The CO2 level in Mars’ primitive atmosphere 3.5 billion years ago was too low for sediments, such as those found by NASA’s Curiosity exploration vehicle in areas like the Gale Crater on the planet’s equator, to be deposited. This and other conclusions are drawn from a paper written with the participation of researchers from the Spanish National Research Council (CSIC) and published in the latest issue of the journal, Proceedings of the National Academy of Sciences (PNAS).