Light Carbon, Organic Molecules, and Habitats with Liquid Water.
The latest paper that’s supportive of possible life on Mars was published by Christopher House's research team in the Proceedings of The National Academy of Sciences. The team found that the carbon isotope ratios from several samples in Gale Crater (taken by the Curiosity Rover) were strongly enriched toward lighter carbon (carbon 12). Both forms of stable carbon have six protons, but the heavier carbon 13 has one neutron more than the more common carbon 12.
It is significant that carbon 12 was highly enriched in the mudstones from Gale crater compared to other carbon in the Martian environment. An isotope ratio such as that would be interpreted on Earth as evidence for life without any further questions. The reason is that life is “lazy” for lack of better words, and it tends to save energy and resources as much as it can. It costs slightly less energy to process the lighter carbon 12 than the heavier carbon 13.
However, the measurements were done on Mars, not Earth, making the interpretation more challenging. Christopher House discussed two additional hypotheses on how the lighter carbon could have come about. One suggestion invokes the interactions of UV irradiation with carbon dioxide in the Martian atmosphere. The other idea speculates on a rare event that might have occurred hundreds of millions of years ago when our Solar System passed through a giant molecular cloud with lighter carbon.
I think that the biological explanation is the more straightforward one. Interestingly, the lighter carbon has been found in association with reduced sulfur compounds, which are typically found in habitats with microbes that utilize methane in their metabolism. This is the case because microbial methane oxidation is often associated with sulfate reduction, resulting in reduced sulfur compounds like sulfides. During diagenesis, which are physical and chemical changes that occur after the sediments are deposited, these sulfides can react with organic material and form organic sulfur compounds such as thiophenes (organic molecules are the building blocks of life, and if they are there, life may be present as well). Interestingly, thiophenes have been detected recently on Mars.
In an earlier paper, Jacob Heinz from the Technical University Berlin and I discussed the potential origin of the Martian thiophenes. We concluded that if a low carbon 13/12 ratio were detected, it would support the hypothesis that these organic molecules were produced biologically (however, it would not be proof of life). The paper by House shows that this is indeed the case.
Furthermore, both thiophenes and similar light carbon isotope ratios have been found in stromatolites, microbial mats in Earth's ancient past. In our planet's geological record, we often find fossilized textures associated with these microbial mats, but these textures have not been found together with the light carbon detections in Gale Crater. If that had been the case, the authors of the paper would have surely made a much stronger case for life. However, the lack of associated textures does not mean there was no life. It is likely that any texture was dissolved by acidic fluids in the Martian environment or that Martian microbes do not produce similar fossil textures.
Curiosity's detection of light carbon is consistent with life. So, is its discovery of organic molecules. The presence of an ancient lake in Gale Crater, and the occurrence of other potentially habitable niche environments elsewhere on Mars (such as the recently discovered huge amounts of water in the valley floor of Valles Marineris) show that opportunities for life were and possibly are still present on Mars. Adding up all the evidence means that the case for life on Mars is getting stronger and stronger.
The study by House will also help the Perseverance rover team decide which samples have high priority for the planned sample return to Earth. It might be even more revealing to sample one of the methane plumes that occasionally emanate from the Martian subsurface and determine whether their carbon isotope ratio is consistent with the life interpretation.