In a new study appearing today in the journal AGU Advances, scientists from MIT and NASA report that seven rock samples collected along the “fan front” of Jezero Crater on Mars contain minerals that normally form in water. The results suggest that the rocks were originally deposited by water or may have formed in the presence of water.

The seven samples were collected in 2022 by NASA’s Perseverance rover as it explored the crater’s western slope, where some rocks are thought to have formed in an ancient lake that has now dried up. Members of the Perseverance science team, including scientists from MIT, have examined the rover’s images and chemical analyses of the samples and confirmed that the rocks do indeed contain traces of water and that the crater was likely once a watery, habitable environment.

Whether the crater was actually inhabited is still unknown. The team found that the presence of organic matter – the raw material for life – cannot be confirmed, at least based on the rover’s measurements. However, judging by the rock’s mineral content, scientists believe the samples are their best chance of finding signs of ancient life on Mars once the rock is returned to Earth for more detailed analysis.

“These rocks confirm the existence of at least temporary habitable environments on Mars,” says the study’s lead author, Tanja Bosak, a professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “We found that there was indeed strong water activity. For how long, we don’t know, but certainly long enough to form these large sedimentary deposits.”

Rock cores recovered during the Fan Front campaign. CacheCam images of the cores in their container tubes are shown on the left. Red symbols on the High-Resolution Imaging Experiment (HiRISE) map on the right show the locations of the sampled outcrops and corresponding cores. Shuyak and Mageik were sampled in Amalik (Figure 2), Hazeltop and Bearwallow in Wildcat Ridge (Figure 3), Kukaklek in Hidden Harbor (Figure 4), and Swift Run and Skyland in Skinner Ridge (Figure 5). Sunset Hill (yellow symbol) was ground down, but the rock collapsed during grinding, so it was not sampled. All cores are 13 mm in diameter. White lines on the map mark the rover’s trajectory during the Fan Front campaign. The green symbol shows the location of the Three Forks Cache.

In addition, some of the samples collected may have originally been deposited in the ancient lake more than 3.5 billion years ago – before the first signs of life even existed on Earth.

“These are the oldest rocks that may have been deposited by water that we’ve ever touched with hands or rover arms,” ​​says co-author Benjamin Weiss, the Robert R. Shrock Professor of Earth and Planetary Sciences at MIT. “That’s exciting because it means these are the most promising rocks that may have fossils and signs of life preserved in them.”

Co-authors of the study at MIT include postdoctoral fellow Eva Scheller and research scientist Elias Mansbach, as well as members of the Perseverance science team.

At the front

The new rock samples were collected in 2022 as part of the rover’s Fan Front Campaign – an exploration phase in which Perseverance traversed the western slope of Jezero Crater, where a fan-like region contains sedimentary, layered rocks. Scientists suspect this “fan front” is an ancient delta formed by sediment that flowed with a river and settled in a now bone-dry lake bed. If life existed on Mars, scientists believe it could be preserved in the sediment layers along the fan front.

NASA’s Perseverance rover collected rock samples at two locations seen in this image of Mars’ Jezero Crater: Wildcat Ridge (bottom left) and Skinner Ridge (top right). Image credit: NASA/JPL-Caltech/ASU/MSSS

In the end, Perseverance collected seven samples from different locations along the fan front. The rover obtained each sample by drilling into the Martian rock and extracting a pencil-sized core, which it then sealed in a tube to one day be recovered and returned to Earth for detailed analysis.

Before collecting the cores, the rover took images of the surrounding sediments at each of the seven sites. The science team then processed the image data to estimate the average grain size and mineral composition of a sediment. This analysis showed that all seven samples collected likely contain traces of water, suggesting that they were originally deposited by water.

In particular, Bosak and her colleagues found evidence of certain minerals in the sediments that are known to precipitate from the water.

“We’ve found a lot of minerals like carbonates that form the reefs on Earth,” says Bosak. “And it’s really an ideal material to preserve fossils of microbial life.”

Interestingly, the researchers were also able to detect sulfates in some samples collected from the base of the fan front. Sulfates are minerals that form in very salty water – another sign that the crater once contained water – although very salty water, says Bosak, “is not necessarily the best for life.” If the entire crater was once filled with very salty water, it would have been difficult for any form of life to thrive. However, if only the bottom of the lake were salty, that could be an advantage, at least for preserving any signs of life that may have lived higher up in less salty layers, eventually died out and sank to the bottom.

“As salty as it was, if organic matter was present, it’s like putting something in salt,” says Bosak. “If life had fallen into the salt layer, it would have been very well preserved.”

This mosaic is made up of several images from NASA’s Perseverance Mars rover and shows a rocky outcrop called “Wildcat Ridge” where the rover took two rock cores and ground down a circular patch to study the rock’s composition. Image credit: NASA/JPL-Caltech/ASU/MSSS

Blurry fingerprints

However, the team stresses that the rover’s instruments could not detect organic matter with certainty. Organic matter can be a sign of life, but it can also be created by certain geological processes that have nothing to do with living matter. Perseverance’s predecessor, the Curiosity rover, had discovered organic matter throughout Mars’ Gale Crater, which scientists suspect may have come from asteroids that have impacted Mars in the past.

And in a previous campaign, Perseverance spotted what appeared to be organic molecules in several places along the floor of Jezero Crater. These observations were made using the rover’s SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) instrument, which scans the Martian surface with ultraviolet light. When organics are present, they can glow, similar to how material glows under a black light. The wavelengths at which the material glows act as a sort of fingerprint for the type of organic molecules present.

During Perseverance’s previous exploration of the crater floor, SHERLOC appeared to detect signs of organic molecules throughout the region and later in some places along the fan front. But careful analysis led by Eva Scheller of MIT revealed that while the observed wavelengths could be signs of organic matter, they could just as easily be signatures of substances that have nothing to do with organic matter.

“It turned out that cerium metals embedded in minerals actually produce very similar signals to the organic substance,” says Scheller. “During the investigation, the potential organic signals correlated strongly with phosphate minerals, which always contain some cerium.”

Scheller’s work shows that the rover’s measurements cannot be clearly interpreted as organic matter.

“That’s not bad news,” says Bosak. “It just tells us that there isn’t a lot of organic material. It’s still possible that it’s there. It’s just below the rover’s detection limit.”

When the collected samples are eventually sent back to Earth, Bosak says the laboratory instruments will be sensitive enough to detect any organic matter they may contain.

“Once we have nanometer-resolution microscopes on Earth and different kinds of instruments that we can’t fit on a rover, we can actually try to look for life,” she says.

This work was supported in part by NASA.

Astrobiological potential of rocks collected by the Perseverance rover from a sediment fan front in Jezero Crater on Mars, AGU Advances (open access)

Astrobiology