Possible signs of ancient life on Mars are rich in complex carbon

NASA’s Perseverance rover has found complex carbon compounds in a Martian crater that had previously shown tantalising possible signs of ancient life. On Earth, these compounds are typically associated with dead organisms, but scientists say it is too early to draw the same conclusion here as these compounds are also found in lifeless environments, like on meteorites.

In 2024, Perseverance entered a rocky outcrop, called Bright Angel, near what appeared to be an ancient riverbed that once fed a lake inside Jezero crater. Several rocks photographed by the rover displayed unusual spotted patterns, which NASA scientists called “leopard spots” and “poppy seeds”. These markings, which are largely or entirely formed of dark, circular blots of up to a millimetre in size, look very similar to the patterns left behind by ancient microbial activity on Earth.

Although non-biological sources couldn’t be ruled out, the markings are some of the best candidates we have for potential ancient life on Mars. But scientists lacked comprehensive information on the chemical make-up of these patterns or how widely distributed they were in the Bright Angel formation.

However, Perseverance carries measurement tools that can provide more chemical context about the rocks it encounters, such as the SHERLOC instrument, which uses the reflected light from an ultraviolet laser to identify elements and compounds in a rock sample.

Now, Ashley Murphy at the Planetary Science Institute in Tucson, Arizona, and her colleagues have used SHERLOC to identify large, complex carbon-containing molecules, called macromolecular carbon, on the surface of the marked rocks in the Bright Angel formation, as well as in a separate rock in the same formation around 100 metres away.

“On Earth, macromolecular carbon is often found in extremely old rocks and, in some cases, it is the only organic evidence of past microbial life,” says Murphy. “Finding these organic macromolecules on Mars and other planetary bodies helps us determine whether the necessary chemical ingredients and environmental conditions to support life have ever existed there.”

The existence of these carbon compounds can’t imply a biological origin by themselves, as they are also found in places like meteorites, says Lewis Dartnell at the University of Westminster in London. However, Murphy and her colleagues also discovered that the compounds were associated with carbonate and sulphate minerals, which tend to form in water-rich environments, another key ingredient for life. “It’s giving us information about the geological context of where those organics are being found,” says Dartnell.

Jezero crater was already suspected to have been water-rich at some point, so the fact that these carbon compounds existed here is unsurprising by itself, says team member Kyle Uckert at NASA’s Jet Propulsion Laboratory in California. However, macromolecular carbon has never been seen on the surface of a rock like this, says Uckert, which might imply that it is unusually resistant and different from other carbon-bearing compounds that have been found on Mars.

“Its ubiquitous presence within mudstones at Bright Angel compared with observations elsewhere in the crater was surprising,” says Uckert. Although it is currently unclear why that should be the case, this is a good sign for the possibility of finding other signs of past life, says Dartnell. “This detection confirms that complex organics, like these macromolecular deposits, can stick around for long periods of time.”

While the SHERLOC instrument can identify macromolecular carbon, it can’t give detailed information on the actual make-up of the compounds beyond saying that they are carbon-rich, says Sean McMahon at the University of Edinburgh, UK. “We would need to get the samples back to Earth to figure out if the carbon in these rocks was of biological origin,” he says.