The planet Mars may once have enjoyed a much milder climate, accompanied by regular and abundant rain. This long-debated hypothesis finds new support thanks to recent observations by NASA's Perseverance rover. It has identified thousands of whitish-colored rocks on the Red Planet, whose presence tells a story very different from the dry, cold environment we currently attribute to Mars.
Since its arrival in Jezero Crater in 2021, the vehicle has spotted a multitude of these scattered rock formations, ranging from the size of a simple pebble to that of a large boulder. Their particularity comes from their composition rich in kaolinite, a clay mineral that, on Earth, forms under prolonged exposure to water. This abundance would indicate that water played a leading role in Martian geological history.
The scientific team compared Perseverance's data with terrestrial kaolinite deposits, such as those present in Southern California or South Africa. The chemical signatures appear very similar, reinforcing the idea of formation by weathering related to precipitation. This type of transformation requires warm and humid conditions, accompanied by repeated rainfall over extremely long periods, potentially millions of years.
These observations help solidify the image of an ancient, more hospitable Mars, where liquid water was omnipresent. Such an environment could have been conducive to the emergence of life, at least in elementary forms. Furthermore, scientists explain that understanding this wet period helps trace how the planet lost its atmosphere to become the arid world we observe today.
Rocks rich in kaolinite, observed by the Perseverance rover, indicate prolonged chemical weathering by water on Mars billions of years ago.
Credit: NASA
However, a major question remains: what is the exact origin of these rocks? No obvious outcrop is visible in the immediate vicinity of the crater. The hypothesis of transport by ancient rivers or projection during meteorite impacts is being considered.
The results of this study, published in
Nature Communications Earth & Environment in December 2025, open a window into Mars's past. They are based on the direct analysis of surface materials, different from previous orbital observations. The next phases of the Perseverance mission will continue to explore these leads to better understand the conditions that prevailed on Mars several billion years ago.