NASA’s Curiosity rover is exploring Gale crater on Mars. The crater contains rock formed from sediments deposited billions of years ago on the bottoms of ancient lakes and rivers of liquid water. Could life have existed in these long-vanished lakes and rivers? Paleontologists looking for evidence of the earliest life on Earth rely, in part, on analysis of the element carbon.
Carbon is a key ingredient in all known living things—molecules with long chains of carbon atoms are their working parts. Carbon atoms come in several different isotopes, that differ in atomic mass. Most have an atomic mass of twelve, but some have an atomic mass of thirteen or fourteen units. Living things prefer the lightest isotope, carbon twelve.
Paleontologists know that carbon found in the fossil remains of living things has a higher fraction of carbon twelve than carbon from non-living sources. An anomalous amount of carbon twelve has been found in two-point-seven-billion-year-old sedimentary rock from Western Australia. Along with additional fossil evidence of ancient mats of microbes, the carbon twelve anomaly is taken as evidence of ancient microbial life.
In 2022 a team of researchers announced an important finding from Curiosity’s analysis of Martian samples. Samples from six different Martian sites contained a strongly anomalous ratio of carbon twelve. On Earth, this would be taken as evidence that the carbon derived from past life. But Mars is an alien planet, whose chemistry is still poorly understood. While the carbon twelve finding is exciting, the researchers believe several possible non-biological explanations must be ruled out before the biological explanation can be considered evidence for life. Finding life on Mars isn’t easy.
