żěè¶ĚĘÓƵs are narrowing down explanations for a potential chemical sign of ancient life on Mars. This sign – marks found on Martian rocks last year – could have been caused by microbes or by a reaction that takes place only at high temperatures. New findings suggest the biological explanation is more likely.
Last year, while exploring an ancient lakebed called Bright Angel in Mars’s Jezero crater, NASA’s Perseverance rover discovered a rock with unusual markings – called “leopard spots” and “poppy seeds” – similar to patterns associated with microbes on Earth. The leopard spots, which are millimetre-sized dark blots with a circular rim, and the poppy seeds, which are smaller dark blotches, were sandwiched between white reams of calcium sulphate, a mineral that typically forms in the presence of water.
On Earth, similar marks are typically associated with the fossilised activity of microbes. That’s because the chemical reactions that produce them also generate energy, and these processes, called reduction and oxidisation (redox) reactions, are an essential fuel source for microbes. Such redox reactions often leave behind telltale chemical signs, such as iron and sulphur in “reduced form”, meaning they have gained electrons.
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Now, at Stony Brook University and his colleagues have used Perserverance’s onboard instruments to work out the chemical composition of the spots on Mars, finding these marks do contain minerals in a reduced form. The poppy seeds showed iron phosphate, with reduced iron, and the leopard spots showed reduced iron, as well as iron sulfide with sulphur in a reduced form. The rock around the spots also contained iron in an oxidised form, suggesting that the redox reactions had indeed taken place.
There are only two ways that we know of to produce minerals with reduced sulphur on Earth, said at Texas A&M University, who with Hurowitz presented their work at the Lunar and Planetary Science Conference in The Woodlands, Texas, on 12 March. The first, which can happen without the presence of microbial life, happens over thousands to millions of years and typically needs temperatures to rise above 120°C. The second, a different chemical reaction related to microbes, can take place at lower temperatures.
If the Martian rocks had been subjected to extremely high temperatures, then they should have shown relatively large crystals that would have formed from melting and resolidifying. But Hurowitz and his colleagues couldn’t see evidence of this, suggesting that the low-temperature scenario was more likely. “As far as we can tell… everything seems to be consistent with low-temperature processes,” said Hurowitz.
“The only mechanism that we know of now is biologically mediated,” said Tice. However, Tice adds that, when the biological reaction takes place on Earth, we normally see additional chemical features, such as large crystals of dolomite, a mineral made from calcium magnesium carbonate. These features seem to be missing from the Martian sample.
Thus, while the evidence hints towards a biological origin, Hurowitz said that we will need to properly analyse a sample of the rocks in the lab. Perseverance collected such samples, but researchers won’t have access to them until they are returned to Earth, which is due to happen in the 2030s.
Though the evidence still has a lot of uncertainty, the way these minerals are interacting chemically is unlike anything else we’ve found on Mars, says at the SETI Institute. “I don’t know if we could say there’s biology acting, but I think we could say that chemistry needed to form amino acids and early molecules necessary for life seem to have been interacting with the minerals, and that’s really cool,” says Bishop.
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Article amended on 21 March 2025
We have clarified how the samples might be returned to Earth.
Article amended on 28 March 2025
We corrected the definition of redox reactions.
