Unusual rocks discovered on Mars contain the clearest evidence yet of potential past life on the Red Planet. A layer of mudstone found in an ancient river valley by NASA's Perseverance rover was found to be dotted with intriguing markings, which scientists have nicknamed "leopard spots" and "poppy seeds". They think the spots may the result of chemical reactions that were carried out, billions of years ago, by micro-organisms living on the river bed. While the scientists stress that the evidence is not definitive proof of past life, they say the spots are far easier to explain by the presence of living things than if geological processes were the cause. For now, the spots are described as "potential biosignatures"—requiring further investigation to determine whether they are biological or abiotic (non-biological) in origin.

​​​​​​​18th September 2025

The Perseverance rover, which touched down on the Martian surface in 2021, was sent to search for signs of past life on Mars. It has spent the last four years exploring a region called the Jezero Crater, once an ancient lake with a river flowing into it. Perseverance has been inching its way across the landscape covering some 300 metres a day, collecting and scanning rock samples. It has scientific instruments on board capable of carrying out a basic chemical analysis of what it finds.

In July 2024, Perseverance explored a series of rocky outcrops that scientists have named the Bright Angel formation. These rocks are found on either side of the Neretva Vallis, a river valley measuring 400 metres (a quarter of a mile) wide that was carved by water flowing into Jezero Crater billions of years ago. The Bright Angel formation is composed of layers of sedimentary rocks that are about 3.5 billion years old. They include mudstone, a fine-grained rock made of silt and clay that was originally deposited by water. Perseverance took a sample from a rock scientists named Cheyava Falls on the northern edge of the valley.

Spots and dots

The rock sample was found to have pale dots ringed with black outlines that look like leopard spots. There are also small black dots resembling poppy seeds. Having examined the results of analysis of these features by Perseverance's instruments, scientists say they show all the signs of having been created by micro-organisms. Could this be proof at last that life once existed on Mars?

In a study, published in September 2025, the surface spots are carefully described as "potential biosignatures." A biosignature is a leftover sign of life rather than life itself. A potential biosignature is a substance or structure that might have a biological origin—that is, produced by living things—but which requires further study before a non-biological origin (such as a natural geological process) can be firmly ruled out.

On Earth, vivianite and greigite are found in sediments around decaying organic matter in wertland environments such as peat bogs. Certain micro-organisms present here take in nutrients, such as organic carbon, sulphur and phosphorus, as part of their metabolism. As a result of chemical reactions, other chemicals such as phosphates and sulphides are produced as waste. These eventually form vivianite and greigite.

The discovery of these minerals in Martian rocks raises the question: did similar processes take place in wetlands on Mars billions of years ago? Or was some unknown, non-biological chemistry responsible for producing the minerals?

This leaves the strong possibility that the minerals were indeed made by micro-organisms, such as bacteria, living in the mud at the bottom of a Martian lake more than three billion years ago. Nevertheless, further investigation to determine whether they are definitely biological in origin is required before this can be proved.

Bringing the rock sample back to Earth would allow scientists to analyse it with instruments far beyond the capabilities of those on board Perseverance. They would then be able to determine conclusively whether high-temperature geological processes could, after all, explain the potential biosignatures, or confirm that the minerals were made by micro-organisms and that life once existed on Mars.

The sample Perseverance collected from the Bright Angel formation is now stored in a sealed tube. It is one of approximately 30 samples that the rover will gather over the course of its mission. They are being deposited at various locations on the surface of Mars awaiting a potential future mission that could return them to Earth.
​​​​​​​