Mars: The past of the crater where Perseverance landed intrigues scientists

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Depuis 18 mois, le rover Perseverance analyse sans relâche la géologie du cratère martien Jezero. © Nasa, JPL-Caltech, ASU, MSSS

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[EN VIDÉO] Perseverance, in search of life on Mars Perseverance, this is how NASA has decided to name the rover it will send to Mars in the summer of 2020. A rover from which researchers expect a lot. It will be the first to collect rock samples to return to Earth. Objective: Find traces of microbial life. The arrival of the Perseverance rover on Mars, which complemented the instruments already in place such as the Curiosity rover or the InSight seismological station, made it possible to explore a new Martian environment: the Jezero crater, made it possible to establish that this crater was occupied formerly by a vast lake, fed by a river forming a delta. The site is therefore particularly suitable for studying the history of water on the Red Planet and looking for possible traces of life. However, these topics require a fine study of the geology of the site: nature of the rocks, mineralogy, sedimentary architecture that testifies to episodes and conditions of the lake, volcanism… Waiting for the return of the samples to Earth, an operation that it is not planned until 2033, however, the analyzes are going well from the data relayed by the rover. Four new studies detailing the nature of the soil and subsurface of Jezero Crater have just been published together. The floor of Jezero Crater is made up of deep-seated igneous rocks. The ancient lake, the Perseverance rover has carried out several analyses. of the rocks that form the bottom of the crater. While everyone expected to find sedimentary rocks deposited on the lake floor or volcanic rocks such as ancient lava flows, the data revealed that the crater floor is often formed from magmatic accumulations. Finding this type of rock in the outcrop is surprising, because they are so-called plutonic rocks, that is to say, they are usually formed at depth, usually in the heart of magma chambers or at the bottom of lava lakes. In fact, they result from a slow cooling of the magma. The crystals that form as the temperature decreases will gradually settle to the bottom of the magmatic reservoir and accumulate giving rise to a stratified rock, in any case, the presence of this type of rock in the outcrop of the bottom of the crater, however, can only mean one thing: all the material that covered them has been removed by the slow process of erosion over billions of years. We’re still talking about a rock mass several hundred meters thick! These results were published in the journal Science under the title An olivine cumulate outcrop on the floor of Jezero crater, Mars, as well as in the article Compositionnaly and density stratified igneous terrain in Jezero crater, Mars published in Science Advances. confirmed by radar images taken by Perseverance. The instrument carried by the rover has indeed made it possible to visualize the subsoil at a depth of about 15 meters, revealing a very stratified architecture, which can be explained by the magmatic origin of the rocks, but also by the presence of sedimentary deposits of the lake . These results are presented in the article Ground penetrating radar observations of subsurface structures in the floor of Jezero crater, Mars, published in the journal Science Advances. One thing is certain, the rocks analyzed by the rover are evidence of a magmatic episode before the formation of the Jezero delta. Therefore, they could allow giving a lower age limit to this sedimentary formation. Different traces of alteration by water Their discovery is doubly interesting since these rocks also show traces of alteration by water. Since igneous rocks are particularly easy to date, the samples taken by Perseverance could thus allow to establish a precise chronology of the different water events at the site and in particular to date the formation of the lake. These data are one of the key elements to better understand the evolution of the Martian climate. The study of these rocks could therefore allow us to know precisely when the climate of the planet allowed the establishment of a water system on the surface and when the situation changed drastically to the cold and arid conditions we observe today . hui.Perseverance is however unable to make these appointments. Therefore, we will have to wait patiently for the samples to return to Earth. Thanks to the instruments on board the rover, however, the detailed study of the accumulated present at the bottom of the crater is possible. Their mineralogical analysis shows that they are composed of small, intricate crystals of olivine and pyroxene, which testify to slow crystallization. But what interests the specialists the most are the traces of water weathering. In fact, the different samples taken by Perseverance at different points in the crater appear to have been altered in different ways. The rocks of the Máaz deposit contain minerals in their pores that would have formed from brackish, very salty water. Conversely, the rocks of the Seitah deposit show traces of reaction with carbonate-rich water. Therefore, both samples testify to a change in lake conditions over time, which may be related to climate change. Once again, we will have to wait for the samples to return to Earth to be able to precisely date these different stages and establish their chronology. The detail of the analysis is available in the article Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars, published in Science. The existence of the lake was relatively short. Valuable samples Apart from the very local case of Jezero , the more detailed study of the olivine-rich cumulates could provide a better understanding of the magmatic activity of Mars. Combined with satellite images, the data reported by Perseverance could therefore help paint a larger-scale picture of the planet’s magmatic history, tasked by the mission to ensure its safekeeping and arrival on Earth in 11 years. In each of the four sites studied, the samples taken were doubled. These duplicates will be stored in a backup location near the delta in case the samples held by Perseverance cannot be retrieved, due to mechanical failure for example. The sedimentary rock samples recently collected by the rover at delta level will also be stored in this site. New samples that should also provide us with valuable information about Mars’ past. Interested in what you just read?
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