Strange ‘leopard spot’ markings on a Martian rock could finally be the sign we’ve been waiting for that alien microbes once lived on the Red Planet.
A detailed study of the rocks has revealed the intricate patterns are “the clearest sign of life we’ve ever found on Mars,” according to NASA’s acting administrator Sean Duffy.
The rock, thought to be around 3.5 billion years old, was discovered by NASA’s Perseverance rover in July 2024. Ever since, planetary scientists have been interrogating possible explanations for what created the markings.
The scientists explain in a recent Nature paper recording their findings that, while the patterns could have formed geologically, ancient Martian microbes are currently the leading theory.
In the hope of obtaining a more definitive answer, Perseverance has taken and stored a fragment of the rock. If all goes to plan, this sample will be returned to Earth one day, where it can be thoroughly examined for signs of past life.
Leopard spots at Bright Angel
Today, Mars is a dry, dead world – but that hasn’t always been the case. Until around three billion years ago Mars’s surface was covered with flowing rivers and vast lakes of liquid water.
Here on Earth, wherever you find water, you can expect to find life somewhere nearby. To uncover whether this was also true on Mars, NASA rovers have spent the last 20 years hunting for clues to whether the Red Planet was once habitable – or even inhabited.
The Perseverance rover is exploring a region known as Jezero Crater, which was a lake in Mars’s distant past. Similar regions on Earth are havens for microbial life.
While exploring an outcrop of rocks known as the Bright Angel formation, Perseverance spotted a rock with a striking pattern resembling leopard spots.
“We took just about every possible observation we could of the [patterned] rock and the Bright Angel formation as a whole,” study lead Prof Joel Hurowitz from Stony Brook University, in the US, told BBC Science Focus.
Perseverance’s cameras were able to capture the patterns in minute detail, while devices known as spectrometers analysed the minerals they were made of. The rover was even able to map out the outcrop’s structure below the surface using radar.
“Basically, we threw everything but the kitchen sink at these rocks,” said Hurowitz.
The analysis revealed the patterns were created by iron-rich minerals known as vivianite and greigite. On Earth, both minerals are often produced by ‘redox reactions’, where the microbes exchange electrons with their surroundings.
“On Earth, redox reactions like these are often driven by microbes living in sediment, which derive energy from these reactions to power their metabolic activity,” said Hurowitz. The traces left behind by these reactions then create these distinctive patterns in the rocks formed from those sediments.
That doesn’t mean it’s time to start celebrating the discovery of alien life just yet. There are other ways the leopard spots could have been created without the influence of life.
For instance, heat could have provided the energy needed to drive the same reactions between mud and organic matter to produce the new minerals.
The team couldn’t find any sign the rocks had been heated, however. Nor did the conditions look quite right for any other non-life-based method they investigated. But, Hurowitz cautions, “we can’t rule them out completely.”
One of the biggest surprises is how young the rock is. As the rock is just 3.5 billion years old, it means the patterns formed when Mars was already beginning to dry up. This means the planet may have been habitable for a lot longer than planetary scientists previously thought.
Unfortunately, Perseverance has an entire planet to explore, and has now moved on to search for life elsewhere.
“If we were to return to Jezero in the future, there are follow-up questions we might look to address with the rover payload,” says Hurowitz.
“But I suspect that these follow-up measurements would not be able to provide a more definitive answer to the question of whether these features were formed by life.
“Ultimately, the determination of whether life was involved will require analysis in laboratories on Earth.”
Bringing a piece of Mars to Earth
Luckily, Perseverance is the first stage of a Mars sample return mission – it’s not just inspecting rocks on Mars, but preparing to bring them back to Earth.
Before it left Bright Angel, the rover collected a sample from the rock and stored it along with dozens of similar fragments taken during its time on Mars.
NASA’s goal is to mount a follow-up mission alongside the European Space Agency to pick up the samples. They will then return them to Earth where they can be analysed in the world’s best labs.
After 3.5 billion years, however, it won’t be easy to find a ‘smoking gun’ clue that definitively answers whether the patterns were created by life or not. The researchers will instead look for other signs that microbes might have left behind.
“I think a logical first step would be analyses of the isotopic composition of the iron, sulphur and carbon in the various mineral and organic components of the rock,” said Hurowitz.
Isotopes can be thought of as different flavours of the same element. Microbes hold on to some isotopes more than others, so measuring how much of one there is compared to another can point towards the presence of life, or at least narrow down the list of alternative options.
“These variations in isotopic composition are key tools in the investigation of biological signals in ancient rocks on Earth, and I suspect that we’d want to apply similar tools to this Martian sample,” said Hurowitz.
The plan was to schedule the return mission for the 2030s, but it risks being cancelled after the Trump administration cut NASA’s planetary exploration budget.
“NASA is looking into how we’re getting this sample back or other samples back,“ a NASA spokesperson told BBC Science Focus. “We’ve been exploring Mars for 60 years and that is going to continue. We are going to look at our budgets and at our timing for how we can get samples back quickly and inexpensively. We will share any updates as we get them.”
“I’m hopeful that these results provide added motivation for a sample return mission,” Hurowitz added. “That will allow us to probe this sample in all of the detail that is needed to understand whether it contains a record of life on Mars.”
“If it is life, that means our planet isn’t the only one where life evolved," Hurowitz concludes. "If it happened twice, where else did it happen?”
About our expert
Joel Hurowitz is an Associate Professor at the Department of Geosciences at Stony Brook University, in New York in the US. He studies the early history of Mars using both measurements taken at the planet and field studies at similar terrains on Earth.
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