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).
The area Curiosity has been analysing since 2012, as part of NASA's Mars Science Laboratory mission, is composed primarily of sedimentary sequences deposited at the bottom of a lake 3.5 billion years ago. These sediments contain various secondary minerals, such as clays or sulphates, which indicate that the primitive surface was in contact with liquid water.
The existence of liquid water requires a warm surface temperature brought about by a minimum content of CO2 in the atmosphere. Yet this was not the case with Mars in its beginnings. "This contradiction has two possible solutions. Either we have not yet developed climatic models which explain the environmental conditions on Mars at the beginning of its history, or the Gale sedimentary sequences really did form in a very cold climate. The second option is the most reasonable", explains CSIC researcher Alberto Fairén, who works at the Centre for Astrobiology near Madrid (a joint centre run by CSIC and Spain's National Institute of Aerospace Technology).
"However, the rover has not found carbonates, thereby confirming the results of studies by all previous probes: carbonates are very scarce on the surface of Mars and, therefore, the CO2 level in the atmosphere was very low", adds. Fairén.