“Astrobiological claims, particularly those related to the potential discovery of past extraterrestrial life, require extraordinary evidence,” cautioned Katie Stack Morgan, project scientist for NASA’s Perseverance rover. That bar is being tested today by a suite of odd minerals and textures found in Jezero Crater’s Bright Angel formation findings that tie together geology, chemistry, and the tantalizing prospect of biology.
Since its landing in 2021, Perseverance has employed its advanced instrument suite to explore the crater’s mudstones rich in clay, chosen for their capacity to preserve ancient biosignatures. In July 2024, the rover drilled into rock designated Cheyava Falls, one of the Bright Angel outcrops, extracting a core sample now dubbed Sapphire Canyon. This rock came into prominence for its millimeter-scale “poppy seeds” and concentric “leopard spots” that PIXL (Planetary Instrument for X-ray Lithochemistry) and SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) detected enriched in iron, phosphorus, and sulfur.
Detailed studies in *Nature* show that these nodules and spots contain vivianite (Fe₂⁺₃(PO₄)₂·8H₂O) and greigite (Fe₃S₄), minerals that elsewhere on the planet typically occur due to microbially catalyzed redox reactions. Vivianite will precipitate in sediments where microbes reduce ferric iron, while greigite can occur as a byproduct of microbial sulfate reduction. The intimate correlation of the minerals with organic carbon deduced from the deep-ultraviolet Raman spectroscopy of SHERLOC through evidence of a distinctive 1,600 cm⁻¹ G band puts extra weight behind their candidacy as putative biosignatures.
Above all, the mineralogical context suggests low-temperature formation close to the time of deposition of sediments, and not high-temperature abiotic formation. Bright Angel mudstones lack evidence of contact metamorphism or acid alteration, circumstances otherwise present to explain these phases without biology. The textures are instead comparable to terrestrial “reduction halos” and “reduction spots” in ancient sediments, where microbial metabolism bleaches iron oxides and induces secondary minerals to precipitate.
The rover’s multi-instrument approach has been invaluable. Micro-XRF mapping of PIXL mapped elemental distributions in sub-millimeter resolution, distinguishing Fe-phosphate rimmed from sulfide-enriched cores in leopard spots. SHERLOC’s simultaneously collected Raman and fluorescence imaging confirmed organic matter within the same microdomains. SuperCam and Mastcam-Z spectroscopy supplemented hydration and oxidation state determination with an inverse correlation of vivianite/greigite content and ferric iron content of host rock. This. trend is a mimic of microbial iron and sulfur cycling in terrestrial analogs, such as the low-sulfate Antarctic lakes and organic sulfur cycle in Lake Superior, where microbes are forced by climatic limitation of sulfate to use organic sulfur compounds.
The geochemical reactions explained at Bright Angel iron reduction with oxidation of organic material, and possible sulfate reduction to sulfides are some of the Earth’s oldest metabolisms. Terrestrial and Martian environments alike can sequester organic carbon in these reactions and seal it within authigenic minerals, isolating it from decomposition for billions of years.
But the authors of the research are cautious, qualifying their interpretations as “possible biosignatures.” Abiotic processes, such as organic compound–catalyzed reduction of ferric iron or magmatic sulfur input, are not precluded. These sources would have to be separated by laboratory analyses much more advanced than can be transported in rover-borne hardware.
The interest in conducting that research brings attention to NASA’s Mars Sample Return (MSR) mission. Sapphire Canyon and 26 other cached cores represent the most promising Martian samples ever collected for life detection studies. Returned to Earth, they would be accessible to analysis by nanoscale imaging, isotopic fractionation analysis, and compound-specific radiocarbon dating technologies with which it is possible to resolve biological from abiotic carbon and mineral forming temperatures with sub-degree precision.
But MSR is faced with daunting challenges. The mission would comprise the first launch from the surface of another world, orbital rendezvous in Mars orbit, and interplanetary sample return tasks never attempted. Budgetary constraints and shifting priorities have pushed the present return date into 2040, and projections range from $8 to $11 billion. NASA is now calling for industry proposals to push the schedule back into the 2030s, trying to reduce complexity without impairing scientific integrity.
For astrobiologists, the stakes are clearly known. As SETI Institute’s Janice Bishop pointed out, “There is no evidence of microbes or other life forms on Mars yet, but our search is just beginning.” The fine-grained mudstones of the Bright Angel formation, authigenic mineral assemblages, and preserved organics all give us one of the best looks yet at Mars’ past habitability. Whether such “leopard spots” are the fingerprints of ancient Martian microbe life or products of lifeless chemistry, only a return to the laboratories of Earth is likely to reveal.
