The leopard spots of Cheyava Falls

In July 2024, NASA’s Perseverance rover drilled a core from an arrowhead-shaped rock the team had nicknamed Cheyava Falls. The rock sits in Neretva Vallis, a channel carved by water that once flowed into Jezero crater, which held a lake more than three billion years ago.

What caught the scientists’ attention were dozens of millimeter-sized pale spots, each ringed with a dark rim. The rover’s instruments found that the rims are rich in iron and phosphate, and in iron sulfide. The rock, a fine-grained mudstone of the kind that preserves fine detail on Earth, also contains organic carbon compounds. In September 2025, after a year of analysis and peer review, the team published its findings in Nature, and NASA described them as a potential biosignature.

Why the spots matter

To see why the pattern is interesting, it helps to think of it as the record of a reaction. Sediments often contain oxidized iron and sulfate, which can accept electrons, and organic carbon, which can give them up. When those ingredients meet, electrons can flow, and energy is released. Microbes on Earth make their living from exactly this kind of exchange. The spots look like the frozen fronts of such reactions, with the minerals left behind, likely vivianite, an iron phosphate, and greigite, an iron sulfide, marking where iron and sulfur were reduced.

The association is what makes the case. The rover’s organic-detecting instrument registered its strongest signal at the spot where the two minerals were most abundant, and the paper argues that the organic carbon took part in the reactions that formed them. In other words, carbon, iron, sulfur and phosphorus were not just lying in the same rock. They appear to have participated in one chemical process.

But the same reactions can also happen without life, especially at higher temperatures. The case for biology rests heavily on geological context indicating that these reactions occurred at low temperatures, where the non-biological routes to these minerals would be slow. That evidence is suggestive. It is not proof, and a rock formed billions of years ago on another planet leaves a lot of room for processes nobody has yet thought to test.

A potential biosignature is a question, not an answer.

A scale for extraordinary claims

Astrobiologists have spent years trying to avoid the cycle of announcement and retraction that followed earlier claims, such as the 1996 report of possible microfossils in the Martian meteorite ALH84001. In 2021, a group of NASA scientists proposed a confidence scale for life detection, with seven levels running from an initial signal to independent confirmation.

No official score has been assigned here, so what follows is our own reading. The first steps look reasonably satisfied: a signal has been detected, and contamination by the rover is very unlikely. The next step, showing that the signal can be produced by biology in analogous settings, has support from Earth. The step after that, ruling out every known non-biological way of making it, is where things stop. That step is close to impossible with the instruments that can fit on a rover, which is exactly why the rock’s location matters so much.

What a laboratory could do

The core from Cheyava Falls, named Sapphire Canyon, is sealed in a tube aboard Perseverance. In a laboratory on Earth, scientists could do things no rover can. They could measure sulfur isotopes in the greigite, since microbes that reduce sulfate tend to favor the lighter isotope and can leave a strong and characteristic imbalance. They could image the mineral rims grain by grain, look for structures on the scale of cells, identify the organic molecules individually, and date the rock’s alteration directly.

Different instruments, in different laboratories, run by people who disagree with one another: that is what would turn a suggestive pattern into a result. It is also the standard Cheyava Falls has not yet been held to.

A ride that has been canceled

Here the story turns from geology to budgets. NASA’s Mars Sample Return program, conceived to bring Perseverance’s tubes to Earth, grew in projected cost until it became politically untenable. The spending bill for fiscal year 2026 provided no money for the existing program, leaving $110 million for related technologies under a new account for future Mars missions. It has not been formally canceled, but many experts regard it as finished in its current form.

Meanwhile China’s Tianwen-3 mission aims to collect its own samples from Mars and return them around 2031. That is a major achievement in prospect, and it is also a different thing: a different site, chosen on different criteria, without the context of Perseverance’s years of mapping. The rock that raised the question would remain where it is.

There is an uncomfortable asymmetry in this. A potential biosignature is a question, not an answer. Humanity has now asked one of the largest questions available, collected the material needed to answer it, and sealed it in a titanium tube on another planet. For the moment, the evidence is waiting for a ride, and there is none on the schedule. Similar patience is being demanded at Europa and in the atmospheres of distant planets, but only on Mars has the sample already been taken.

Sources

• Hurowitz et al., “Redox-driven mineral and organic associations in Jezero Crater, Mars”, Nature, 2025
• Green et al., “Call for a framework for reporting evidence for life beyond Earth”, Nature, 2021
• NASA: Mars 2020 Perseverance mission
• Universe Today: on the end of Mars Sample Return funding