Researchers call for their return to Earth

For 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 hold evidence of past life on Mars.

But the information they contain cannot be extracted without more detailed 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 was launched,” said study 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 that collected the samples. “This is exactly what everyone hoped to accomplish. And we accomplished it. This is what we were looking for.”

The critical importance of these rocks, taken from river deposits of a dried-up lake that once filled a crater called Jezero, is detailed in a study to be published Aug. 14 in AGU Progress.

“These are the first and only sedimentary rocks 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 then altered by chemistry that implied the presence of liquid water on the surface of Mars at some point in the past. The main reason we came to Jezero was to study these types of rocks. These are absolutely fantastic samples for the overarching mission objectives.”

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 sampled from a known environment that could have supported life,” Bosak said. “When we bring them back to Earth, they will be able to tell us a lot about when, why, and for how long Mars contained liquid water, and whether organic, prebiotic, and potentially even biological evolution could have occurred on this planet.”

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

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

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

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

Sediments hold the answers

Shuster noted that Jezero and the sediment fan left by the river that once flowed into it likely formed 3.5 billion years ago. That abundant water is now gone, 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 ubiquitous.

“At that time, 3.5 billion years ago, life already existed on Earth,” he explained. “The fundamental question is: Did life also exist on Mars at that time?”

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

Shuster and Bosak acknowledge that the organic analysis equipment aboard the rover did not detect organic molecules in the four samples taken from the sedimentary fan. Organic molecules are used and produced by the type of life we ​​know of on Earth, although their presence is not definitive proof of life.

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

To date, Perseverance has collected a total of 25 samples, including duplicates and atmospheric samples, as well as three “witness tubes” that capture potential contaminants around the rover. Eight duplicate rock samples, along with an atmospheric sample and a witness tube, have been placed in the Three Forks cache on Jezero’s surface as a backup in case the rover encounters problems and the samples on board cannot be retrieved. The remaining 15 samples, including the Cheyava Falls sample collected on July 21, remain on 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 meteor impact struck the surface and carved out the crater. Those results were reported in a 2023 paper, based on analyses from instruments aboard Perseverance.

The new study is an analysis of seven other samples, three of which are duplicates now hidden on the surface of Mars, collected between July 7 and November 29, 2022, from the front of the western sedimentary fan of 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 Jezero’s western fan and collected a sample suite consisting of eight carbonate sandstones, a sulfate-rich mudstone, a sulfate-rich sandstone, and a pebble-sand conglomerate,” Bosak said. “The rocks collected at the front of the fan are the oldest, while those collected at the top of the fan are likely the youngest rocks produced during aqueous activity and sediment deposition in the western fan.”

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

“Through laboratory analysis of these detrital minerals, we could make quantitative statements about when the sediment was deposited and what the chemistry of that water was. What was the pH (acidity) of that water when these secondary phases precipitated? When did this chemical alteration take place?” he said.

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

Answers to these questions rely on analyses of materials returned to Earth-based laboratories to uncover the organic, isotopic, chemical, morphological, geochronological and paleomagnetic information they record, the researchers stressed.

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