After ten years of work, a new map of the surface of Mars has been produced. It provides information on the history of liquid water over 3.5 billion years ago – the time of the oldest known traces of life on Earth. Cette carte, faite à partir des sondes orbitales de l’ESA Mars Express et de la NASA Mars Reconnaissance Orbiter, recense des centaines de milliers de dépôts de minéraux « hydratés » – formés en contact avec de l’eau liquide – à la surface de Mars.
Wet minerals have been one of the priority science goals for the past two decades, and future re-sampling missions (Mars sample return, persistence rover and ExoMars external analyzes in situ (ExoMars) will study them in more detail. Finally, potentially exploitable water resources and construction materials for manned missions future to Mars.
For all these reasons, it was necessary to understand their distribution, the context of their formation, their abundance on the surface. Our knowledge was previously limited to a few thousand sites scattered across Mars, and details of only a few have been identified. This time around, the new map of Hydrometallurgy provides a global view with a high spatial resolution (about 200 meters per pixel) of the entire planet (except for its poles, which are covered in permanent ice or seasonal frost).
Do hydrated minerals “store” Mars’ water?
These minerals have the peculiarity of being formed by a chemical reaction between the crust of Mars, especially of volcanic origin, and liquid water. Most of them retain a small part of the stored water in molecular or structural form, which can amount to several tens of percent of the mass of weathered rocks.
[Plus de 80 000 lecteurs font confiance à la newsletter de The Conversation pour mieux comprendre les grands enjeux du monde. Abonnez-vous aujourd’hui]These hydrated minerals, such as clays or salts, are excellent tracing tools for ancient, water-rich geological environments. They provide information on chemical conditions and give an indication of water availability and potential stability over time. It is also suspected that some aquatic minerals are able to trap and preserve organic matter over geologically long periods of time, some of which may belong to living organisms.
An amazing fact emerges from these maps: the oldest surface of Mars, more than 3.5 billion years old, shows remnants of liquid water everywhere, thanks to the presence of these minerals. Previously, the discovery of such a deposit was a rare curiosity of Mars. The model has since changed and it is rare that these minerals are not seen when observing the oldest terrain.
Detailed knowledge of their geographical distribution makes it possible to link mineralogy and morphology to better constrain the context of their formation. This is how the mudflats and deltas were discovered at the site of Oxia Planum, proposed as a landing site for the ExoMars mission. However, the fate of this major mission to explore outer Mars is still uncertain, but it may leave for Mars before the end of this decade.
On the other hand, a An innovative approach to modeling the infrared spectra of Mars It makes it possible to estimate not only the exact composition of these minerals but also their abundance. It thus becomes possible to estimate the mass fraction of minerals of interest in the Martian outcrops and then the possible water fraction available. This information is key to determining the intensity of a chemical change by water, but also for estimating its potential resources.
Climate evolution of Mars
One of the main questions that can now be addressed concerns the climatic evolution of Mars.
If Mars had an active, water-rich surface in its first period, it is well established that a Major irreversible climate change radically Changed the conditions of the surface of Marswhich has become arid and cold globally.
The change in the atmosphere of Mars is not entirely explained by the flight of its components into space. Much of the water and carbon dioxide could have “drank” from its rocks during the chemical reactions that form the hydrated minerals. In the absence of recycling mechanisms as on Earth, particularly plate tectonics, the equivalent of a global layer of water hundreds of meters thick could be trapped in rocks, and still exist there today.
