The Sample Analysis at Mars (SAM) instrument suite onboard the NASA Curiosity rover has been analyzing drilled rock and scooped-up soil samples taken from the martian surface since Curiosity landed in Gale Crater in August 2012. The laser spectrometer included in SAM heats these geological samples to approximately 1000°C to extract gases for analysis, particularly focusing on gases that are essential for life as we know it (i.e., water vapor and carbon dioxide). Recently, the SAM team discovered that some drilled rock samples were surprisingly enriched in light isotopes of carbon which, on Earth, would be a strong indicator of ancient microbial activity.

There are two stable isotopes of carbon, carbon-12 and carbon-13. Atoms of carbon-13 have an extra neutron in their nuclei, compared with carbon-12. It is more difficult to divide molecules that include the heavier carbon-13 atom, so life prefers to utilize molecules with the lighter carbon-12, enriching this isotope in organic compounds created by life. A study led by Christopher House from the Pennsylvania State University, University Park, found that six sites drilled by Curiosity showed over 70 parts per thousand higher carbon-12 relative to carbon-13, compared with an Earth-based reference standard. Although the intervention of lifeforms is the most likely cause of light carbon isotope enrichment on Earth, a different process may have caused this enrichment on Mars, and the SAM team is currently exploring alternative hypotheses. The strongest signals of carbon-12 enrichment in Gale Crater occur in rocks situated in present-day topographic highs, which suggests that the enriched carbon may have been deposited out of the atmosphere billions of years ago. This does not necessarily rule out the involvement of life in creating the carbon-12 signal on Mars, but until conclusive supporting evidence of life is also found in the rock record, it remains only a tantalizing hint at the possibility of life on ancient Mars. READ MORE