If Martian life ever existed, it should have sufficient amounts of organic carbon. Hence, Curiosity rover is testing rock samples for measuring organic carbon in them.
Gale crater samples tested
Curiosity rover drilled samples from 3.5-billion-year-old mudstone rocks with organic carbon in the Yellowknife Bay of Gale Crater. This site also had an ancient lake.
About the experiment
Curiosity’s Sample Analysis at Mars instrument heated the rock samples to high temperatures and converted it into CO2. Scientists used that data to measure carbon isotope ratios.
Importance of organic carbon
Organic carbon is created and used by all forms of life. Although its presence itself is not proof, this gives scientists more insight into the question of life on Mars.