“If life ever appeared on Mars billions of years ago… chemical traces of this ancient life could still be present today for us to detect,” analytical chemist Caroline Freissinet of the French National Centre of Scientific Research said. That concept was sharpened following the discovery of the longest organic molecules ever found on Mars by NASA Curiosity rover in a rock dug in Gale Crater. The molecules are straight chains of hydrocarbons consisting of 10, 11 and 12 carbon atoms that were found in a mudstone sample found in an ancient lakebed deposit at Yellowknife Bay. In 2013, curiosity attracted the sample; the chemistry, though, had to wait over ten years to be given the proper twist of the analytical analysis.
The compounds, decane, undecane and dodecane are simple but their context is not. The discovery of them within the SAM, which was a miniature chemistry laboratory aboard Curiosity, happened during the search of other areas: amino acids, proteins building blocks. The revised method applied in the new run was that the powder was pre-heated first to force out oxygen prior to the actual analysis. No amino acids appeared. But rather the spectrum indicated unexpected peaks which were characteristic of decane, undecane, and dodecane indicating that Mars is still able to retain organics greater than the small grains that Curiosity had already observed.
The chains are generally understood as fragments of fatty acids that break away, on Earth, to be the key molecules in cell membranes and numerous metabolic pathways. Nonetheless, fatty acids can also be generated without biology as well as by water-rock chemistry in hydrothermal systems. The subtlety lies in the chain lengths: the Curiosity group associated the fragments with the potential parent fatty acids containing 11 to 13 carbons, a size range that is typically easier to form abiotically in relatively high concentration in short chains than longer ones. Even the limitations of curiosity itself have made the story open-ended, since SAM is not designed to identify longer organics, so it is not known whether the original rock supports still larger molecular scaffolds.
Geology assists in understanding the reason as to why Yellowknife Bay continues to pay dividends. The sample of the drill of the Cumberland fine-grained mudstone was deposited in standing water, and previous studies found the sample contains clays, sulfur-bearing chemistry, and even nitrates, materials that not only preserve the information about the environmental conditions but also help preserve the delicate organics. Co-author Daniel Glavin pointed out earlier, liquid water in the Gale Crater had existed many millions of years, which was enough time to have allowed complex prebiotic reactions to proceed in the sediments that eventually became rock. Even were biology to have never come on the stage, the environment of the lake might have concentrated compounds with carbon and buried them within mineral matrices that retarded their dissolution.
The search is still complicated by radiation and reactive salts. The surface of Mars is bombarded by energy which fractures organics over time and the perchlorate salts are able to scramble the signals when the samples are heated. Studies that have been attributed to Curiosity in its datasets have suggested the possibility of another one in that some of the ancient organics may still remain as organic salts, chemically modified residues which may be more recalcitrant than the molecules out of which they were built. The heating-based technique of SAM cannot easily identify which salts they are, since the gases they produce can resemble the gases of other ingredients of Mars.
It is the reason why the deeper message of the discovery is procedural rather than chemical. The method of the Cumberland sample remained the same; the sample changed. A minor modification in the treatment of an ancient powder by Curiosity opened a family of molecules which had been long concealed in view, confirming the idea that Mars can preserve records of life by billions of years and that the crucial evidence can be provided as much by the treatment of samples as by their locality.
