March 26, 2023

A year ago, NASA’s Perseverance rover successfully took its first samples from Martian rocks.

To this day, Perseverance has collected 38 samples of Martian rock and soil. NASA plans to bring about 30 samples back to Earth in 2033 for analysis by more sophisticated instruments than what can be brought to Mars today. They will provide a long-awaited timeline of Earth’s geological and water history.

Still, the scientists remain enthusiastic about what they have found so far about the samples.

Jezero CraterLocated north of the Martian equator, it is the target of NASA’s Mars 2020 mission and its Perseverance rover, because it contains river deltas that appear to have formed within the lake bed, and thus may tell scientists when water is flowing on the Earth’s surface.

“When this delta was deposited is one of the main goals of our sample return program, as this will quantify when the lake existed and when there were environmental conditions that might have been suitable for life,” explained geochemist david schusterwho was part of NASA’s sample collection science team.

The main surprise, Schuster said, was that the rocks collected from the four sites at the bottom of Jezero Crater were igneous—that is, they formed from the cooling of molten magma, and once the samples were available, they were The best rocks for precise geochronology. back to earth. They also show evidence of being altered by water.

“From a sampling standpoint, it’s huge,” he said. “In fact, we have evidence of water alteration in igneous rocks — these are ingredients that people are very excited about, about understanding the environmental conditions that might have supported life at some point after these rocks formed.”

Before the mission, geologists expected the bottom of the crater to be filled with sediment or lava, which is lava that spilled to the surface and cooled rapidly. But at two sites known as Séítah—the Navajo word for “in the sand”—rock appears to have formed underground and cooled slowly. Apparently, over the past 250 to 3.5 billion years, the things covering them have eroded away.

“For the first nine months, as we drove at the bottom of the crater, we were really debating whether the rock we were looking at was sediment deposited in the lake, or igneous rock,” he said. “In fact, they are igneous rocks. And the morphology of the igneous rock we found was quite surprising, because it didn’t look like a simple volcanic rock flowing into the crater. Instead, it looked like something that formed at depth and was in a Gradually cooling in larger magma chambers.”

The crystal structure of igneous rocks – with Granite in the Sierra Nevada, but with a different composition and finer grains — showing millimeter-sized olivine grains symbiotic with pyroxene, which can only be formed by slow cooling. Coarse-grained olivine is similar to that seen in some meteorites thought to have originated on Mars and ended up falling to Earth. Data supporting this comes from multispectral images and X-ray fluorescence analysis of the instrument on Perseverance.

“Either the rock cooled underground and came out of it, somehow, or something like lava lake filled the crater and gradually cooled,” Schuster said.

Another nearby sample called Máaz — Mars in Navajo — is also igneous, but with a different composition. Because this layer overlies Séítah’s exposed layers of igneous rock, the Máaz rocks may be the upper layers of the lava lake. In magma lakes on Earth, denser minerals sink downward as they crystallize, forming layers of different compositions. These types of igneous rocks are called accumulationswhich means they were formed by the deposition of iron- and magnesium-rich olivines and the subsequent multistage cooling of thick magma bodies.

Slow cooling rocks of Séítah and possibly more rapidly cooling rocks of Máaz Shows water changes, albeit in different waysAccording to chemical analysis on the rover, the Máaz rocks contain pockets of minerals that may have condensed from the salty brine, while the Séítah rocks react with carbonated water.

It’s unclear when and how long liquid water persisted in the crater, either from flash floods that filled the crater and dried up in just a few years, or groundwater that served as a source of lake water for millions of years.

The exact timing can only be revealed by laboratory analysis on Earth, because the geochemical analysis tools needed for the dating are too large to fit on Perseverance.

Duplicate rock samples were collected at each of the four sampling sites and will be stored in a safe location near the delta, used only if the primary sample onboard Perseverance is inaccessible due to mechanical failure. Future caches will also include recently collected sediment samples from the delta itself. Sedimentary rocks may also contain traces of microfossils, if there is life on Mars.

Materials provided University of California Berkeley.

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