Carbon is critical to life, as far as we know. So anytime we detect a strong carbon signature somewhere like Mars, it could indicate biological activity.
Does a strong carbon signal in Martian rocks indicate biological processes of some type?
Any strong carbon signal is intriguing when you’re hunting for life. It’s a common element in all the forms of life we know of. But there are different types of carbon, and carbon can become concentrated in the environment for other reasons. It doesn’t automatically mean life is involved in carbon signatures.
Carbon atoms always have six protons, but the neutron count can vary. Carbon atoms with different numbers of neutrons are called isotopes. Three carbon isotopes occur naturally: C12 and C13, which are stable, and C14, a radionuclide. C12 has six neutrons, C13 has seven neutrons, and C14 has eight neutrons.
When it comes to carbon isotopes, life prefers C12. They use it in photosynthesis or to metabolize food. The reason is relatively simple. C12 has one fewer neutron than C13, which means that when it bonds with other atoms into molecules, it makes fewer connections than C13 does in the same situation. Life is essentially lazy, and it will always seek the easiest way to do things. C12 is easier to use because it forms fewer bonds than C13. It’s easier to get at than C13, and life never takes the hard way when an easier way is available.
The Curiosity rover is hard at work in Mars’ Gale crater, searching for signs of life. It drills into rock, extracts a pulverized sample, and places it into its onboard chemistry laboratory. Curiosity’s lab is called SAM which stands for Sample Analysis at Mars. Inside SAM, the rover uses pyrolysis to bake the sample and convert the carbon in the rock into methane. The pyrolysis is done in a flow of inert helium to prevent any contamination in the process. Then it probes the gas with an instrument named the Tunable Laser Spectrometer to find out what carbon isotopes are in the methane.
The Sample Analysis at Mars tool is called SAM. SAM is made up of three different instruments that search for and measure organic chemicals and light elements that are important ingredients potentially associated with life. Credit: NASA/JPL-Caltech
The team behind Curiosity’s SAM looked at 24 rock samples with this process and recently discovered something noteworthy. Six of the samples showed elevated ratios of C12 to C13. Compared to an Earth-based reference standard for C12/C13 ratios, the samples from these six sites contained greater than 70 parts per thousand more C12. On Earth, 98.93% of the carbon is C12 Earth, and C13 forms the remaining 1.07%.
A new study published in the Proceedings of the National Academy of Sciences (PNAS) presented the findings. Its title is “Depleted carbon isotope compositions observed at Gale crater, Mars.” The lead author is Christopher House, a Curiosity scientist at Penn State University.
It’s an exciting finding, and if these results were obtained on Earth, they would signal that a biological process produced the abundance of C12.
On ancient Earth, surface bacteria produced methane as a byproduct. They’re called
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