Since 2012, the Curiosity rover has been surveying Gale Crater, which may have housed a large lake several billion years ago. In this formerly aqueous environment, the main objective of its mission was to probe the potential for habitability of the planet Mars. However, we could have been wrong.
A few days ago, Curiosity celebrated its nine years on the red planet. The rover evolves inside the Gale crater, with many scientists suggesting that the structure housed an ancient lake more than three billion years ago. During its long mission, Curiosity carried out numerous surveys and took a lot of photos. So far, all this data analyzed suggests that the researchers were right, evidenced by the many layered sediments of the Murray Formation and Mount Sharp, in the center of the crater, visibly formed from meltwater. , depositing sand and silt to the bottom.
A recent study published by a team from the Department of Earth Sciences at the University of Hong Kong today overturns this idea. Their work, published in Science Advances, indeed proposes that the sediment measured by the rover during most of the mission did not actually form in a lake, but from sand and silt carried by the wind. . According to their analyses, water would still be involved, but in lesser quantities, probably in the form of acid rain. All this water would then have accumulated in lakes, but these would probably have been much smaller and shallower than had been theorized.
The key point is that some elements are said to be mobile (they can be easily dissolved in water) while others are said to be immobile (indissoluble). Whether an element is mobile or immobile depends not only on the type of element, but also on the properties of the fluid with which it interacts (acid, saline, oxidant, etc.).
What this work shows is that immobile elements are correlated to each other and strongly enriched at higher altitudes in the rock profile. According to the authors, this suggests a downward alteration. If these rocks had formed in a lacustrine environment, these elements would resemble each other at the bottom (no longer at the top, but at the bottom). Furthermore, this work points out that iron is depleted as weathering increases, which means that the atmosphere at the time was reducing (devoid of oxygen), and not oxidizing.
"Their data challenges existing assumptions about both the repository environment of these unique rock formations and the atmospheric conditions in which they were formed “Summarizes Dr. Ryan McKenzie, from the same university. "Specifically, the authors show evidence of weathering processes under a reducing atmosphere in a subareal desert-like environment, rather than formation in a watery lacustrine environment “.
Naturally, it remains difficult to establish a diagnosis precise and without appeal on rocks analyzed tens of millions of kilometers away. Nevertheless, it is worth asking. This work could indeed help paint a better picture of ancient Mars and, by extension, ancient Earth as well.
