“The findings reported in this paper present the best chance we have seen for identifying the remains of life on Mars,” said Caltech professor of geology and geochemistry John Eiler. That is within the scientific community as NASA’s Curiosity rover discovers its largest organic molecules since it landed on the Red Planet. These buried hydrocarbons in Martian mudstone 3.7 billion years ago offer us a peep into Mars’ distant past—a period when life was just beginning on Earth.
The decane, undecane, and dodecane molecules are carbon atom chains of 10, 11, and 12 atoms. These molecules are most frequently found on Earth in fatty acids, which are structural molecules in cell membranes and biochemical reaction substrates. But the same molecules can also be synthesized through non-biological chemistry, like hydrothermal chemistry or chemical reactions between minerals and water. That double functionality is only one of a number of reasons Martian organic chemistry is so difficult to read. “These molecules can be made by chemistry or biology,” wrote CNRS chemist Dr. Caroline Freissinet, who led the study. “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 finding is predicated on an analogy from the ancient lake bed of Gale Crater, Yellowknife Bay. The site has fascinated scientists with its sedimentary clay rich rock, sulfur, and nitrates ingredients to stabilize organic molecules for billions of years. The Sample Analysis at Mars (SAM) lab on the Curiosity used gas chromatography and mass spectrometry to examine the sample and discovered evidence for these intricate carbon chains. Perhaps most of all, SAM’s advanced methods allowed the fragile molecules to survive in their complete forms all the way to analysis, which paves the way for future missions.
The scale of the discovery is colossal. Not evidence of life itself, but that there was chemical building blocks of life on Mars at some point or other. Freissinet highlighted the importance “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,” Freissinet explained to ScienceAlert.
To yet again increase the suspense, the researchers found a weak trend in the molecular abundance. Enzymes like to form fatty acids with an even number of carbon atoms on Earth since they insert two carbon atoms at once. Interestingly, the same was seen in the Martian sample as well, and dodecane (12 carbon atoms) occurred more often than its counterparts. Freissinet savored the tease of this finding “Cumberland is teasing us. 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.”
Scientists are keen to know where the molecules are coming from in subsequent missions. The 2028 ESA ExoMars and NASA-ESA Mars Sample Return will bring back Mars samples to Earth for more detailed investigation. The samples may be able to allow scientists to probe the carbon hydrogen isotopic ratios, the primary constrains on abiotic or biologic origin of molecules. To Eiler, “At present, there is no plausible path to making such measurements using an in-situ instrument on Mars. That will have to wait for a Mars sample return mission.”
The Curiosity discovery makes it also sensible to search for evidence of past life in near-surface sedimentary rocks on Mars. Daniel Glavin, a senior scientist with NASA’s Goddard Space Flight Center, added further, “The discovery of long chain hydrocarbons that were preserved (and not completely destroyed by exposure to ionizing radiation) in ancient sedimentary rocks on Mars bolsters the current strategy of searching for ancient signs of life in the Martian near-surface that may share similar characteristics with life on Earth.”
Scientists marvel and are intrigued by that which remains unknown but still today. Is the molecule evidence for life on early Mars millions of years ago, or might it have been generated non-biologically? How many Martian fossils’ rocks hold secrets? As Freissinet herself, in a happy turn of phrase, described it, “Our study proves that, even today, by analyzing Mars samples we could detect chemical signatures of past life, if it ever existed on Mars.” Until the next installment in this drama is on the anvil, this finding alone is enough proof that man’s endless curiosity about where we are in the universe will never stop existing.
