A six-wheeled geologist named Perseverance may have just found the first extraterrestrial life form. A year ago, the rover came across mineral patterns that resembled the chemical signature of bacteria. Now, after one year of careful analysis, NASA confirmed its findings.

“After a year of review, they have come back and they said, listen, we can’t find another explanation,” Acting NASA Administrator Sean Duffy, said during a news briefing following the announcement. “So, this very well could be the clearest sign of life that we’ve ever found on Mars, which is incredibly exciting.”

Not Just Another Rock

The Perseverance rover has been methodically exploring Jezero Crater since its landing in 2021. The mission can’t directly detect bacteria, but it can look for indirect signs of life, especially ancient life.

Mars was once much warmer, wetter, and more hospitable than the frigid desert it is today, and it may have had conditions suitable for life. Jezero Crater was chosen specifically because billions of years ago, it held a deep, standing lake fed by a winding river delta; precisely the kind of environment where life might have thrived.

After exploring the crater floor and the main river delta, Perseverance journeyed to the crater’s western edge, entering a channel. There, it encountered a distinctive formation of pale, fine-grained mudstones named Bright Angel. And in these rocks, the mission found something extraordinary.

Etched into the mudstone are spotted, leopard-like patterns. Perseverance’s analysis revealed the presence of organic matter, as well as iron, phosphorus, and sulfur. All of these are consistent with Martian microbes munching in the mud.

“We have designed the route for Perseverance to ensure that it goes to areas with the potential for interesting scientific samples,” said Nicola Fox, associate administrator, Science Mission Directorate at NASA Headquarters in Washington in 2024. “This trip paid off as we found something we’ve never seen before, which will give our scientists so much to study.”

Indeed, this gave NASA researchers lots of work. While we can’t say for sure that these are signs of life, it’s exactly what you’d expect to find if bacteria had been there. This is the first time something like this has been discovered outside of the Earth.

“These spots are a big surprise,” said David Flannery, an astrobiologist and member of the Perseverance science team from the Queensland University of Technology in Australia. “On Earth, these types of features in rocks are often associated with the fossilized record of microbes living in the subsurface.”

The Leopard Print Analysis

In several targets, particularly one named “Cheyava Falls,” the rover’s cameras revealed peculiar spots, ranging from less than a millimeter to a full millimeter across. Dubbed “leopard spots” by the science team, they featured light-toned cores surrounded by dark, crenulated rims. Nearby, the rock was peppered with even smaller, dark circular masses the team called “poppy seeds”.

“Cheyava Falls is the most puzzling, complex, and potentially important rock yet investigated by Perseverance,” said Ken Farley, Perseverance project scientist of Caltech in Pasadena. 

The rover drilled a small rock sample, about half the size of an adult finger, and analyzed it with its suite of instruments. It found that these discolorations were actually zones of intense chemical alteration. Using the PIXL instrument, which maps elements with pinpoint accuracy, the team discovered that these spots and nodules were packed with specific minerals that suggest chemical transformation.

The dark rims and “poppy seed” nodules were rich in a type of iron phosphate, likely a mineral called vivianite. The lighter-toned cores of the “leopard spots,” meanwhile, contained an iron sulfide mineral, likely greigite. You’ve probably never heard of those two minerals before, but here’s why they’re so exciting: on Earth, both are frequently the handiwork of microscopic life.

“When the rover entered Bright Angel and started measuring the compositions of the local rocks, the team was immediately struck by how different they were from what we had seen before,” said Tice, a geobiologist and astrobiologist in the Department of Geology and Geophysics. “They showed evidence of chemical cycling that organisms on Earth can take advantage of to produce energy. And when we looked even closer, we saw things that are easy to explain with early Martian life but very difficult to explain with only geological processes.”

“It’s not just the minerals, it’s how they are arranged in these structures that suggests that they formed through the redox cycling of iron and sulfur,” Tice said. “On Earth, things like these sometimes form in sediments where microbes are eating organic matter and ‘breathing’ rust and sulfate. Their presence on Mars raises the question: could similar processes have occurred there?”

Early Life on Mars

The processes that could have formed these minerals are well-known. To survive without oxygen, bacteria evolve to “breathe” other elements. Some of them perform what’s called iron reduction. Such organisms use organic matter for energy and use oxidized iron (rust) from nearby minerals the way we use oxygen. As a byproduct, they release a different type of iron (basically transforming Fe³⁺ into Fe²⁺). This is the process that can create vivianite nodules.

Other microbes are sulfate-reducers. They, too, consume organic matter, but they “breathe” sulfate instead of iron or oxygen and their waste product is sulfide. This is how greigite nodules are formed.

The rocks at Bright Angel contain all the necessary ingredients for this to have happened. They contain organic matter (the food), oxidized iron and sulfate (the “air” for microbes), and water (the environment). The study proposes that after the mud was deposited, a process of “redox reactions” (the same type of chemistry that powers a battery) took place. This process stripped the reddish rust color from the rock in spots, creating bleached halos, and concentrated the newly formed vivianite and greigite into the nodules and reaction fronts we see today.

It fits perfectly. But we’re missing a smoking gun.

Hypotheses and Next Steps

The researchers concede that these features could be created by purely abiotic (non-biological) chemistry. Some organic compounds can react and create similar structures. However, this process would require conditions that are unlikely to have taken place in Jezero Crater. Another option is that sulfide-rich fluids from a volcanic system elsewhere seeped into the rocks, but there’s no nearby evidence for such a system.

The bacteria option fits best with the data we have. But until we bring samples for Earth analysis, or send another rover with different capabilities, we won’t be able to confirm this theory.

“We have zapped that rock with lasers and X-rays and imaged it literally day and night from just about every angle imaginable,” said Farley. “Scientifically, Perseverance has nothing more to give. To fully understand what really happened in that Martian river valley at Jezero Crater billions of years ago, we’d want to bring the Cheyava Falls sample back to Earth, so it can be studied with the powerful instruments available in laboratories.”

For now, the verdict is out, but the evidence is compelling. The spots in the Bright Angel mudstone remain one of the most intriguing clues ever found in our long search for neighbors in the cosmos.

The study www.doi.org/10.1089/ast.2022.0065 in the journal Astrobiology.