Key Takeaways:
- NASA's Perseverance rover discovered a "potential biosignature" within a Martian rock, Cheyava Falls, containing organic molecules and minerals associated with biological processes on Earth. This finding is detailed in a peer-reviewed Nature publication.
- The rock's composition, analyzed using SHERLOC and PIXL instruments, revealed the presence of vivianite and greigite minerals, often byproducts of microbial activity, alongside organic matter exhibiting a strong G-band signal. This suggests a possible ancient redox reaction.
- The rock's age (2-3 billion years old) expands the timeframe for potential Martian habitability, potentially extending beyond previously considered periods.
- While a biological origin is plausible and cannot be ruled out, non-biological explanations remain possible. Further analysis, requiring sample return to Earth for more sophisticated testing, is necessary for definitive conclusions.
NASA announced Wednesday that its Perseverance rover discovered what scientists are calling a “potential biosignature” in a rock on Mars last year. This signature, which may have a biological origin but requires further study, represents the most compelling evidence to date for ancient microbial life on Mars, scientists and agency officials said in a press conference.
The findings, detailed in a peer-reviewed paper published in the journal Nature, center on a unique, leopard-spotted rock named Cheyava Falls, found within the ancient Jezero Crater that Perseverance has been exploring since 2021. The rock contains a cocktail of organic molecules and minerals strongly associated with biological processes on Earth.
The announcement adds a layer of scientific rigor to a discovery first released by NASA last summer. The key development is the comprehensive analysis and peer review of data from Cheyava Falls.
“This very well could be the clearest sign of life that we’ve ever found on Mars, which is incredibly exciting,” said Acting NASA Administrator Sean Duffy at Wednesday’s press conference.
A surprising discovery in an ancient riverbed
The story of this potential biosignature began in July 2024. After years of exploring the floor of Jezero Crater, an ancient lakebed, the Perseverance rover drove into Neretva Vallis, the river valley that once fed the now-dry lake. There, it encountered the peculiar Cheyava Falls rock within the Bright Angel formation — a set of rocky outcrops along the northern and southern edges of the river valley. The rock was immediately striking, adorned with small, dark “poppy seed” flecks and distinctive, white, ringed “leopard spots.”
Initial analysis with the rover quickly confirmed the presence of organic molecules — carbon-based molecules that form the basic building blocks of life. But the true significance lay hidden in the chemistry of the spots themselves, which contained minerals thought to indicate the presence of microbial life billions of years ago.
While NASA shared initial findings last summer, Wednesday’s announcement is significant because the research has now survived the gauntlet of formal peer review. “To the scientific community, [the discovery] is only worth so much without having the independent vetting and scrutiny of the peer-review process,” Melissa Rice, a planetary scientist at Western Washington University and a member of the Perseverance science team, tells Astronomy. “Many people I know in the community were almost dismissive of the presentation of the results last summer because it doesn’t really mean anything … until it’s gone through the rigor of peer review.”
NASA’s Perseverance Mars rover took this selfie on July 23, 2024. To the left of the rover near the center of the image is the arrowhead-shaped rock nicknamed Cheyava Falls, which has features that may bear on the question of whether Mars was home to microscopic life in the distant past. The small dark hole in the rock is where Perseverance took a core sample, which is now in a sample tube stored in the rover’s belly. Credit: NASA/JPL-Caltech/MSSS
Unpacking the chemical clues
The results from the Nature paper led by Joel Hurowitz, a planetary scientist at Stony Brook University in New York, reinforce the findings from last summer.
That reinforcement comes not only from the validation of the peer-review process itself, but also from the new, deeper level of scientific detail revealed in the paper. This includes a step-by-step look at the chemical analysis, a process that began once the rover maneuvered its robotic arm for a close-up look.
The first crucial step was to analyze the rock with the SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instrument. SHERLOC detected a strong G-band signal, which, during Wednesday’s press conference, Hurowitz called a “smoking gun indicator for the presence of organic matter in this mud.”
Rice says it was the strongest organic detection the mission had made yet, a finding that “caused us to pause and really have the discussion about, ‘Is this sample worthy of a sample tube?'” With only a limited number of sample tubes available for the entire multiyear mission, the decision was a critical one. “And absolutely it was,” Rice concludes, “because this is exactly the type of organic-bearing, fine-grain rock that we had come to Jezero Crater to find.” The team elected to use one of those precious tubes to drill a core from the Cheyava Falls rock, collecting the sample now known as Sapphire Canyon.
With organics confirmed and the decision made to sample, the team turned to the PIXL (Planetary Instrument for X-ray Lithochemistry) instrument to map the precise chemical composition of the intriguing spots. The results were striking. PIXL’s data revealed that the dark rims of the “leopard spots” and the smaller “poppy seeds” were rich in an iron-phosphate mineral — most likely vivianite. On Earth, vivianite is a common byproduct of microbial activity in sediments. Even more compellingly, PIXL found that the lighter-colored cores of the spots contained an iron-sulfide mineral identified as likely greigite, another mineral associated with microbes.
These findings allowed scientists to piece together a hypothesis. The paper posits that ancient microbes could have used the organic matter in the mud as an energy source leading to a process known as a redox reaction. In this chemical reaction, electrons are transferred from one substance to another; here, microbes would have taken electrons from the organic matter and transferred them to the iron and sulfate minerals in the mud. This process would have changed the chemical state of the iron and sulfur, precipitating the vivianite and greigite minerals as waste products.
If such a redox reaction were responsible for the mineral spots, scientists would expect to find clear evidence of it in the rock’s appearance. The reaction would consume the rust-colored, oxidized ferric iron that gives Mars its signature hue. Therefore, the areas where the reaction occurred should be visibly less red, or bleached. And that is what Perseverance’s instruments show. The rock is least oxidized in the same leopard spots where the organic matter and the mineral byproducts are located.
A wider window for life?
Adding another layer of intrigue is the rock’s age. Because the mudstones of the Bright Angel formation were deposited inside a river valley that had already been carved, they are geologically younger than much of the terrain Perseverance has explored. At perhaps 2 billion to 3 billion years old, they suggest that habitable environments could have existed on Mars for longer than previously thought. “This has the huge significance of broadening the habitability window on early Mars,” says Rice.
Previously, scientists believed Mars’s most habitable period was in its distant past, around 4 billion years ago, a time when the planet had a thicker atmosphere, more abundant water, and a protective magnetic field. That ancient timeline aligns with the period when the earliest fossil evidence for life first appears on Earth. The younger age of the Cheyava Falls rock suggests this window may have remained open much longer, raising the possibility that niches of habitability, like the ancient lake that once filled Neretva Vallis, could have supported microbial life for a billion years or more after the planet’s surface became largely inhospitable.
Weighing the evidence
While the science team has rigorously tested for nonbiological explanations using all the relevant tools available in Perseverance’s payload, primarily PIXL, they cannot be entirely ruled out with the rover’s current toolkit. For example, the minerals could theoretically form abiotically (without life) under high heat, but the surrounding rock shows no evidence of having been “cooked.” Hurowitz explained, “When we see features like this in sediment on Earth, these minerals are often the byproduct of microbial metabolisms. … But there are nonbiological ways to make these features that we cannot completely rule out on the basis of the data that we collected.”
Duffy’s enthusiastic claim during Wednesday’s press conference that scientists “can’t find another explanation” is tempered by a more nuanced scientific reality. The key conclusion from the year-long analysis is not that nonbiological causes have been disproven, but that a biological explanation has withstood intense scrutiny and cannot be dismissed. “At this point, we can’t rule out biology,” explains Rice. This stands in contrast to previous martian discoveries that were later attributed to known geological processes.
However, independent scientists were quick to point out that plausible nonbiological pathways still exist. In their commentary, Janice Bishop, chemist and planetary scientist at the SETI Institute, and Mario Parente, associate professor at University of Massachusetts Amherst, note that “slower, non-biological reduction of iron from organic compounds or other sources probably led to vivianite formation,” suggesting the features could have formed without life over long geological timescales.
The limits of remote science
To bring structure to such extraordinary claims, scientists use a framework called the Confidence of Life Detection (CoLD) scale. Last summer’s initial announcement likely met the criteria for Level 1: detection of a signal known to result from biological activity. The extensive analysis in the new paper, which places the signal in its environmental context, helps push the finding toward Level 2. But to move any higher on the seven-step scale requires a level of analysis Perseverance cannot perform. “We really can’t move much beyond level two until we get samples back here on Earth and can have multiple instruments with a higher level of sophistication and repeatability in Earth laboratories,” Rice says.
Marked by seven benchmarks, the Confidence of Life Detection, or CoLD, scale outlines a progression in confidence that a set of observations stands as evidence of life. Credit: NASA
Ultimately, the final answer lies locked inside the Sapphire Canyon sample tube. During the press conference, Katie Stack Morgan, research scientist at the Jet Propulsion Laboratory, confirmed that the Perseverance team “basically threw the entire rover science payload at this rock” and are at the limits of what can be determined remotely.
The next step, according to Rice, is to “rule out these abiotic mechanisms,” a task that requires analysis far beyond the rover’s capabilities. “Something that always limits us in our exploration of another planet with a single spacecraft is we have one instrument, and so we’re not able to reproduce the same results with other instrumentation,” she says. Confirming the discovery and ruling out contamination will require “multiple instruments with a higher level of sophistication and repeatability in Earth laboratories.”
As Ken Farley, a Perseverance project scientist, stated when the rock was first discovered: “To fully understand what really happened … we’d want to bring the Cheyava Falls sample back to Earth.” The sentiment that rings even truer today.
The finding therefore adds urgency to the politically fraught Mars Sample Return (MSR) mission to retrieve Perseverance’s cached samples, which NASA is developing jointly with the European Space Agency. After blowing past its budget, falling behind schedule, and undergoing significant revisions, NASA last year hit pause on the mission’s development and solicited proposals from private industry to find a cheaper, faster option.
The administration’s recent budget request called for an end to the current MSR mission, zeroing out its funding. However, during the press conference, Duffy suggested this was not an abandonment of the goal, but a strategic pivot. “We believe there’s a better way to do this, a faster way to get these samples back,” Duffy said, explaining that NASA is analyzing how to retrieve the samples more “cost-effectively”
When asked by a reporter whether the new findings would reshuffle the Trump administration’s priorities for returning the samples, Duffy said that the rover’s findings did not “change the mission,” but were “consistent with the president’s vision and mission of continuing the science to support human exploration beyond Earth.”
“Listen, we are about … human exploration, but as part of human exploration, you need science,” Duffy said. “We study our own atmosphere to learn from what we have here on Earth … what we can learn about other atmospheres potentially in our galaxy and beyond.”
Duffy previously made comments in an Aug. 14 TV appearance appearing to back away from NASA Earth science entirely, saying the agency’s mission was “to explore, not to do all of these earth sciences.”
