NASA’s Perseverance rover discovers rocks that may contain extraterrestrial microfossils

In a dry river valley on Mars, NASA’s Perseverance rover may have finally found its purpose: evidence of ancient extraterrestrial life and a vital link in the space agency’s ambitious but difficult plan to bring Martian material back to Earth.

This potentially space-shaking piece of evidence may look just like a plain rock, but it’s unlike anything seen on Mars so far. Named “Cheyava Falls” after a rock in Earth’s Grand Canyon, the rock looks like a coffee-table-sized, arrowhead-shaped outcrop of mudstone. But its most notable visual feature is the reddish and whitish stripes, the former studded with light-colored spots with dark edges that resemble leopard spots. According to the Perseverance team, the red color is likely due to the iron mineral hematite. The rover’s studies revealed that the whitish stripes are veins of calcium sulfate deposited in the water, while the dark edges of the strange “leopard spots” contain molecules of iron phosphate, which could be a potential food source for hungry subsurface microbes.

Perseverance’s instruments also show that the rocks contain organic compounds, carbon-based molecules that are the building blocks of life as we know it — an unusual discovery for the rover, which has been exploring Mars’ Jezero Crater and surrounding areas since landing in February 2021. (Perseverance’s predecessor, the Curiosity rover, also found organics while exploring a different area, Gale Crater.)

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Taken together, the Perseverance data reveal not only that water long ago seeped into Cheyaba Falls, but that the mudstone once held other conditions typically associated with microbial life. In sedimentary rocks on Earth that contain hematite, chemical reactions can produce similar pale ring-like spots. These reactions release energy that can support single-celled life within the rocks.

This was more than enough for the Perseverance team to instruct the rover to excavate some of the rock and store it in one of its sample tubes to be retrieved and transported to Earth on a future mission. The retrieval mission, called Mars Sample Return (MSR), is a collaboration between NASA and the European Space Agency (ESA). It is managed by NASA’s Jet Propulsion Laboratory (JPL), which also built and operates Perseverance. But significant schedule delays and billions of dollars of budget overruns have plunged the project into uncertainty. Now, MSR is being replanned to find faster, cheaper ways to bring Perseverance’s precious samples back to Earth.

“Cheyava Falls are the most puzzling, complex, and potentially important rocks Perseverance has ever explored,” Ken Farley, Perseverance project scientist at the California Institute of Technology, said in a July 25 NASA statement announcing the discovery. “On the one hand, we have the first strong detection of organic material, distinctive colorful specks of chemical reactions that microbes could use as an energy source, and clear evidence that water necessary for life once flowed through the rock. On the other hand, we are unable to pinpoint exactly how the rocks formed, or to what extent nearby rocks may have heated Cheyava Falls and contributed to these features.”

How did the rock get its spots?

The Perseverance team noticed and targeted the rock in late June as the rover was slowly making its way along the northern slope of the Neretva Valley, a 500-meter-wide trough carved out hundreds of millions of years ago by a river flowing into Jezero Crater, which was once the site of a large lake and delta. As Perseverance’s initial observations of the rock slowly traveled back to Earth via radio waves, the increasingly astonished team began a frantic, around-the-clock effort to collect as much data and crucial samples as possible before the rover could move on to its other planned targets.

Cheyaba Falls may have originally formed from hardened mud in the riverbed, with the prominent calcium sulfate veins deposited later by mineral-rich liquids seeping through cracks in the rock. But this simple scenario is complicated by something else Perseverance glimpsed in the rock veins: tiny crystals of olivine, a greenish mineral that forms from magma. The rover’s scientists say the mineral’s presence in the veins could mean that both olivine and sulfate were carried into the rock by harsh, hot water currents, forming the leopard spots through a completely abiotic, high-temperature chemical reaction.

“Rocks above and north of Cheyaba Falls have been identified as ‘marginal’ and are very distinct and rich in olivine,” said Katie Stack Morgan, Perseverance deputy project scientist at JPL. “The olivine seen in the Cheyaba Falls veins may be related to the emplacement of nearby olivine-rich marginal units.”

Stack Morgan said previous observations by Perseverance had revealed that the exposed rock beneath the slopes of Cheyaba Falls has a very similar composition, including organic matter and tiny chunks of dark minerals, but other rocks studied so far have not shown the distinctive leopard-print spots.

Despite their murky origins, there’s no denying that the Cheyaba Falls spots bear an uncanny resemblance to structures that, when found in deep Earth rocks, are usually considered signs of life, so-called biosignatures. Technically called “reduced spherules,” these features usually form from organic matter buried in sedimentary rocks. Sedimentary rocks are rich in iron oxide, or rusty iron, which is why the rocks are reddish in color. Running water drives a “redox” reaction between the organic matter and the surrounding rocks, reducing (stealing electrons from) the iron oxide and bleaching it white. Microorganisms within the rocks can use the flow of nutrient electrons to fuel their metabolism, fueling or piggybacking on this process. The end result is a pale sphere of reduced iron mixed with other trace metals and possibly microbial microfossils, a kind of oasis buried deep inside.

“The key thing is that these (reduced spherules) are likely the only biosignature of subsurface microbial life that we can see with the naked eye, or even with the rover’s cameras,” says David Flannery, an astrobiologist at Queensland University of Technology in Australia and a member of the Perseverance science team. But because these structures can also result from purely abiotic processes, they’re not solid evidence of past life, especially when they’re being remotely probed by alien robots millions of kilometers away. Even on Earth, Flannery says, “there’s surprisingly little work on (reduced spherules) because they’re so difficult to understand.”

Take out

While Cheyaba Falls isn’t anywhere close to being evidence of Martian life, it’s the next best thing for scientists eager to study it further, and it’s the most promising rock found yet for finding Martian fossils.

“It’s certainly exciting,” said Caleb Scharf, a senior astrobiology scientist at NASA’s Ames Research Center, who was not on the Perseverance team. “The discovery of Cheyaba Falls is one of the more compelling arguments for returning samples from Mars and for sending new instruments and devices that expand the way we examine Martian evidence.”

“We bombarded the rock with lasers and X-rays, taking images from nearly every angle imaginable, literally day and night,” Farley said in a recent NASA statement. “Scientifically, there is nothing more Perseverance can provide. To fully understand what really happened in the Martian river valley in Jezero Crater billions of years ago, we need to bring samples of Cheyaba Falls back to Earth and study them with the powerful instruments available in our laboratories.”

Stack Morgan says these powerful instruments include things like electron microscopes and mass spectrometers, which are large and sensitive and not suitable for placement on an interplanetary robot. They will scrutinize the samples for various biological signatures, pinpointing details like the age of the leopard spots’ dark edges, when the rocks reacted with water, and even the temperature at which some of the minerals formed. Perhaps most importantly, while Perseverance can’t yet tell us anything about the nature of the organic compounds in Cheyaba Falls, instruments on Earth can reveal the types and complexity of molecules in the material and determine whether it’s biological in origin.

But first, NASA and ESA must figure out how the MSR can get it back to Earth.