Leopard spots in a Martian mudstone
Perseverance drilled a rock ringed with vivianite and greigite. On Earth microbes make those minerals; on Mars nobody has yet found a plain chemical route that fits.
In July 2024 Perseverance was working the Bright Angel outcrops on Neretva Vallis, the 400-metre-wide channel that once poured water into Jezero crater. An arrowhead-shaped slab a metre by sixty centimetres carried markings new to the team: millimetre-scale dark rings with pale centres — leopard spots — and a scatter of smaller black specks, poppy seeds. The rock was named Cheyava Falls, after an intermittent Grand Canyon waterfall.
PIXL, which fires X-rays and maps elements point by point, found the rings rich in iron and phosphorus; SHERLOC found organic carbon in the surrounding mudstone. An analysis led by Joel Hurowitz of Stony Brook University, in Nature on 10 September 2025, reported minerals consistent with vivianite, a hydrated iron phosphate, and greigite, an iron sulfide.
On Earth vivianite turns up in waterlogged sediment, peat bogs and lake beds, and greigite comes routinely from bacteria feeding electrons from organic matter to iron and sulfate. They resemble what forms when a microbial community sets up a chemical gradient in wet mud and precipitates minerals around it. Both can form without life, but the known abiotic routes require sustained high temperature or strong acid, absent here, or catalysis by organic compounds, untested at low temperature.
NASA classifies the find as a "potential biosignature": a substance or structure that might have a biological origin, but needs more data before anyone can say whether life was involved. It is not a fossil: no cells, no organism-like shapes. Hurowitz notes that the laboratory catalogue of abiotic mineral chemistry is thin: the absence of a demonstrated non-biological pathway is partly a statement about what is known.
Settling it needs laboratory instruments that will never fly: mass spectrometers measuring sulfur and carbon isotope ratios to a precision no rover can reach, and electron microscopes resolving mineral textures at the nanometre scale. Perseverance drilled a core and sealed it in a titanium tube: sample 25, Sapphire Canyon, sitting inside a rover on Mars with no funded mission coming for it.