While we go about our daily lives on Earth, a nuclear-powered robot the size of a small car is wandering around Mars in search of fossils. NASA’s Perseverance rover, unlike its predecessor Curiosity, is explicitly intended to “search for potential evidence of past life,” according to the mission’s official mission goals.

Crater Lake was chosen as the landing site because it contains the remains of ancient alluvium and other sediments deposited where the river emptied into the lake more than 3 billion years ago. We don’t know if there is life in this lake, but if there is, Perseverance may find evidence of it.

We can imagine Perseverance stumbling upon large, well-preserved fossils of microbial colonies; these fossils resemble cabbage “stromatolites,” perhaps produced by solar-powered bacteria on Earth’s ancient shores. Fossils like this would be large enough to be seen clearly by the vehicle’s cameras, and could also contain chemical evidence of ancient life that the vehicle’s spectroscopic instruments could detect.

But even in such optimistic scenarios, we cannot be completely sure that we have found fossils until we see them under a microscope in laboratories on Earth. This is because geological features formed by non-biological processes can resemble fossils. These are called false fossils. That’s why Perseverance not only searches for fossils in situ, but also collects samples. If all goes well, around 30 samples will be returned to Earth in the next mission planned in collaboration with the European Space Agency (ESA).

Earlier this month, NASA announced that a particularly intriguing sample had joined the growing collection of Perseverance’s 24th rover, unofficially called Comet Geyser. It comes from an outcrop called Bunsen Peak, which is part of a rocky deposit called the Margin Unit located near the edge of the crater.

Perhaps this rock was formed on the shoreline of an ancient lake. The rover’s instruments showed that Bunsen peak is dominated by carbonate minerals (the main component of Earth’s rocks such as limestone, chalk and travertine).

Small carbonate grains are cemented by pure silica (similar to opal or quartz). “This is the type of rock we expected to find when we decided to explore Jezero Crater,” Perseverance project scientist Ken Farley was quoted in NASA’s press release as saying.

So what makes carbonates so special? So what makes the Bunsen Peak model particularly fascinating for astrobiology, the study of life in the universe? First, this rock may have formed under conditions we consider habitable: capable of sustaining the metabolism of life as we know it.

One of the components of life is the presence of water. Carbonate and siliceous minerals can form by direct precipitation of liquid water. Sample 24 may have precipitated from lake water at temperature and chemical conditions compatible with life, but there may be other possibilities that need to be tested. In fact, carbonate minerals are surprisingly rare on Mars, which has always contained abundant CO₂.

In the humid environment of early Mars, this CO₂ would have dissolved in water and reacted to form carbonate minerals. Analyzing the Bunsen peak and sample 24 when sent to Earth may help us finally solve this mystery. There is some interesting rough and striated texture on one face of the outcrop that may explain its origin, but is difficult to interpret without additional data.

Second, we know from terrestrial samples that ancient sedimentary carbonates can produce remarkable fossils. Such fossils include stromatolites, which consist of carbonate crystals precipitated directly by bacteria. Perseverance has not seen convincing examples of this.

There are some concentric circular patterns on the edge unit, but these are almost certainly the effect of weathering. However, even where stromatolites are absent, some ancient carbonates on Earth contain fossil colonies of microbial cells that form ghostly sculptures in which the original cellular structures have been replaced by minerals.

The small grain size of the Comet Geyser sample indicates a higher potential for preserving sensitive fossils. Under certain conditions, fine-grained carbonates may even contain organic matter (modified residues of the oils, pigments, and other compounds that make up living things). Silica cement makes such preservation more likely: Silica is generally harder, more inert, and less permeable than carbonate, and can protect fossil microbes and organic molecules within rocks from chemical and physical changes over billions of years.

When my colleagues and I wrote a scientific paper called “A Field Guide to Finding Fossils on Mars” in preparation for this mission, we strongly recommended sampling fine-grained, silica-cemented rocks for these reasons. Of course, to unlock this sample and discover its secrets, we need to bring it back to Earth.

An independent review recently criticized NASA’s plans to send samples from Mars as too risky, too slow and too expensive. Changing mission architectures are currently being evaluated to address these challenges. Meanwhile, hundreds of brilliant scientists and engineers at NASA’s Jet Propulsion Laboratory in California lost their jobs after the U.S. Congress failed to provide the necessary support, effectively cutting off funding for the return of samples from Mars.

Returning samples from Mars remains NASA’s highest priority in planetary science and is strongly supported by the global planetary science community. Examples from Perseverance may change our perspective on life in the universe. Even if they contain no fossils or biomolecules, they will spur decades of research and give future generations an entirely new perspective on Mars. Let’s hope NASA and the US government can live up to their rover’s name and use it for a long time.