Researchers urge to bring them back to Earth

Over the course of nearly five months in 2022, NASA’s Perseverance rover collected rock samples from Mars that could rewrite the history of water on the Red Planet and even contain evidence of past life on Mars.

But the information they contain cannot be extracted without further analysis on Earth, requiring a new mission to the planet to retrieve the samples and bring them back. Scientists hope to have the samples on Earth by 2033, although NASA’s sample return mission could be delayed.

“These samples are the reason our mission happened,” said paper co-author David Shuster, a professor of earth and planetary sciences at the University of California, Berkeley, and a member of NASA’s science team for the sample collection. “This is exactly what everyone was hoping to accomplish. And we accomplished it. This is what we were aiming for.”

The critical importance of these rocks, mined from river deposits in a dry lake that once filled a crater called Jezero, is detailed in a study to be published August 14 in AGU Advances.

“These are the first and only sedimentary rocks that have been studied and collected on a planet other than Earth,” Shuster said. “Sedimentary rocks are important because they were transported by water, deposited in a standing body of water, and subsequently modified by chemistry involving liquid water on the surface of Mars at some point in the past. The main reason we came to Jezero was to study this type of rock. These are absolutely fantastic samples for the overall mission goals.”

Shuster co-authored the paper with first author Tanja Bosak, a geobiologist at the Massachusetts Institute of Technology (MIT) in Cambridge.

“These rock cores are likely the oldest material ever obtained from any known environment that could have supported life,” Bosak said. “When we bring them back to Earth, they can tell us a lot about when, why, and for how long Mars contained liquid water, and whether some organic, prebiotic, and even biological evolution may have occurred on that planet.”

Importantly, some of the samples contain very fine-grained sediments, which are the type of rock most likely to retain evidence of past microbial life on Mars, if there ever was or is life on the planet.

“Liquid water is a key element in all of this because it’s the key ingredient for biological activity, as far as we know,” said Shuster, a geochemist. “Fine-grained sedimentary rocks on Earth are the most likely to preserve signals of past biological activity, including organic molecules. That’s why these samples are so important.”

On July 25, NASA announced that Perseverance had collected new rock samples from an outcrop called Cheyava Falls that could also hold signs of past life on Mars. The rover’s science instruments detected evidence of organic molecules, while “leopard spot” inclusions in the rocks are similar to features often associated with fossilized microbial life on Earth.

In a statement, Ken Farley, Perseverance project scientist at Caltech, said: “Scientifically, Perseverance has nothing else to offer. To fully understand what really happened in that Martian river valley at Jezero Crater billions of years ago, we would want to bring the Cheyava Falls sample back to Earth, so we could study it with the powerful instruments available in the labs.”

Sediments contain the answers

Shuster noted that Jezero and the fan of sediment left behind by the river that once flowed into it likely formed 3.5 billion years ago. That abundant water is gone now, either trapped underground or lost to space. But Mars was wet at a time when life on Earth, in the form of microbes, was already everywhere.

“At that time, 3.5 billion years ago, life was already present on Earth,” he said. “The basic question is: was there life on Mars at that time as well?

“Anywhere on Earth over the last 3.5 billion years, if you give me the scenario of a river flowing into a crater and transporting materials to a standing body of water, biology would have taken up residence there and left its mark, one way or another,” Shuster added. “And in fine-grained sediment, specifically, we would have a very good chance of recording that biology in laboratory observations that we can make of that material on Earth.”

Shuster and Bosak acknowledge that the organic analysis team on board the rover did not detect any organic molecules in the four samples from the sedimentary fan. Organic molecules are used and produced by the kind of life we ​​are familiar with on Earth, although their presence is not unequivocal proof of life.

“We didn’t clearly see organic compounds in these key samples,” Shuster said. “But just because that instrument didn’t detect organic compounds doesn’t mean they’re not present in these samples. It just means they weren’t in a concentration detectable by the rover’s instrumentation in those particular rocks.”

To date, Perseverance has collected a total of 25 samples, including duplicate and atmospheric samples, plus three “core tubes” that capture potential contaminants around the rover. Eight duplicate rock samples plus one atmospheric sample and one core tube were deposited in the so-called Three Forks cache on the surface of Jezero as a backup in case the rover experiences problems and the onboard samples cannot be retrieved. The other 15 samples, including the Cheyava Falls sample collected on July 21, remain aboard the rover awaiting retrieval.

Shuster was part of a team that analyzed the first eight rock samples collected — two from each site on the crater floor — all of which were igneous rocks likely created when a meteorite struck the surface and carved out the crater. Those results were reported in a 2023 paper, based on analyses from the instruments aboard Perseverance.

The new paper is an analysis of seven more samples, three of them duplicates now found on the surface of Mars, collected between July 7 and November 29, 2022, at the front of the western sediment fan at Jezero. Bosak, Shuster and their colleagues found that the rocks were composed primarily of sandstone and mudstone, all created by fluvial processes.

“Perseverance encountered aqueously deposited sedimentary rocks at the front, top, and margin of the western Jezero Fan and collected a sample set consisting of eight carbonate-bearing sandstones, a sulfate-rich shale, a sulfate-rich sandstone, and a sand-pebble conglomerate,” Bosak said. “Rocks collected at the front of the fan are the oldest, while rocks collected at the top of the fan are likely the youngest rocks produced during aqueous activity and sediment deposition on the western fan.”

While Bosak is most interested in potential biosignatures in the fine-grained sediments, the coarse-grained sediments also hold key information about water on Mars, Shuster said. Although they are less likely to preserve organic matter or potential biological materials, they do contain carbonate materials and detritus carried from upstream by the now-vanished river. They could therefore help determine when water actually flowed on Mars — the main emphasis of Shuster’s own research.

“With laboratory analysis of those detrital minerals, we could make quantitative statements about when the sediments were deposited and the chemical composition of that water. What was the pH (acidity) of that water when those secondary phases precipitated? At what point did that chemical alteration occur?” he said.

“We have this combination of samples in the sample set that will allow us to understand the environmental conditions when liquid water was flowing into the crater. When was that liquid water flowing into the crater? Was it intermittent?”

The answers to these questions depend on the analysis of materials returned to terrestrial laboratories to discover the organic, isotopic, chemical, morphological, geochronological and paleomagnetic information they record, the researchers emphasized.

“One of the most important goals of planetary science is to bring back these samples,” Shuster said.