To take the measurement, Curiosity drilled into rocks at a geological formation named Yellowknife Bay in Gale Crater. The location was chosen as the rover’s landing site because it was once home to an ancient martian lake.
The rocks themselves are mudstone, formed when very fine sediment settled at the bottom of a lake and was buried. They are an estimated 3.5-billion years old. Using the onboard Sample Analysis at Mars (SAM) instrument, Curiosity baked the rock. Then, by sprinkling in some oxygen, the organic carbon was converted into carbon dioxide (CO2).
This resource-intensive experiment has only been performed once by Curiosity in its nearly 10 years on Mars.
But while organic carbon is the basis for all (known) forms of life, it isn’t necessarily a smoking gun, as it could be delivered to Mars by non-biological means. Meteorites, volcanoes, and even reactions on the martian surface could all be responsible for creating the organic carbon.
To help clarify the possible sources of the organic carbon, SAM also measured the carbon isotopes present in the sample, providing key information on where the element may have come from.
Isotopes are kind of like different flavors of an element. Each isotope has a different number of neutrons — and, therefore, weight — allowing scientists to distinguish them from one another. Carbon-12 is lighter than carbon-13, for example. And because being lighter means that it’s quicker to react, life tends to prefer to use carbon-12.