The past of the abyss into which Perseverance has descended intrigues scientists

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] Persistence, searching for life on Mars
Perseverance, NASA has decided to name the rover it will send to Mars in the summer of 2020. The most anticipated rover by researchers. He was the first to collect rock samples brought back to Earth. Objective: To detect traces of microbial life.

Arrival of the Rover Persistence On Mars, it complemented instruments already on site Curiosity rover or the Insite Seismic StationMade it possible to explore a new Martian environment: the Jezero Crater.

The first observations quickly established that the crater was once occupied by a large lake. A delta is formed by the river. The site is therefore well-suited to studying the red planet’s water history and searching for possible traces of life. However, a more detailed study of these themes is needed Geography of place: sedimentary architecture testifying to the character, mineralogy, episodes and conditions of the rocks Lake, Volcano

While waiting for samples to return to Earth, A move scheduled only for 2033However, analyzes are well advanced from published data Round. Four new studies, published together, describe the soil and subsoil characteristics of Jezero Crater.

The floor of Jezero Crater is composed of magmatic rocks of deep origin

During its journey through the core of the old lake, the Perseverance rover made several analyzes of the rocks that actually make up the crater’s floor. When everyone expected to find out Sedimentary rocks At the bottom of the lake or Volcanic rocksLike the old runs WashingThe data revealed that the crater floor actually exists Mostly formed by magmatic cumulates. Find this type of rockExtrovert Surprising, because these are called rocks PlutonicThat means they usually form deep down, usually in the heart Magma chambers Or at the bottom of volcanic lakes. They actually result from slow cooling Magma. As the temperature drops, the formed crystals gradually settle to the bottom of the magma reservoir and accumulate to form a layer of rock.

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In both cases, the presence of these types of rocks at the bottom of the crater can only mean one thing: all the material covering them has been removed by a slow erosion process over the years. billion years. We are still talking about a thickness of several hundreds of meters of rock! These results were published in the journal Science Under the title An olivine accumulation on the floor of Jezero CraterMarch As well as in the article Jezero crater, a compositional and density stratified igneous landscape on Mars appeared Scientific advances.

This is Stratification The base of the Jezero Crater was confirmed by diligent radar imagery. The instrument carried by the rover has imaged the subsoil at a depth of about 15 meters, which can be explained by the magmatic origin of the rocks, but reveals a more layered architecture by the presence of lake sediment deposits. . These results are presented in the article Ground-penetrating radar observation of subsurface structures on the floor of Jezero Crater on MarsPublished in the magazine Scientific advances.

One thing is certain, the rocks analyzed by the rover bear witness to a magmatic episode that preceded the formation of the Jezero Delta. They can therefore provide a lower age limit for the formation of these sediments.

Different traces of weathering by water

Their discovery is doubly interesting because these rocks also bear traces of water weathering. The Volcanic rocks Due to the ease with which the samples have been diligently taken, it is possible to establish a precise chronology of the various hydrological events of the site and in particular the formation of the lake to date. This data is one of the key elements that allow a better understanding of evolution Climate Mars planet. Therefore, by examining these rocks, it is possible to determine precisely when the climate of the planet allowed a hydrous system to be established on the surface and the conditions changed dramatically towards the cold and dry conditions we observe today.

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However, diligence is not in a position to meet such dates. So we have to wait patiently for the samples to return to Earth. Thanks to the instruments on board the rover, a detailed study of the bulges at the bottom of the crater was possible. Their mineralogical analysis shows that they are composed of small complex crystalsOlivine and pyroxene, indicating slow crystallization. But of most interest to experts are traces of water weathering. Different samples taken by diligence at different points in the crater seem to have actually been altered in different ways.

The rocks of the mass floor are in their holes Minerals formed from a brackish water, very salty. Conversely, rocks from the Cheetah site show traces of reaction with carbonate-rich water. Two samples testify to a change in lake conditions over time, which may be related Climate change. Again, we will have to wait until the samples are returned to Earth to precisely date these different stages and establish their chronology. The analysis is detailed in the article Samples of hydrous-altered igneous rocks on the floor of Jezero Crater on MarsPublished in Science.

However, the low abundance of minerals due to the weathering of igneous rocks suggests that the lake’s period of existence was relatively short.

Valuable models

Apart from the local case of Jezero, a detailed study of olivine-rich cumulates will help to better understand the magmatic activity of Mars. Combined with satellite images, the data reported by Persistence will help paint a large-scale picture of the planet’s magmatic history.

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We fully understand the value of diligent sampling and the precautions taken by the scientists responsible for ensuring them. Backup And in 11 years they came to earth. At each of the four sites surveyed, samples were taken in duplicate. These copies are stored at a backup site near Delta if, for example due to mechanical failure, diligently stored samples cannot be retrieved. The site will store sedimentary rock samples recently collected by the rover at delta level. The new models should provide us with valuable information about Mars’ past.

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