NASA's Perseverance rover, which has spent five years poking around Jezero Crater looking for chemical clues about what Mars was up to billions of years ago, has made a discovery that is either very exciting or a total coincidence, depending on how you feel about extraterrestrial life. The rover detected complex macromolecular carbon sitting right on the surface of a rock at an outcrop called Bright Angel, near the edge of an ancient river channel named Neretva Vallis. This is, according to lead author Ashley E. Murphy of the Planetary Institute in Tucson, Arizona, “the shallowest detection of organic matter on Martian surface to date.” On Earth, this much macromolecular carbon usually means something was once alive. But on Mars, it could also mean rocks just do weird stuff.

The detection came from SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), a UV Raman spectrometer attached to Perseverance's robotic arm. SHERLOC fires a deep-ultraviolet laser at a target and reads the returning light to identify molecular bonds. Between Martian sols 1180 and 1218, the rover zapped four targets at Bright Angel. Three of them - Cheyava Falls, Apollo Temple, and Walhalla Glades - returned a spectroscopic signal called the graphitic band (G-band), indicating a tangled network of reduced carbon atoms that is highly resistant to breakdown. The control rock, Steamboat Mountain, showed nothing, because of course it didn't.

The material roughly matches terrestrial kerogen, which on Earth is made almost exclusively from fossilized microbes. But the researchers decided not to use the word “kerogen,” because that would imply they know it came from life. “The term kerogen implies biogenic source,” Murphy explained. “Macromolecular carbon implies we don’t know whether its origin is biotic or abiotic.” So they went with the less fun but more accurate term.

The team had to rule out two major concerns. First, that the signal was light bouncing off SHERLOC's own fused-silica front window - a worry because Bright Angel was the first site examined after a dust-cover anomaly disabled the focusing mechanism. The team confirmed SHERLOC was working properly by testing spare optics in the lab and pointing it at nothing on Mars. The lack of signal from control rock Steamboat Mountain sealed the deal: the signal was real, not hardware. Second, contamination: perhaps the rover dragged Earth organics to Mars? The abrasion bit was sterilized before launch and had cut into other rocks without producing a strong G-band. Plus, Cheyava Falls was never touched; the rover just blew dust off with a nitrogen puff. And again, Steamboat Mountain came up clean.

Once convinced the finding was real, the team looked at what minerals surrounded the carbon. At Apollo Temple, the carbon clustered with carbonate and sulfate minerals - stuff that precipitates from water moving through old rock. At Walhalla Glades, it sat within silicate-rich sediment. Murphy sees this as evidence for at least two separate events: first, organic matter settling into mud at the bottom of an ancient lake and getting buried; later, groundwater moving through the buried rock and leaving behind new carbonate and sulfate minerals.

But the big question - whether this carbon is a remnant of ancient Martian life - will remain unanswered for now. “The science payload of the Perseverance rover was not designed to distinguish between abiotic and biotic processes but to identify compelling rocks to be collected for possible return to Earth,” says deputy principal investigator Kyle Uckert at NASA's JPL. “Perseverance rover has an incredible instrument payload, but those instruments pale in comparison to world-class techniques that could be used to analyze these samples when they get back to Earth,” added principal investigator Kevin P. Hand. He's particularly interested in the isotopic signature and chirality - a preference for one molecular handedness over another that strongly suggests biology. Also, “We could also use some of the most powerful microscopes on Earth to search for ancient microbial fossils,” Hand said.

There are plenty of abiotic explanations: fluid-rock reactions can synthesize organic compounds without life. But Hand is optimistic about what Perseverance might still find. “Now we are exploring the region outside of Jezero crater - the rocks over which we’re currently roving are perhaps some of the oldest rocks ever investigated by a rover on Mars. There’s a chance that if life arose early on in the history of Mars, we might find some hints of it in the rocks we’re looking at now.” The study appears in Science Advances.