There is new proof that life on Mars was possible in the distant past: the robot Curiosity discovered the fossil witness of a cyclical climate alternating dry and wet seasons, an environment similar to that of the Earth and therefore conducive to the appearance of living organisms, according to a study published Wednesday in Nature.
The red planet, whose current climate is extremely arid, had abundant rivers and lakes billions of years ago, which have now evaporated. But unlike that of Earth, the surface of Mars is not renewed by plate tectonics, and the traces of these ancient terrains have been well preserved.
The rover Curiosity, from NASA, has been exploring one of these terrains since 2012, the huge Gale crater and its 6 km high mountain made of sedimentary layers. “We quickly realized that we were working in lake and river deposits, but we didn’t know what type of climate to link them to,” says William Rapin, CNRS researcher and lead author of the study.
Mars could, for example, have been a frozen planet, where a volcanic eruption would have suddenly warmed the climate and triggered the formation of liquid water, adds this planetologist from the Institute for Research in Astrophysics and Planetology (University of Toulouse III – Paul Sabatier / CNES), who conducted the research with the Lyon Geology Laboratory and American and Canadian colleagues.
As he slowly ascended the slope of the mountain, Curiosity fell on salt deposits forming hexagonal patterns, in soil dating from 3.8 to 3.6 billion years ago.
Analysis of the rock by the robot’s American MastCam and Franco-American ChemCam instruments showed that they were cracks of dried mud.
“When a lake dries up, the mud cracks, and when it rehydrates, the cracking ‘heals’”, explains William Rapin. If this process is repeated on a regular basis, the cracks arrange themselves in such a way as to form hexagons, similar to patterns observed in ancient land basins which dry up seasonally. The modeling of terrestrial mud subjected to dry and wet cycles has further demonstrated “mathematically” this particular hexagonal formation.
From inert to living
It is therefore “the first tangible proof that Mars had a cyclical climate”, according to the researcher. As on Earth, dry and wet seasons followed each other at regular intervals, more than three billion years ago. And over a long enough period—several million years—for life to develop.
Such a climate is one of the conditions for organic matter to go from inert to living.
” Curiosity had already detected the presence of simple organic molecules that can be formed by geological or biological processes”, details the CNRS in a press release. For example amino acids, which sometimes combine to form more complex and constitutive molecules of living organisms, such as RNA or DNA. However, such a process needs cycles to form, as independent experiments in the laboratory have shown, continues the research organization. “In a world that is too dry, these molecules never have the opportunity to form; in a world that is too humid either,” adds the planetologist.
The red planet therefore possessed the balance necessary for the development of life forms. What type? Scientists think of primitive single-celled microorganisms such as archaea or bacteria, which are our most distant ancestors.
How they appeared on Earth remains a mystery, as plate tectonics erased the trace of the oldest fossils. “What we lack on Earth is the odyssey of the origins of life at the molecular level”, says William Rapin.
Mars has recorded its remains and could allow scientists to understand on a small scale what happened on our planet in its infancy. If it turned out, of course, that life forms did indeed appear on Mars, or aborted, which exploration missions like Curiosity And Perseverance try to find out.
