After rock samples return to Earth in 2033, analysis will answer lingering questions about when water sank on the planet

During its first year of exploring the Jezero crater on Mars, the Perseverance rover collected rock samples that scientists believe will provide a long-awaited timeline of the planet’s geological and water history.

They’ll just have to wait a decade to find the answer, until the samples can be retrieved from the surface and sent back to Earth for 2033 dating.

Scientists are nevertheless excited about what they have discovered so far on the samples. These findings are described in an article that will appear on August 25 in the journal Sciencewith more detailed analyzes in a second Science article and two other articles published online simultaneously in Scientists progress.

Jezero Crater, just north of the Martian equator, was a target for NASA’s Mars 2020 mission and its Perseverance rover because it contained what looked like a river delta that formed inside a lake bed and could therefore potentially tell scientists when water was flowing on the planet. surface. According to geochemist David Shuster, a professor of earth and planetary sciences at the University of California, Berkeley, rocks collected from the crater floor underlie the delta sediments, so their crystallization ages will provide an upper limit. for the formation of the delta.

“The time this delta was deposited is one of the primary goals of our sample return program, as it will help quantify when the lake was present and when environmental conditions were present that could have been conducive to life. “said Shuster, who is a member of NASA’s science team for sample collection, one of the three main authors of the Science article summarizing the work and co-author of two of the other three articles.

The other two main authors of the abstract Science article are geochemist Kenneth Farley of Caltech, project scientist Perseverance, and assistant Mars 2020 project scientist Katherine Stack Morgan of NASA’s Jet Propulsion Laboratory (JPL).

The main surprise, Shuster said, is that the rocks collected from four sites at the bottom of Jezero Crater are cumulated igneous rocks – that is, they were formed by the cooling of molten magma and are the best rocks for accurate geochronology once samples return to Earth. They also show evidence of having been altered by water.

“From a sampling perspective, it’s huge,” he said. “The fact that we have evidence of aqueous weathering of igneous rocks – those are the ingredients that get people excited, in terms of understanding the environmental conditions that could have potentially supported life at some point after these formed. rocks.”

“A big value of the igneous rocks we collected is that they will tell us when the lake was present at Jezero. We know it was there more recently than the igneous rocks at the bottom of the crater formed,” said Farley. “It will answer some major questions: When was the climate of Mars suitable for lakes and rivers on the planet’s surface? And when did it change to the very cold and dry conditions we experience today. ?”

Prior to the mission, geologists expected the crater floor to be filled with sediment or lava, which is molten rock that has spread to the surface and cooled rapidly. But at two sites called Séítah – the Navajo word for “in the middle of the sand” – the rocks appear to have formed underground and slowly cooled. Obviously, everything covering them has eroded away over the past 2.5 to 3.5 billion years.

“We literally debated for the first nine months, as we drove across the crater floor, whether the rocks we’re looking at are sediments that were deposited in a lake or igneous rocks,” he said. “In fact, they are igneous rocks. And the shape of the igneous rocks that we found is quite surprising, because it does not look like a simple volcanic rock that has flowed into the crater. Instead, it looks like to something that formed deep down and gradually cooled in a large magma chamber.”

The crystal structure of the igneous rock – much like the Sierra Nevada granite, but with a different composition and a much finer grain – showed millimeter-sized grains of olivine interwoven with pyroxene that could not be formed only by slow cooling. The coarse-grained olivine is similar to that seen in some meteorites thought to have originated on Mars and eventually crashed to Earth. Supporting data comes from multispectral imagery and X-ray fluorescence analysis by instruments onboard Perseverance and is detailed in a second step. Science article by lead author Yang Liu, planetary geologist at JPL.

Sites of Séitah and Máaz

According to Shuster, the data allows for a few scenarios that explain the igneous rocks at the bottom of the crater.

“Either the rock cooled underground and came up from below somehow, or there was something like a lake of magma that filled the crater and gradually cooled. “, did he declare.

Samples from a second nearby site called Máaz – Mars in the Navajo language – are also igneous, but of a different composition. Because this layer overlies the layer of igneous rock exposed at Séítah, the Máaz rock could have been the top layer of the magma lake. In magma lakes on Earth, the densest minerals are deposited as they crystallize, creating layers of different compositions. These types of igneous formations are called cumulative, which means that they formed by the sedimentation of iron- and magnesium-enriched olivine and the subsequent multi-stage cooling of a thick magma body.

The igneous rocks of Máaz could also come from a later volcanic eruption.

In either case, the top layer that partially eroded could have been on the order of hundreds of meters thick, Shuster said.

Slowly cooled rocks at Séítah and potentially faster cooled rocks at Máaz showed weathering by water, albeit in different ways. The Máaz rocks contained pockets of minerals that could have condensed from salty brine, while the Séítah rocks had reacted with carbonated water, according to chemical analyzes aboard the rover.

The precise times when these different layers formed will only be revealed by laboratory analysis on Earth, as the geochemical analysis tools needed for dating are too bulky to have been placed aboard Perseverance.

“There are a variety of different geochemical observations we can make in these rocks when we send them back to Earth. It will give us all kinds of information about this igneous environment,” he said. “We can determine when the rock crystallized, which is one of the things I’m most excited about for providing a delta time constraint. But it also gives us information about when the igneous activity s “is being produced inside the planet. Combined with the satellite imagery, we can then relate this to some of the more global and regional igneous activity.”

Shuster noted that duplicate rock samples have been taken from each of the four sites and that within a year they will be cached with other duplicate samples at an emergency site near the delta at use only if primary samples aboard Perseverance become inaccessible due to mechanical failure. This future cache will also include sediment samples recently collected from the delta itself – details of which are being prepared for a future scientific paper.