“Astrobiological claims, particularly those related to the potential discovery of past extraterrestrial life, require extraordinary evidence,” declared Katie Stack Morgan, project scientist for NASA’s Perseverance rover. That guardedness shaped the declaration that a Martian core rock, known as Sapphire Canyon, could have the strongest possible biosignature yet discovered on the Red Planet.
Harvested in July 2024 from an arrowhead-shaped outcrop named Cheyava Falls in the Bright Angel formation of Neretva Vallis, the sample is pockmarked by millimeter-sized “poppy seeds” and “leopard spots.” High-resolution imaging and in situ spectroscopy showed these to be iron-phosphate and iron-sulfide minerals presumably vivianite and greigite distributed in characteristic reaction fronts. On Earth, such minerals tend to form at low temperatures in water-saturated sediments as products of microbial metabolisms that burn organic matter and “breathe” rust or sulfate.
Perseverance’s PIXL (Planetary Instrument for X-ray Lithochemistry) imaged the elemental make-up of the spots, detecting rims of iron, phosphorus, and zinc, and cores of sulfur, nickel, and copper just like greigite. SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) observed a Raman G-band signature of organic carbon colocated with these minerals. The local co-occurrence of organics with redox-sensitive phases is remarkable: on Earth, such combinations can suggest biological mediation of electron-transfer reactions.
The geological setting adds to the fascination. Neretva Vallis previously channeled water into Jezero Crater’s ancient lake. Sedimentology mapping by mission scientists, including researchers from Imperial College London, indicates that sections of the valley floor previously supported a tranquil, low-energy lake setting exactly the type of environment that would be capable of supporting microbial life. At Bright Angel, the finely grained silica-rich and clay-rich mudstones were deposited within this water-rich environment and then subsequently altered by fluids that deposited calcium sulfate veins and allowed for the development of the nodules and spots.
Vivianite occurs on Earth in freshwater or marine sediments in which microbial iron reduction releases Fe²⁺ and phosphate into pore waters. Greigite can form through microbial sulfate reduction, usually in close proximal association with vivianite. The Bright Angel minerals also exhibit zinc enrichment, a characteristic favorable to repeated cycles of sulfidation and oxidation in microbially driven diagenetic systems. However, as lead author Joel Hurowitz of Stony Brook University noted, “there are nonbiological ways to make these features that we cannot completely rule out,” such as abiotic redox reactions between iron and organic matter at low temperatures.
The null hypothesis geochemical origin is in trouble. Most of the abiotic paths involve high temperatures or acidic environments not present in rocks of Bright Angel. Sulfate reduction in the absence of biology is kinetically slow below 150°C, and burial or heating to these temperatures is not evidenced. However, Martian organic material may be abiotic in origin, brought by meteorites or in situ produced, and could fuel some of the reactions observed without the presence of life.
The preservation of textures is partly due to Mars’ geologically stable history. Plate tectonics and metamorphism on Earth destroy such fine-scale textures in rocks of similar age. Here, 3.5-billion-year-old reaction fronts are sharp, providing a glimpse into the chemistry of ancient sediments that is exceptionally unusual.
To verify whether or not these are genuine biosignatures, however, will take lab methods far beyond the rover’s capabilities high-resolution isotopic analysis, nanoscale mineralogy, and directed microfossil searches. That means bringing the sample home. Perseverance has amassed 30 cores, including Sapphire Canyon, but the NASA Mars Sample Return (MSR) program is confronted with daunting challenges. The current design, which consists of several spacecraft and a Mars Ascent Vehicle, has been estimated to cost as much as $11 billion and may slip into the 2040s if not redesigned. Budget considerations have already forced the agency to seek industry proposals for quicker, less expensive solutions based on tried hardware.
Acting NASA Administrator Sean Duffy called Sapphire Canyon “the closest we’ve actually come to discovering ancient life on Mars,” but stressed the need for “Gold Standard Science.” Until the cores are in terrestrial labs, the leopard spots in a Martian mudstone will remain an enigma either the silent work of ancient microbes or the intricate artistry of chemistry alone.
