Scientists maintain a Mars rock that was just analyzed holds the most compelling evidence yet to show that the Red Planet was once littered with and possibly still inhabited by microbial life. The Perseverance rover drilled the core from a speckled mudstone in the river-carved mouth of the Jezero Crater; together, a suite of chemical and textural clues in it look uncannily like fossilized remnants of some of the most ancient microbial metabolisms on Earth, according to a peer-reviewed paper published Thursday in the journal Nature.
What Perseverance Discovered in Jezero
The rock, from a unit informally named Cheyava Falls, was formed in a delta system where an ancient river fanned out into a lake. In such Earthly environments, slow, fine-grained sedimentation, plentiful clays and fluctuating redox conditions provide prime real estate for microbes. Close-up images showed dark, leopard-like spots and nodules — indicators of chemical alteration typically associated with fluid movement and energy gradients in the rock.
Perseverance’s instruments detected a curious mix of ingredients. The SHERLOC instrument identified organic carbon associated with sulfur and phosphorous-containing minerals. PIXL, an X-ray spectrometer, resolved iron-rich phases, including iron sulfides that are consistent with minerals like greigite, which on Earth is associated with the activity of sulfate or iron “breathing” microbes. SuperCam and Mastcam-Z offered complementary context in terms of composition and texture. Of more than two dozen cores cached by the rover, this one — called “Sapphire Canyon” by the team — is unique for the intensity and proximity of these signals.
Why this looks biological — but may not be
Here on Earth, similar iron–sulfur clusters and spotty textures form abundantly whenever microbes harness energy by shuffling electrons between iron and sulfur compounds. Think delta muds along the Gulf Coast, Black Sea sediments, or salt-lake margins: places where organic matter, salts and light circulating fluids meet. In these analogs, greigite and to other associated minerals can precipitate as a direct consequence of microbial metabolism, often retaining the subtle “fossils” of those processes.
What’s critical is this Mars sample is not just organics, but specifically it has organics at the spot where you would expect them to survive […] metabolic processes—near mineral edges and, within a rock, a nodule that records the effects of oxygen.” That spatial choreography is one of the most important biosignature criteria mentioned by the authors of the study, including planetary geochemist Joel Hurowitz of Stony Brook University.
But Mars provides non-biological paths to the same place. Greigite and other sulfides can also arise when warmed fluids react with basaltic rocks, or when burial transforms sediments without biology. Telling biology apart from chemistry requires measurements that Perseverance cannot take in situ, including ultra-precise isotope ratios of carbon and sulfur, the chirality of organic molecules and nanometer-scale textures that might suggest cell walls or chains of magnetite created by bacteria.
Peer-reviewed, not proof
The Nature study is a significant milestone: Other experts have looked closely at the data and methods and concluded that the work was good. Officials at NASA emphasized that the rock does not have living organisms. Instead, it captures what may be the chemical traces of ancient metabolism. In the cautious parlance of astrobiology, the sample registers as a “biosignature candidate,” not a discovery of life.
And this is just the latest candidate to add to a growing tally of Martian hints. Curiosity famously found organics in mudstones at Gale Crater and detected seasons of cyclical changes in the air chemistry — tantalizing but ambiguous signals that have chemical as well as biological explanations. The Jezero core is unique in that it packages several, context-rich signals inside rocks that were laid down in a once-inhabitable lake–delta system.
The sample return stakes
On Mars, the core would be wrapped in a titanium tube. “Scientists need it on Earth to solve the puzzle.” Only on terrestrial labs can you pile on the tests that could be determinative: compound-specific isotope analysis for the “light” carbon of life, sulfur and iron isotope fractionations that crop up with microbial cycling, nanoscale imagery for structures suggestive of cells, high-sensitivity mass spectrometry for complex organics that degrade under radiation.
NASA and the European Space Agency are rethinking the architecture for Mars Sample Return following an independent review that found cost and schedule risks. The options being investigated are designed to simplify the caching process of the tubes round and to ensure that they could be retrieved and launched from the Martian surface, with ESA’s Earth Return Orbiter (ERO) still a main player in the rendezvous.Forming part of an overall program to prepare for the potential arrival of the 2026 Fetch rover delivered by NASA’s Mars Sample Return lander, its hardware may be more pivotal than ever — with the Fetch rover being tasked with collecting and transferring the tubes to the collaboration and retrieval lander. The scientific argument for bringing back Jezero mudstones—now buoyed by this biosignature candidate—provides the program greater focus on which tubes should hitch a ride home first.
Meanwhile, Perseverance will keep mapping the delta’s layers, comparing facies and chasing patterns—do organics and iron sulfides spike along certain ancient channels, or drop off with distance away from the lake? Teams here on Earth are also re-creating the rover’s readings, on Mars-analog sediments, to explore how non-biological pathways potentially could generate similar signals to those we detect in the core.
What would seal the case
For astrobiologists, three lines of evidence generally have to converge: a plausible habitat, clear chemical signatures of metabolism, and textures or structures that could reasonably be attributed to biological activity — and they all have to hang together within the rock’s geological narrative. Perseverance has sent the first and pieces of the second. The rest are probably waiting in Earth labs where a single tube from Jezero could finally give us an answer: whether Mars was ever alive, if only in minuscule, hardy forms.