According to NASA’s Perseverance rover, ancient rocks in Jezero Crater formed in the presence of water. These sedimentary rocks are more than 3.5 billion years old and may predate the appearance of life on Earth. When and if these samples are returned to Earth, scientists hope to determine if they hold evidence of ancient Martian life.

In 2022, the Perseverance Rover worked its way along Jezero Crater’s western slope and sampled rocks from a feature called the ‘fan front.’ Scientists hypothesized that some of the rocks in this region were formed in the ancient lakebed when the crater was filled with water. Perseverance analyzed the rocks’ chemistry and captured images of their surroundings. Members of the Perseverance science team studied this data and have published their results.

“These rocks confirm the presence, at least temporarily, of habitable environments on Mars.”

Professor Tanja Bosak, MIT

Their work is titled “Astrobiological Potential of Rocks Acquired by the Perseverance Rover at a Sedimentary Fan Front in Jezero Crater, Mars.” It’s published in the journal AGU Advances, and the lead author is Tanja Bosak, professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).

“These rocks confirm the presence, at least temporarily, of habitable environments on Mars,” said lead author Bosak. “What we’ve found is that indeed there was a lot of water activity. For how long, we don’t know, but certainly for long enough to create these big sedimentary deposits.”

Perseverance collected seven samples from the fan front. Each of the samples is of a sedimentary rock, and some of them may predate life on Earth. “The samples include a sulphate- and clay-bearing mudstone and sandstone, a fluvial sandstone from a stratigraphically low position at the fan front, and a carbonate-bearing sandstone deposited above the sulphate-bearing strata,” the authors explain.

Sulphates and clays typically form in the presence of water, and so do carbonates. Depending on the types of sulphates, it reveals clues about the ancient water’s chemistry, temperature, and acidity. Carbonates are similar and can also reveal things about Mars’ atmosphere when they formed, like how much carbon dioxide it contained.

“The hydrated, sulphate-bearing mudstone has the highest potential to preserve organic matter and biosignatures, whereas the carbonate-bearing sandstones can be used to constrain when and for how long Jezero crater contained liquid water,” the authors explain.

While the samples were placed in sealed tubes for eventual return to Earth, Perseverance also abraded the rock next to each sample location, allowing the rover to analyze the mineral content of the rocks.

Mars rovers have found other rocks that were deposited by water, but none this old. These ancient Martian rocks are the oldest sedimentary rocks ever studied, and they likely formed when the Jezero Crater was a habitable lake. Because they’re sedimentary rocks, they could hold ancient organic matter. But that determination will have to wait until they make it safely to labs on Earth.

“These are the oldest rocks that may have been deposited by water, that we’ve ever laid hands or rover arms on,” said 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 preserved fossils, and signatures of life.”

Most sedimentary rock has two components: grains, which are like the building blocks for sedimentary rock, and cement, which are mineral deposits that come along later and bind the grains together. Over time, pressure forces cement into the rock pores, filling them and creating solid rock in a process called lithification. The researchers think that both the grains and the cement in the fan front sedimentary rocks likely formed in aqueous environments. During lithification, organic matter from ancient life could’ve been trapped in the rock.

The fan front is a prime place to search for evidence of ancient life. “We found lots of minerals like carbonates, which are what make reefs on Earth,” Bosak says. “And it’s really an ideal material that can preserve fossils of microbial life.”

Though sulphates form in the presence of water, the water tends to be very salty, which isn’t necessarily great for life. But it could work out for the best because of salt’s preservative effect. If the brine was restricted to the lake bottom, life could’ve persisted in the upper portions of the ancient lake. When lifeforms died, they could’ve sunk to the bottom. In this case, the brine would’ve acted to preserve signs of ancient life.

“However salty it was, if there were any organics present, it’s like pickling something in salt,” Bosak says. “If there was life that fell into the salty layer, it would be very well-preserved.”

It’s fairly well-established that Mars was once warm and wet. The next question is, did life ever exist there? To answer that, we need to find organic matter. But even that can be tricky since some organic matter can be produced geologically without life. The Curiosity Rover found organic carbon in Gale Crater, but scientists showed that UV fractionation is responsible.

Previously, Perseverance also found evidence of organic matter on the floor of Jezero Crater. Subsequent analysis showed that it could be matter that had no connection to life. This is a cautious reminder of the rovers’ limitations. Though they’re powerful, and it’s an amazing feat to have them roam around on another planet studying rocks, they can’t do the same science that’s possible in labs here on Earth.

That’s why the Mars Sample Return is so critical. Only by finally bringing pieces of Mars back to Earth can we fully understand the evidence that Perseverance is collecting.

“On Earth, once we have microscopes with nanometer-scale resolution, and various types of instruments that we cannot staff on one rover, then we can actually attempt to look for life,” Bosak says.