“It’s not life itself,” warned Nicky Fox, NASA’s Science Mission Directorate associate administrator. But the new discoveries from the Perseverance rover’s investigation of Jezero Crater have driven the hunt for Martian life into one of its most thrilling stages. In a mudstone core called Sapphire Canyon, retrieved from the Cheyava Falls outcrop in the Bright Angel area of Neretva Vallis, researchers have found patterns of minerals vivianite and greigite whose shape and chemistry closely resemble biosignatures of microbial activity on our planet.
The SHERLOC and PIXL instruments aboard Perseverance, intended to search for fine-scale biosignatures, imaged organic carbon with phosphate, iron, and sulfur in recurring, bullseye-shaped patterns. The rims of Martian “leopard spots” are vivianite-rich, an iron phosphate, and their centers are filled with greigite, an iron sulfide. Terrestrially, vivianite tends to occur in phosphorus-trapping, water-saturated sediments reduced by microbes, and greigite is characteristic of sulfate-reducing bacteria in anoxic muds. The combination in Martian mudstone is striking: concentric fronts of reaction that mirror sequences of electron transfer reactions seen in terrestrial microbial mats.
Geological setting adds to the mystery. Neretva Vallis was once the feeder of Jezero’s ancient lake, leaving behind fine-grained sediment at low-temperature conditions temperatures that would maintain fragile chemical gradients and textures destroyed by high heat. PIXL micro-XRF measurements reveal the mudstone is silica- and aluminosilicate clay-rich, magnesium- and manganese-poor, and undisturbed by high-grade metamorphism. Calcium sulfate veins and nodules dissect the rock, yet there is no hydrothermal overprinting evidence that might complicate the signal.
SHERLOC instrument deep-UV Raman spectroscopy identified a strong G-band at around 1,600 cm⁻¹ in multiple Bright Angel targets, indicative of aromatic organic material. The strongest signals correlate with the most reduced mudstone and maximum inferred concentration of vivianite and greigite. This correlation indicates a connection between organic carbon and redox-controlled iron and sulfur mineralization though whether or not it is biological or abiotic is still to be determined.
NASA’s Confidence of Life Detection (CoLD) framework puts these results at the beginning of the ladder toward an established biosignature. The organics and minerals might result from chemically driven pathways only. Slow sulfate reduction under favorable pH or abiotic reduction of ferric iron through organic molecules can produce analogous phases. But laboratory simulations on our own planet demonstrate that greigite frequently defies the ability to form without microbially catalyzed processes, and vivianite’s presence in authigenic nodules indicates in-situ precipitation from Fe²⁺ and phosphate-rich porewaters microbially controlled conditions in numerous Earth analogs.
The comparisons with terrestrial settings are not conceptual. Acidic, iron- and sulfur-rich waters on our own planet, like those examined in Mars analog research, retain biosignatures in minerals following simulated diagenesis. Laboratory pyrolysis–GC–MS of such analogs discloses that iron phases may protect hopanes, fatty acids, and other molecular fossils from oxidation, particularly if minerals that oxidize, such as jarosite, are removed before analysis. This highlights the need for sample return: only in Earth laboratories can isotopic fractionation, molecular structure, and microtextures be analyzed at the sensitivity required to separate metabolism from mimicry.
Perseverance’s payload was designed with that objective in mind. SHERLOC’s spectroscopic mapping combined with PIXL’s elemental micro-imaging can identify and quantify possible biosignatures in place, directing core selection for caching. The Sapphire Canyon sample is hereby sealed for possible return under the Mars Sample Return campaign, which would allow isotope ratio measurements like δ³⁴S in sulfides or δ⁵⁶Fe in phosphates that on Earth have been definitive for distinguishing biological cycling.
The engineering task of returning that core to Earth is daunting. Mars Sample Return will involve the inaugural launch from another planet, independent rendezvous in Mars orbit, and rigorous planetary protection regulations. But the reward could be significant: deciding whether the vivianite–greigite patterns in Bright Angel mark nothing more than ancient chemistry, or the energy metabolism of life in a Martian river delta billions of years ago.
