Martian Organic Material Suggests Origin of Life’s Building Blocks

In the Martian atmosphere billions of years ago, the Sun’s ultraviolet radiation broke down carbon dioxide (CO2) molecules through a process called photolysis. This photochemical reaction split CO2 into carbon monoxide (CO) and oxygen. However, the process occurred more rapidly for the lighter carbon-12 isotope than for the heavier carbon-13 isotope. As a result, the carbon monoxide produced by photolysis was depleted in carbon-13, while the remaining CO2 in the atmosphere became enriched with carbon-13.

This photochemical process was the starting point for the synthesis of complex organic molecules on Mars. The carbon monoxide gradually reacted with other chemicals in the atmosphere, forming the building blocks of life – carbon-based organic compounds. This mechanism could explain the origin of the organic material found on Mars, without the need for biological processes.

Using quantum chemical simulations, researchers from the University of Copenhagen and the Tokyo Institute of Technology made precise predictions about the ratio of carbon isotopes after photolysis on Mars. They identified distinctive isotopic fingerprints that could serve as evidence for this photochemical process being the source of Martian organic compounds.

The study presented compelling evidence from two different Martian samples that support the photochemical origin of organic material on Mars. One of these samples is a meteorite called Allan Hills 84001, which was knocked off Mars by an asteroid impact and later landed on Earth in Antarctica.

Analysis of the carbonate minerals in the Allan Hills meteorite revealed an enrichment of carbon-13 relative to the baseline composition of volcanic CO2 emitted on Mars. This enrichment matches the predictions of the researchers’ quantum chemical simulations, suggesting that the carbonates formed from the leftover CO2 in the Martian atmosphere after photolysis had removed the lighter carbon-12.

The second piece of evidence comes from the organic material recently discovered by NASA’s Curiosity rover on the surface of Mars. Measurements of this material showed a depletion of carbon-13 compared to the baseline composition of Martian volcanic CO2. This depletion is the mirror image of the enrichment observed in the Allan Hills meteorite, and it corresponds to the carbon monoxide produced by photolysis, which served as the starting point for the synthesis of organic compounds.

Together, these two samples provide complementary isotopic fingerprints that support the photochemical origin of Martian organic material. The researchers argue that there is no other way to explain both the carbon-13 depletion in the surface organic matter and the enrichment in the meteorite carbonates, other than through the photolysis of atmospheric CO2 and subsequent reactions involving the resulting carbon monoxide.