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As the Perseverance rover traversed an ancient river valley in Mars’ Jezero Crater back in July 2024, it drilled into the surface and extracted a sample from of a unique, striped rock called Chevaya Falls. The rover’s instruments then analyzed the sample, which is called Sapphire Canyon, and surveyed the surrounding rock.

When scientists started looking into the data, they found two types of iron-rich minerals arranged on the rock in a distinctive, spotted pattern. Both these minerals are associated with life on Earth. One is found around decomposing organic matter on Earth, while the other is produced by certain microbes.

A team of researchers determined in a study published Sept. 10, 2025, that the sample contains a potential biosignature – which could suggest the red planet once hosted microbial life.

These minerals may have formed on the rock when ancient microbes used chemical reactions to produce energy. But chemical reactions not related to life can also produce these minerals under certain conditions.

To learn more, The Conversation U.S. asked Amy J. Williams, an astrobiologist at the University of Florida, about biosignature hunting on Mars and what’s so special about this Sapphire Canyon sample.

What are biosignatures?

A biosignature is any characteristic, element, molecule, substance or feature that serves as evidence for past or present life. It must be something that cannot be produced without life. Some examples include fossils, organic molecules derived from a biological process, or mineral patterns that form only through microbial activity.

A potential biosignature, which is how the Sapphire Canyon finding is described, is a substance or structure that might have a biological origin but requires more data or further study before scientists can make a conclusion about the absence or presence of life.

How do scientists determine whether something could be a biosignature on Mars?

Biosignatures come in many different flavors – chemical, physical or structural. Some are rather obvious, like a dinosaur fossil on Earth, but most are far more nuanced.

The search for ancient life on Earth partially informs the search for biosignatures on Mars. Researchers rely on subtle clues preserved in the rock record to address questions such as how long ago microbial life arose on Earth. We search for that evidence in environments such as craters and lake beds with high preservation potential, meaning those that are likely to preserve the biosignatures.

Scientists can apply these techniques to the search for life on Mars. That is why Perseverance was sent to Jezero Crater. In the ancient past, the crater hosted a river-fed lake, which on Earth would represent a habitable environment: one where life would want to live if it ever arose.

This crater was an ideal location to search for ancient life preserved in the rock record on Mars. Astrobiologists then search for chemical, textural and mineral patterns that resemble processes influenced by life back on Earth.

What makes this sample unique and interesting?

The Sapphire Canyon sample is unique because Perseverance’s PIXL and SHERLOC instruments revealed distinctive textures that were dubbed “leopard spots.” These spots are concentric reaction fronts – places where chemical and physical reactions occur – enriched in the minerals vivianite, which contains iron phosphate, and greigite, which is made of iron sulfide.

On Earth, vivianite often forms in environments with lots of decaying organic matter, while certain microbes that use sulfate for energy can produce greigite. Compounds in both these minerals are part of a chemical process called redox gradients, which refers to a series of gradual changes over physical space where chemicals can oxidize (lose electrons) or reduce (gain electrons).

One example is leaving your metal bike out in the rain. Over time, the reduced iron (Fe2+) will lose an electron and oxidize to rust (Fe3+). This process can happen nonbiologically, as exposure to water and oxygen drive the chemical changes that take your new bike to a rusty bike – I suggest not leaving it in the rain.

But some oxidation and reduction processes are so slow on their own that the only way they can occur is with living organisms that push the reactions forward. This process is how many microbes, such as bacteria, get the energy to live. Because these two minerals in the Sapphire Canyon sample both occur in redox gradients, scientists predict that microbial life, if it was ever present, could have played a role in the reactions that created these mineral signatures.

Now, scientists are looking into the explanations that wouldn’t require life to form these features on the sample.

Did scientists expect to find a sample like this?

This was a finding that we had hoped for. However, it was somewhat unexpected in this particular location. This sample came from some of the youngest sedimentary rocks the mission has investigated to date. An earlier prediction had assumed signs of ancient life would come from older Martian rock formations.

Finding these features in younger rocks widens the window of time that Mars was potentially habitable and suggests that Mars could have been habitable later in the planet’s history than scientists previously thought, and older rocks might also hold signs of life that are simply harder to detect.

What are the next steps to tell whether the sample indicates signs of past life, or whether the signature is from a nonbiological process?

The mineral associations are a potential fingerprint for those redox reactions that can occur when microbes drive the reaction forward – but abiotic processes, such as sustained high temperatures, acidic conditions and binding by organic compounds, could also explain them.

However, the Cheyava Falls rock shows no signs that it’s been exposed to the high heat or acidity usually required for greigite and vivianite to form nonbiologically. Still, the only definitive way to answer this question is to return the sample to Earth, where scientists can use advanced laboratory techniques to distinguish biological from nonbiological origins.