Much like how carbon exists as several allotropes—diamond, graphite, plus other engineered carbons like fullerenes—carbon also exists as several kinds of isotopes. What separates one isotope of carbon from another is its number of neutrons; the more neutrons there are in an isotope, the heavier it is.
Carbon, for instance, has three different naturally-occurring isotopes, all with differing amounts of neutrons inside them: carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C). Carbon-14 is the rarest of the three, only making up less than 0.0001% of all the carbon on Earth, according to the US National Oceanic and Atmospheric Administration; carbon-13, on the other hand, comprises roughly 1.1%. Naturally, carbon-12 is the most abundant of them all, making up some 98.9% of all carbon on Earth. Plus, it’s the most abundant form of carbon in all life forms, including you and me.
However, things took a unique turn when the Curiosity rover drilled rock cores several times on Mars. The scientists responsible expected to find, if any, carbon-13 to be the dominant form of carbon on the Red Planet; instead, they found surprisingly abundant amounts of carbon-12 instead, NASA reported.
Naturally, questions point towards the possibility that something alive left all that carbon-12 in there—but scientists remind the public to exercise caution before jumping to conclusions, mentioning that vast amounts of data collection are needed. This is especially relevant considering previous reports of organic molecules being found on the planet just a few months earlier.
“We’re finding things on Mars that are tantalizingly interesting, but we would really need more evidence to say we’ve identified life,” said NASA’s then-Sample Analysis at Mars (SAM) laboratory principal investigator Paul Mahaffy. (Mahaffy retired back in December 2021.) “So we’re looking at what else could have caused the carbon signature we’re seeing, if not life.”
Said lead author Christopher House, whose work was published in the Proceedings of the National Academy of Sciences, to New Atlas: “On Earth, processes that would produce the carbon signal we’re detecting on Mars are biological. […] We have to understand whether the same explanation works for Mars, or if there are other explanations because Mars is very different.”
As of right now, there are three competing explanations that aim to give reason to the unexpectedly abundant presence of carbon-12 in the area. First, ultraviolet (UV) radiation from the Sun may have reacted with carbon dioxide (CO2) present in the Martian atmosphere, creating carbon-rich molecules that settle on the surface afterward. The second explanation goes that the Solar System passed through a so-called “molecular cloud” rich with carbon-12 several hundred million years ago, giving the Red Planet the carbon it needed to shower down on its surface.
And the third reason, and perhaps the most striking if proven true, is that ancient Martian bacteria may have released methane (CH4) gas into the atmosphere, which interacted with incoming UV light. Of course, the scientists caution that the first two explanations are increasingly more likely to be true compared to the third, but ultimately time and research will tell.
“The hardest thing is letting go of Earth and letting go of that bias that we have and really trying to get into the fundamentals of the chemistry, physics, and environmental processes on Mars,” said co-author Jennifer Eigenbrode to New Atlas.
For now, more research is needed to further identify markers that would clue scientists into one explanation over the others. Additionally, Curiosity will need the help of its co-worker on the Red Planet, the Perseverance Rover, in locating true signs of life, if any, on a planet that apparently used to hold at least some amounts of surface water.
References
- House, C. H., Wong, G. M., Webster, C. R., Flesch, G. J., Franz, H. B., Stern, J. C., Pavlov, A., Atreya, S. K., Eigenbrode, J. L., Gilbert, A., Hofmann, A. E., Millan, M., Steele, A., Glavin, D. P., Malespin, C. A., & Mahaffy, P. R. (2022). Depleted carbon isotope compositions observed at Gale crater, Mars. Proceedings of the National Academy of Sciences, 119(4). https://doi.org/10.1073/pnas.2115651119
- Irving, M. (2022, January 19). Curiosity detects carbon signature on Mars that, on Earth, indicates life. New Atlas. https://newatlas.com/space/curiosity-carbon-12-signature-mars-life/
- Nield, D. (2022, January 19). Nasa’s curiosity rover drilled holes into mars, and found something very strange. ScienceAlert. https://www.sciencealert.com/really-weird-carbon-has-been-found-on-mars-and-it-could-point-to-biological-life
- NOAA & US Department of Commerce. (n.d.). Global monitoring laboratory—Carbon cycle greenhouse gases. Global Monitoring Laboratory; Earth System Research Laboratories. Retrieved 27 January 2022, from https://gml.noaa.gov/ccgg/isotopes/chemistry.html
- Shekhtman, S. (2022, January 14). Nasa’s curiosity rover measures intriguing carbon signature on mars [Text]. NASA. http://www.nasa.gov/feature/goddard/2022/nasa-s-curiosity-rover-measures-intriguing-carbon-signature-on-mars
- Wall, M. (2022, January 19). Curiosity rover finds ‘tantalizing’ signs of ancient Mars life. LiveScience. https://www.livescience.com/mars-organics-curiosity-rover-possible-biosignature
