Data collected from the Perseverance rover last year has just delivered what might be the best evidence yet for microbial life on Mars.
The best explanation for leopard-spot speckles on a rock named Chevaya Falls (and others like it in the Bright Angel formation) is biological processes, according to exhaustive analysis led by geoscientist and planetary scientist Joel Hurowitz of Stony Brook University in the US.
Of course, we won't know for sure until the sample collected by Perseverance has been brought to Earth for comprehensive study. Non-biological processes are still on the table – but the evidence compiled by Hurowitz and his colleagues is tantalizingly compelling.
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Chevaya Falls was found by Perseverance just over a year ago as it trundled its way across the floor of the Jezero Crater, a vast basin once filled with liquid water. Scientists were immediately rapt: here on Earth, features similar to those seen on the rock are often associated with fossilized microbes.
However, the data provided by Perseverance's suite of science instruments from Chevaya Falls and two other rocks in the Bright Angel formation, Sapphire Canyon and Apollo Temple, required further study.
"The combination of chemical compounds we found in the Bright Angel formation could have been a rich source of energy for microbial metabolisms," Hurowitz says. "But just because we saw all these compelling chemical signatures in the data didn't mean we had a potential biosignature. We needed to analyze what that data could mean."
According to Perseverance's data, the samples it analyzed contained organic, carbon-rich material. There is a lot of organic material on Mars, and there are also a lot of non-biological processes that can produce organic material, meaning the presence of such is not diagnostic in and of itself.
However, when other potential biomarkers are present, organic material becomes more significant. The Bright Angel formation is rich in clay, which indicates the presence of water. That's one box ticked. The rock also contained calcium sulfate separated by seams of an iron-rich mineral called hematite.
Meanwhile, the leopard spots were particularly rich in iron phosphate and iron sulfide, probably the minerals vivianite and greigite. Phosphates are of great biological importance here on Earth, and both minerals could be the product of electrochemical reduction and oxidation, or 'redox', reactions involving organic carbon, either biological or non-biological (abiotic).
"It's not just the minerals, it's how they are arranged in these structures that suggests that they formed through the redox cycling of iron and sulfur," says geobiologist and astrobiologist Michael Tice of Texas A&M University.
"On Earth, things like these sometimes form in sediments where microbes are eating organic matter and 'breathing' rust and sulfate. Their presence on Mars raises the question: could similar processes have occurred there?"
This is where it gets really interesting. The team modeled different processes that can produce the observed mineral composition of the Bright Angel samples. While they were able to identify an abiotic process that reduces sulfate to sulfide to produce a result similar to what is observed in the rocks, that process is extremely slow, and requires either high acidity or temperatures in excess of 150 to 200 degrees Celsius.
Mars is certainly capable of producing acidic conditions and high temperatures through volcanism. However, the Bright Angel rocks show no other signs of being subjected to that level of heat, nor ever being exposed to a low pH.
It's going to be difficult to learn more without studying the rocks themselves. Perseverance's suite of instruments is extremely limited compared to what geologists can accomplish here on Earth, and the researchers are itching to get their hands on the collected samples.
Meanwhile, the team suggests researchers here on Earth conduct further investigations into the various biological and abiotic processes that could give rise to the specific features observed in the Bright Angel formation.
"What's fascinating is how life may have been making use of some of the same processes on Earth and Mars at around the same time," Tice says.
"We see evidence of microorganisms reacting iron and sulfur with organic matter in the same way in rocks of the same age on Earth, but we'd never be able to see exactly the same features that we see on Mars in the old rocks here. Processing by plate tectonics has heated all our rocks too much to preserve them this way. It's a special and spectacular thing to be able to see them like this on another planet."
The research has been published in Nature.