Billions of years ago, the face of Mars was likely very different from its current appearance. A recent study examined clay minerals and detected traces of mild and wet periods that persisted over a long duration, contradicting the hypothesis of a planet uniformly cold since its origins.
To trace this evolution, scientists rely on a chronology dividing the planet's history. One of these phases, the Noachian epoch, covering the period from approximately -4.1 to -3.7 billion years ago, is of particular interest. It coincides with an era of violent meteorite bombardment in the Solar System, which left giant imprints on Mars like the Hellas and Argyre basins.
NASA's Perseverance rover, which landed in Jezero Crater in 2021, is providing valuable information about that distant era. This site was selected due to the past presence of a lake, evidenced by fan-shaped deposits and channels carved by liquid flows. The examination of samples collected on site, particularly aluminum-rich clay pebbles called kaolinite, provides clues about the conditions that prevailed then.
A specific chemical composition characterizes these clay minerals, with a low iron and magnesium content but high levels of titanium and aluminum. This signature reveals alteration by water at moderate temperatures, not by very hot hydrothermal sources associated with impacts or volcanism. Researchers see in this evidence of prolonged alteration under the effect of significant rainfall.
Credit: ESA/DLR/FU Berlin (G. Neukum) Parallels with terrestrial clays formed during warm and humid periods support this interpretation. In Nature Communications Earth & Environment, the authors consider that these conditions may have extended over thousands, even millions of years, shaping environments potentially conducive to the emergence of life.
Perseverance has also gathered samples that may show signs of ancient life. These precious elements are stored in sealed containers, awaiting a return mission to Earth. Logistical and financial obstacles have led to the postponement of this project, thereby delaying the detailed analyses that could validate or refute these possibilities.
The continued exploration of Mars, first by robots and perhaps later by humans, will allow for deeper investigation of these questions. Each new discovery, such as that of clays altered by rain, helps to paint a more precise picture of our neighbor's past climate, which was likely far more hospitable than it is today—perhaps even to the point of having been able to develop life.
Illustration of the Perseverance rover on the floor of Jezero Crater.
Credit: NASA