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

The arrival of the rover Perseverance on Mars, which complemented the instruments already on site such as the Curiosity rover or the InSight seismic stationmade it possible to explore a new Martian environment: the Jezero crater.

The first observations quickly made it possible to establish that this crater was once occupied by a vast lake, fed by a river forming a delta. The site is therefore particularly suitable for studying the water history of the Red Planet and looking for potential traces of life. However, these themes require detailed study of the geology of the place: nature of the rocks, mineralogy, sedimentary architecture testifying to the episodes and the conditions lake, volcanism…

While awaiting the return of samples to Earth, an operation that is only planned for 2033the analyzes are however progressing well from the data relayed by the rover. Four new studies have just been published together and detail the nature of the soil and subsoil of the Jezero crater.

The floor of the Jezero crater is composed of magmatic rocks of deep origin

During its journey in the heart of the old lake, the Perseverance rover has indeed carried out several analyzes of the rocks forming the bottom of the crater. When everyone expected to find sedimentary rocks deposited at the bottom of the lake or volcanic rockslike old flows of washthe data revealed that the crater floor is actually often formed by magmatic cumulates. Find this type of rockoutcrop is surprising, because these are rocks called plutonicthat is to say that they usually form at depth, generally at the heart of magma chambers or at the bottom of lava lakes. They result in fact from a slow cooling of the magma. The crystals that form as the temperature decreases will gradually settle to the bottom of the magma reservoir and accumulate to give rise to a stratified rock.

In both cases, the presence of this type of rock at the outcrop in the bottom of the crater can only mean one thing: all the material that covered them has been removed by the slow process of erosion over the years. billions of years. We are still talking about a thickness of rock several hundred meters thick! These results were published in the journal Science under the title An olivine cumulate outcrop on the floor of Jezero craterMarch as well as in the article Compositionnaly and density stratified igneous terrain in Jezero crater, Mars appeared in Science Advances.

This stratification of the basement of the Jezero crater was confirmed by the radar images carried out by Perseverance. The instrument carried by the rover has indeed made it possible to image the subsoil to a depth of around 15 meters, revealing a highly stratified architecture, which can be explained by the magmatic origin of the rocks, but also by the presence of lake sedimentary deposits. These results are presented in the article Ground penetrating radar observations of subsurface structures in the floor of Jezero crater, Marspublished in the journal Science Advances.

One thing is certain, the rocks analyzed by the rover bear witness to a magmatic episode prior to the formation of the Jezero delta. They could therefore make it possible to give a lower age limit for this sedimentary formation.

Different traces of weathering by water

Their discovery is doubly interesting since these rocks also bear traces of weathering by water. The igneous rocks Being particularly easy to date, the samples taken by Perseverance could thus make it possible to establish a precise chronology of the various hydric events of the site and in particular to date the formation of the lake. This data is one of the key elements allowing a better understanding of the evolution of the climate Martian. The study of these rocks could therefore make it possible to know precisely when the climate of the planet allowed the establishment of a hydrous system on the surface and when the situation changed dramatically towards the cold and arid conditions that we observe today. today.

Perseverance is not, however, in a position to perform such dates. We will therefore have to wait patiently for the samples to return to Earth. Thanks to the instruments on board the rover, the detailed study of the cumulates present at the bottom of the crater is however possible. Their mineralogical analysis shows that they are composed of small intricate crystals ofolivine and pyroxene, indicating slow crystallization. But what most interests specialists are the traces of weathering by water. The different samples taken by Perseverance at different points of the crater indeed seem to have been altered in different ways.

The rocks of the Máaz site contain in their pores minerals which would have formed from a brackish water, very salty. Conversely, the rocks from the Seitah site show traces of reaction with water rich in carbonates. The two samples thus 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 return of the samples to Earth in order 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, Marspublished in Science.

The low abundance of minerals resulting from the weathering of igneous rocks, however, suggests that the period of existence of the lake was relatively short.

Valuable samples

Apart from the very local case of Jezero, the more detailed study of olivine-rich cumulates could help to better understand the magmatic activity of Mars. Combined with satellite imagery, the data reported by Perseverance could therefore help paint a larger-scale picture of the planet’s magmatic history.

We better understand the value of the samples taken by Perseverance and the precautions taken by the scientists in charge of the mission to ensure their backup and their arrival on Earth in 11 years. On each of the four sites studied, the samples taken were duplicated. These duplicates will be stored on a backup site near the delta in case the samples kept by Perseverance cannot be recovered, due to a mechanical failure for example. On this site will also be stored the samples of sedimentary rocks recently collected by the rover at the level of the delta. New samples which should also provide us with valuable information on the past of Mars.