The news reports have come to sound almost routine: “Scientists Find Organic Material on Mars.” Case in point: Just last week, stories broke about how the Perseverance rover found specific mineral assemblages associated with organic compounds on the red planet, with NASA describing the findings as a potential biosignature. Cool, you say. But, really, what does that mean? How much closer does it get us to discovering life beyond Earth?
Some findings are, of course, more significant than others, and in fact do get us closer to that long-hoped-for announcement. First, though, we have to understand all the ways that nature — especially Martian nature — might deceive us.
In a recent article in Scientific Reports, a research team led by Felix Arens from the Technical University Berlin in Germany described an experiment wherein they exposed biomolecules and microorganisms to the Mars-like conditions of Chile’s Atacama Desert, one of Earth’s driest places, for eight months. Most previous investigations along these lines have been in laboratories, so real-world experience in a harsh place like the Atacama, and for such a long time, is something new.
Using custom-built exposure plates, the researchers monitored how the presence of salts and minerals, combined with factors like humidity and ultraviolet radiation levels, affected the preservation of well-known terrestrial biomolecules such as ATP and chlorophyll. The result was sobering: No ATP was left after eight months in the desert. Zero. The only traces left of chlorophyll were its decay products: pheophytin-a, pyropheophytin-a, and phytol. And these survived only because clay and salt in the surrounding soil protected them from further decay.
It’s been more than a decade since NASA’s Curiosity rover first detected chlorinated hydrocarbons on Mars. The discovery of sulfur-rich organics followed in 2018, and just this year, more complex, long-chain alkanes were found in Martian mudstones. To date, the Perseverance rover has detected many significant organic compounds. Meanwhile, previous notions of an organics-free planet have been discarded: The discovery of chloromethane and dichloromethane on Mars by the Viking Life Detection Mission half a century ago, initially interpreted as terrestrial contamination, is now accepted by many scientists as the first detection of indigenous organic compounds on Mars.
These findings pose a problem, however. Most organic compounds are extremely fragile when exposed to harsh environmental conditions — and that’s especially true for biomolecules, most of which are large organic compounds. Some even decay rapidly in relatively benign conditions, so the multiple detections of Martian organic compounds are all the more surprising. The surface of Mars is a harsher environment than any desert on Earth, given the enormous temperature swings and punishing UV radiation. So, why should there be any biomolecules left? Shouldn’t they all have disintegrated, as they did in the Atacama?
This stirs an uneasy thought in the astrobiologist’s mind. Maybe the organic molecules we’ve been seeing on Mars are not produced by biological processes at all. We’ve often assumed they’re the breakdown products of larger organic compounds — and size matters here. Small, simple organics can form through both biology and nonbiological chemistry, which makes them ambiguous. Larger, more complex organics are harder to generate without life and would be more telling, but they’re also relatively fragile and far less likely to survive on Mars’s harsh surface. That leaves us with an open question: Are the compounds we’re detecting the breakdown products of larger organic molecules (which could be signs of past life), or simply the durable products of nonbiological chemistry?
Several years ago, Jacob Heinz and I took a close look at sulfur-rich organics (thiophenes) detected on Mars by the Curiosity rover in 2018. We pointed out that on Earth, some truffles contain thiophenes produced by certain bacteria (which are what give the truffles their strong aroma that’s so attractive to pigs). But the thiophenes don’t necessarily come from bacteria. They could have simply derived from sulfur-rich organic matter. In fact, our study showed more than one pathway to producing the thiophenes — some biological, some non-biological. And it was hard to say which one was the most likely without doing a carbon isotope analysis (life generally favors lighter isotopes because less energy is used up during metabolic reactions).
To complicate matters further, there’s no easy way to know how long any of the organic compounds we’ve discovered on Mars have been there. Given the harshness of the environment and the fact that the compounds seen so far are generally small and relatively stable, they could have been there for mere decades or for billions of years.
The apparent abundance of organic compounds on Mars may favor the biological explanation. But we have to be careful. Such compounds can form by other means, such as, for example, the electrochemical reduction of carbon dioxide.
Fortunately, there may be a way to solve this scientific mystery. We could include instruments on the next Mars rover mission that can also detect larger organic molecules, which would make it easier to establish the source of the smaller remnants discovered so far, and would improve our chances of detecting a known biomolecule directly. We should search for these molecules in clay- and salt-rich regions of Mars, because it now appears that’s an important factor in their long-term preservation. Fifty years after Viking, it’s time to launch another life-detection mission, so we don’t have to keep guessing about the evidence we’ve gathered so far.
This article Biosignatures? Why organics on Mars don’t necessarily signal life is featured on Big Think.
The post “Biosignatures? Why organics on Mars don’t necessarily signal life” by Dirk Schulze-Makuch was published on 09/17/2025 by bigthink.com