in 2013; although the rock was found to be rich in clay minerals, sulfur, and nitrates, no hydrocarbons were found at the time.

It wasn’t until a research team went looking for amino acids—the building blocks of proteins—that the hydrocarbon molecules were discovered. Using a new technique that required pre-heating the sample to 1,100°C (2,012°F) to rid the compounds of oxygen for the analysis, the spectrometers in Curiosity’s Sample Analysis at Mars (SAM) found the hydrocarbons instead.

“The excitement was super high when I saw the peaks on the spectrum for the first time,” Freissinet remarked. “It was both surprising and not surprising. Surprising because those results were found on the Cumberland sample that we had already analyzed many times in the past. Not surprising because we have defined a new strategy to analyze this sample.”

“New method, new results,” Freissinet added.

Freissinet’s team believes that these molecules may have broken off from longer organic molecules called undecanoic acid, dodecanoic acid and tridecanoic acid; to test their theory, they mixed undecanoic acid into clay similar to what is found on Mars, then conducted an analysis similar to what Curiosity’s SAM module performed. As expected, the experiment resulted in the formation of decane, the molecules consisting of 10 carbon atom chains, meaning they could have come from biologically-generated fatty acids.

Although there are other sources for these fatty acids, non-biological processes typically only result in chains of less than 12 carbon atoms. It is also worth noting that the spectrometers on the SAM module are not optimized to detect longer molecules, meaning chains of 13 carbon atoms or more could have been present but remained undetected—compounds that are more likely to have been produced by an organism, if they were indeed present.

“There is evidence that liquid water existed in Gale Crater for millions of years and probably much longer, which means there was enough time for life-forming chemistry to happen in these crater-lake environments on Mars,” according to co-author Daniel Glavin, a researcher at NASA’s Goddard Space Flight Center.

“We are ready to take the next big step and bring Mars samples home to our labs to settle the debate about life on Mars,” Glavin added.