A recent scientific publication suggests that the abundance of discovered organic molecules by the Curiosity rover is too high to be accounted for by any known process other than life. While this doesn’t entirely rule out an as-yet-unknown mechanism, this represents the second instance where ancient life emerges as the most plausible explanation for the measured composition. The findings of this new study have been featured in the journal Astrobiology.
In March 2025, the detection of the largest organic molecules ever found on Mars was announced, originating from Cumberland mudstone collected by Curiosity in a location dubbed Yellowknife Bay. These specific molecules, identified as long-chain alkanes, are recognized as biological byproducts, but they can also arise from certain chemical reactions that do not involve living systems.
At the time of that discovery, the most compelling aspect was the molecules’ age: 3.7 billion years old, aligning with a period when Mars was wet and likely habitable. This implied that if the Red Planet supported organisms during its “blue” phase, evidence of it should persist today.
Six months later, that work was shadowed by news that the Perseverance rover had located a rock in the Jezero Crater area whose chemical makeup scientists could only attribute to the presence of ancient life forms. The sample from Jezero was swiftly labeled “the clearest sign of life we have ever found on Mars,” yet the Yellowknife Bay sample might also warrant a spot on that pedestal. The reason is that while non-biological pathways exist for generating molecules similar to those in the mudstone, they likely wouldn’t leave behind such a considerable quantity.
Alexander Pavlov from NASA’s Goddard Space Flight Center and his colleagues set out to estimate the quantity of long-chain alkanes originally present in the sample. Prior studies of Gale Crater revealed the mudstone was buried for 3.6 billion years and only reached the surface 78 million years ago. The cosmic radiation it encountered on the surface slowly degrades such alkanes, so the team factored in the rate of loss to calculate their abundance at the time irradiation began.
Given that the current level of long-chain alkanes in the sample registers at 30–50 parts per billion, Pavlov and his co-authors concluded that their prevalence was more than 2,000 times greater before radiation exposure. They estimate their content during the late Cretaceous period on Earth to be around 120–7,700 parts per million (ppm), whether referring to the long-chain alkanes themselves or the fatty acids they transform into. This broad range accounts for the uncertainty regarding whether common Martian chemicals accelerate radiation’s effect on the alkanes, but even the lower bound has significant implications.
“Such high concentrations of large organic molecules in Martian sedimentary rock cannot be easily accounted for by either the accretion of organic material from carbon-rich interplanetary dust particles and meteorites, or by the deposition of hypothetical organics formed from the haze of the ancient Martian atmosphere,” the authors state.
To ensure thoroughness, the authors also investigated the potential for alkane formation via hydrothermal processes during Mars’ wetter, more volcanic epoch. The authors note they lack the means to calculate how many long-chain alkanes this type of process might have generated. Nevertheless, a concentration of 120 ppm derived from this source does not align with the proportion of carbonates in the surrounding environment.
On the other hand, alkanes in such quantities are observed on Earth, having been generated by organisms, so the same could be true on Mars; the authors are careful to point out they are not making that definitive claim.
These estimations still await review by the broader astrobiology community. However, the dating component relies on work conducted using three independent isotopes 12 years prior and is unlikely to be contested.
If the remainder of Pavlov and his co-authors’ findings hold up, the possibility of processes we haven’t observed or conceived of remains open. Nonetheless, the more numerous the discoveries that strongly suggest the byproducts of living organisms and are difficult to explain otherwise, the stronger the case for ancient Martian life becomes.
When the Curiosity and Perseverance rovers were designed, they likely lacked the capacity to find conclusive evidence of life. This is why the Mars Sample Return mission was planned; terrestrial laboratories were anticipated to conduct analyses that could prove truly definitive. However, with that program nearly entirely defunded, it appears the pair of rovers will continue to uncover artifacts that look like probable indicators of ancient life, but without any definitive confirmation.
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