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 contain evidence of past life on Mars.

However, the information they contain cannot be extracted without more detailed analysis on Earth, which will require a new mission to the planet to retrieve and return the samples. Scientists hope to have the samples on Earth by 2033, although NASA’s sample return mission may be delayed.

“These samples are the reason our mission was carried out,” 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 sampling science team. “This is exactly what everyone wanted to accomplish. And we did it. This is what we were looking for.”

The crucial importance of these rocks, which were taken from river deposits in a dried-up lake that once filled a crater called Jezero, is detailed in a study published August 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 subsequently altered by chemical processes that involved liquid water on the surface of Mars at some point in the past. The only reason we came to Jezero was to study this type of rock. These are absolutely fantastic samples for the overall goals of the mission.”

Shuster is co-author of the article with lead author Tanja Bosak, a geobiologist at the Massachusetts Institute of Technology (MIT) in Cambridge.

“These rock cores are probably the oldest materials we have recovered from a known environment where life was possible,” Bosak said. “If we bring them back to Earth, they can tell us a lot about when, why and how long liquid water existed on Mars, and whether organic, prebiotic and possibly even biological evolution could have occurred on this planet.”

Importantly, some of the samples contain very fine-grained sediments. This type of rock is the most likely to contain traces of microbial life on Mars – if there ever was or is life there.

“Liquid water is a key element in all of this because it is, as far as we understand it, the key ingredient for biological activity,” said Shuster, a geochemist. “Fine-grained sedimentary rocks on Earth are the ones most likely to preserve signatures 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 a rocky outcrop called Cheyava Falls that may also contain signs of past life on Mars. The rover’s science instruments detected evidence of organic molecules, while “leopard spot” inclusions in the rocks resemble features often associated with fossilized microbial life on Earth.

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

Sediments contain the answers

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

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

“If you imagine the scenario of a river flowing into a crater and carrying material to a standing body of water over the last 3.5 billion years, everywhere on Earth, biology has taken hold and left its mark in one way or another,” Shuster added. “And particularly in the fine-grained sediment, 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 on board the rover did not detect any organic molecules in the four samples from the sediment fan. Organic molecules are used and produced by the life forms we know on Earth, although their presence is not definitive evidence of life.

“We were unable to clearly detect organic compounds in these key samples,” Shuster said. “But just because this instrument did not detect organic compounds does not mean that they are not present in these samples. It just means that they were not present in these particular rocks at a concentration that could be detected by the rover instruments.”

So far, Perseverance has collected a total of 25 samples, including duplicates and atmospheric samples, as well as three “witness read tubes” that record possible contaminants around the rover. Eight duplicate rock samples, as well as an atmospheric sample and a witness tube, have been deposited in the Three Forks cache on Jezero’s surface as backups in case the rover has problems and the onboard samples cannot be recovered. The other 15 samples – including the Cheyava Falls sample collected on July 21 – remain onboard the rover awaiting recovery.

Shuster was part of a team that analyzed the first eight rock samples collected, two from each location on the crater floor. They were all igneous rocks, likely formed when a meteorite impact smashed into the surface and gouged the crater. Those results, published in a 2023 paper, are based on analysis from the instruments aboard Perseverance.

The new paper is an analysis of seven other samples, three of which are duplicates, now stored on the Martian surface, collected from the front of the western sedimentary fan in Jezero between July 7 and November 29, 2022. Bosak, Shuster and their colleagues found that the rocks are mostly sandstone and mudstone, all formed by fluvial processes.

“Perseverance encountered water-deposited sedimentary rocks at the front, top and edge of the western Jezero fan and collected a sample collection consisting of eight carbonate-rich sandstones, a sulfate-rich mudstone, a sulfate-rich sandstone and a sand-pebble conglomerate,” Bosak said. “The rocks collected at the front of the fan are the oldest, while the rocks collected at the top of the fan are likely the youngest rocks, formed during water activity and sediment deposition in the western fan.”

While Bosak is primarily interested in possible biosignatures in the fine-grained sediments, Shuster says the coarse-grained sediments also contain important information about water on Mars. Although they probably don’t contain organic matter or potential biological materials, they do contain carbonate materials and detritus that were brought upstream by the now-vanished river. They could therefore help determine when water actually flowed on Mars, which is the main focus of Shuster’s own research.

“Using laboratory analysis of these detrital minerals, we were able to make quantitative statements about when the sediments were deposited and what the chemistry of the water was. What was the pH (acidity) of the water when these secondary phases precipitated? At what point did this chemical change occur?” he said.

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

The answers to these questions depend on analyses of the returned materials in terrestrial laboratories to uncover the organic, isotopic, chemical, morphological, geochronological and paleomagnetic information recorded therein, the researchers emphasized.

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