In Its First Year Exploring Rocks On Mars, What Did Perseverance Find?

One year ago, NASA’s Perseverance Mars rover successfully collected its first sample from a Martian rock.

To this day, Perseverance collected 38 samples of Martian rock and dirt. NASA plans to bring around 30 samples back to Earth in 2033, to be analyzed by instruments that are much more sophisticated than those that can be brought to Mars at present. They will provide a long-awaited timeline for the planet’s geologic and water history.

The scientists are nevertheless enthused by what they’ve discovered so far about the samples.

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 thus could potentially tell scientists about when water flowed on the planet’s surface.

“When that delta was deposited is one of the main objectives of our sample return program, because that will quantify when the lake was present and when the environmental conditions were present that could possibly have been amenable to life,” explains geochemist David Shuster, who is a member of NASA’s science team for sample collection.

The main surprise, Shuster said, is that the rocks collected from four sites on the floor of Jezero Crater are igneous cumulate rocks—that is, they were formed by the cooling of molten magma and are the best rocks for precise geochronology once the samples have returned to Earth. They also show evidence of having been altered by water.

“From a sampling perspective, this is huge,” he said. “The fact that we have evidence of aqueous alteration of igneous rocks—those are the ingredients that people are very excited about, with regard to understanding environmental conditions that could potentially have supported life at some point after these rocks were formed.”

Before the mission, geologists expected that the floor of the crater was filled with either sediment or lava, which is molten rock that spilled onto the surface and cooled rapidly. But at two sites referred to as Séítah—the Navajo word for “amidst the sand”—the rocks appear to have formed underground and cooled slowly. Evidently, whatever was 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 were driving around on the crater floor, whether the rocks that we’re looking at are sediments that were deposited into a lake, or igneous rocks,” he said. “In fact, they are igneous rocks. And the form of the igneous rocks that we found is quite surprising, because it doesn’t look like a simple volcanic rock that flowed into the crater. Instead, it looks like something that formed at depth and cooled gradually in a largish magma chamber.”

The crystal structure of the igneous rock—not unlike the granite of the Sierra Nevada, but with different composition and much more finely grained—showed millimeter-sized grains of olivine intergrown with pyroxene that could only have been formed by slow cooling. The coarse-grained olivine is similar to that seen in some meteorites that are thought to have originated on Mars and eventually crashed into Earth. The data supporting this came from multispectral images and X-ray fluorescence analysis by instruments aboard Perseverance.

“Either the rock cooled underground and came up from below, somehow, or there was something like a magma lake that filled up the crater and cooled gradually,” Shuster said.

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

Both the slow-cooled rocks at Séítah and the potentially more rapidly-cooled rocks at Máaz showed alteration by water, though in different ways. The Máaz rocks contained pockets of minerals that may have condensed from salty brine, while the Séítah rocks had reacted with carbonated water, according to chemical analyses onboard the rover.

It is not clear when and how long liquid water persisted inside the crater, either as flash floods, filling the impact crater and drying up in the space of just a few years, or as a lake feed by groundwater for millions of years.

The precise time will be revealed only by lab analysis on Earth, since the geochemical analysis tools required for dating are too large to have been placed aboard Perseverance.

Duplicate rock samples were taken at each of the four sampling sites and will be stored at a safe site near the delta, to be used only if the primary samples onboard Perseverance become inaccessible because of mechanical failure. That future cache will also include recently collected samples of sediments from the delta itself. Sedimentary rocks could also hold traces of microfossils, if ever life existed on Mars.

Material provided by University of California – Berkeley.