Ancient rocks in Mars' Jezero crater confirm habitability

According to NASA's Perseverance rover, the ancient rocks in Jezero Crater were 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 these samples return to Earth, if they ever do, scientists hope to determine if they contain evidence of ancient Martian life.

In 2022, the Perseverance rover drove along the western slope of Jezero Crater and sampled rocks from a formation called the “fan front.” Scientists hypothesized that some of the rocks in this region formed on the ancient lake bed when the crater filled with water. Perseverance analyzed the chemical composition of the rocks and captured images of their surroundings. Members of the Perseverance science team studied this data and published their results.

His work is entitled “Astrobiological potential of rocks acquired by the Perseverance rover from a sedimentary fan front in Jezero Crater, Mars.It is published in the journal AGU Advances, and the lead author is Tanja Bosak, professor of geobiology in the Department of Earth, Atmospheric, and Planetary Sciences (EAPS) at MIT.

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

Perseverance collected seven samples from the front of the fan. Each sample is a sedimentary rock, and some of them may predate life on Earth. “The samples include a shale and sandstone containing sulfates and clays, a fluvial sandstone from a stratigraphically low position at the front of the fan, and a carbonate-bearing sandstone deposited over the sulfate-bearing strata,” the authors explain.

Sulfates and clays typically form in the presence of water, as do carbonates. Depending on the type of sulfates, they reveal clues about the chemical composition, temperature, and acidity of ancient water. Carbonates are similar and can also reveal information about the atmosphere of Mars when it formed, such as how much carbon dioxide it contained.

“Hydrated sulfate-bearing shale has the greatest potential for preserving organic matter and biosignatures, while 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 weathered rock next to each sample location, allowing the rover to analyze the rocks' mineral content.

Mars explorers 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 likely formed when Jezero Crater was a habitable lake. Because they are sedimentary rocks, they could contain ancient organic matter. But that determination will have to wait until they arrive safely at laboratories on Earth.

“These are the oldest rocks that could have been deposited by water, that we’ve ever laid our hands on or the arms of an explorer,” said co-author Benjamin Weiss, the Robert R. Shrock Professor of Earth and Planetary Sciences at MIT. “It’s exciting, because it means these are the most promising rocks that may have preserved fossils and signs of life.”

Most sedimentary rocks have two components: grains, which are like the building blocks of sedimentary rocks, and cement, which are mineral deposits that appear later and bind the grains together. Over time, pressure forces cement into the pores of the rock, filling them and creating solid rock in a process called lithification. Researchers believe that both the grains and cement in fan-front sedimentary rocks likely formed in watery environments. During lithification, organic matter from ancient life could have been trapped in the rock.

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

Although sulfates form in the presence of water, the water tends to be very salty, which is not necessarily good for life. But it might work better because of the preservative effect of salt. If the brine was confined to the bottom of the lake, life might have persisted in the upper parts of the ancient lake. When life forms died, they might have sunk to the bottom. In this case, the brine would have acted to preserve signs of ancient life.

“As salty as it was, if there was any organic component present, it was like pickling something in salt,” Bosak says. “If there was life that fell into the salty layer, it would have been very well preserved.”

It's pretty 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 have shown that UV fractionation is responsible.

Previously, Perseverance also found evidence of organic matter on the floor of Jezero Crater. Further analysis showed that it could be matter that had no connection to life. This is a cautious reminder of the limitations of rovers. While they are powerful and it is an amazing feat to have them roaming another planet studying rocks, they cannot do the same science that is possible in laboratories here on Earth.

That's why sample return from Mars is so important. Only if we eventually bring pieces of Mars back to Earth will we be able to fully understand the evidence Perseverance is collecting.

“On Earth, once we have microscopes with nanoscale resolution and multiple types of instruments that we can’t put on a single rover, then we can really try to look for life,” Bosak says.