For more than a decade, NASA’s Curiosity rover has been crawling across the dusty, windswept plains of Mars, drilling into rocks and, in the process, revealing the Red Planet’s geological history. Now, it has stumbled upon its most compelling chemical clues yet: large organic molecules resembling fatty acids. These are, in fact, the largest organic molecules ever detected on the Red Planet.

Though scientists have stopped short of declaring they’ve discovered signs of past life, these compounds fuel fresh speculation about whether the Red Planet once harbored biology billions of years ago.

A Chemical Relic from an Ancient Lake

The organic molecules were found in a mudstone sample nicknamed Cumberland, drilled from Yellowknife Bay, a dried-up lake bed in Mars’ Gale Crater. Nearly 3.7 billion years ago, this region was likely filled with liquid water—a place where life, if it ever existed, could have thrived.

Using its onboard laboratory, the Sample Analysis at Mars (SAM) instrument, Curiosity detected three long-chain organic molecules: decane, undecane, and dodecane, made up of 10, 11, and 12 carbon atoms, respectively. These compounds are thought to be broken-down remnants of even larger molecules—possibly fatty acids, which on Earth are some of life’s building blocks, making up cell membranes.

However, fatty acids can form in hydrothermal vents or through reactions in meteorites, meaning their presence alone isn’t proof of past organisms.

“These molecules can be made by chemistry or biology,” said Dr. Caroline Freissinet, the study’s lead author and a researcher at the French National Centre for Scientific Research.

“If we have long-chain fatty acids on Mars, those could come—and it’s only one hypothesis—from membrane degradation of cells present 3.7 billion years ago.”

A Smoking Gun—But for What?

Scientists had long sought such complex organics in the Martian soil but faced frustrating setbacks. Curiosity’s onboard chemistry lab works by heating rock samples to extreme temperatures to identify their chemical composition. Previously, SAM only detected small organic molecules, leaving scientists uncertain whether larger molecules had ever existed—or simply decayed over eons in Mars’ harsh environment.

But in a striking reversal of fortune, Freissinet and her team revisited old data from Cumberland. Initially overlooked due to analytical difficulties, new experiments conducted in an Earth-bound replica of SAM allowed scientists to finally decode the chemical signals hidden in the rover’s data.

Ultimately, the researchers were convinced they had emerged from the Martian haystack with the metaphorical needle. “There’s no question about it. We have three needles,” said Daniel Glavin, senior scientist at NASA’s Goddard Space Flight Center and co-author of the study.

More than 3.7 billion years ago, water filled Gale Crater, forming sedimentary rocks rich in clay, sulfur, and nitrates. These conditions provided a perfect recipe for preserving organic material over geological time.

“There is evidence that liquid water existed in Gale Crater for millions of years and probably much longer, which means there was enough time for life-forming chemistry to happen in these crater-lake environments on Mars,” Glavin said in a press release.

However, definite answers look out of reach, considering the rover’s limitations. The rover’s aging systems, after 13 years exposed to Martian elements, restrict what scientists can still achieve on-site. Even when it was brand new, Curiosity could only do so much.

To determine whether these molecules truly came from ancient Martian life, scientists will need to study pristine samples in Earth’s labs—a goal of NASA’s Perseverance rover, which is currently collecting rocks for a future return mission to Earth.

In the meantime, Curiosity still holds one final opportunity. A second, untouched portion of the Cumberland sample—set aside as a backup—remains onboard. Scientists now carefully consider how best to analyze this precious material.

“We are ready to take the next big step,” said Glavin. “Bringing Mars samples home will settle the debate about life on Mars.”

The findings appeared in the journal PNAS.