👽 How to recognize extraterrestrial life through chemistry?

How to recognize extraterrestrial life if space chemistry produces the same building blocks as biology? This fundamental question is gaining relevance following the analysis of samples from the asteroid Bennu. Scientists have discovered a treasure trove of organic molecules there, including amino acids and nucleobases, without any evidence of past life forms being associated with it.

These samples, brought back by the OSIRIS-REx mission, had a notable particularity: the amino acids were present in nearly equal proportions between their so-called "left" and "right" mirror forms. On Earth, however, life almost exclusively uses the left-handed version. Consequently, the lack of a marked preference on Bennu suggests that this molecular asymmetry specific to our biology likely developed later, through processes specific to our planet.


Faced with this situation, a team of researchers proposes a new approach called LifeTracer, described in the journal PNAS Nexus. Instead of targeting a single molecule, this method studies the entire set of chemical patterns present in a sample. The starting hypothesis is based on the fact that life assembles molecules for a specific purpose, such as storing energy or transmitting information, whereas non-living geochemical processes follow different logics.

To develop LifeTracer, scientists compared materials of biological origin, like terrestrial soils, with abiotic samples from carbon-rich meteorites. Each sample, containing tens of thousands of organic molecules, was analyzed by fragmenting the compounds to examine their properties. A machine learning model then learned to separate the two groups by relying on the global distribution of chemical footprints.

This approach revealed general trends. The meteorite samples, for example, contained more volatile compounds, reflecting the cold environments of space. A sulfur-containing compound, 1,2,4-trithiolane, emerged as a robust marker for non-biological samples. Thus, the model does not rely on a single clue, but on how an entire collection of molecules is structured.

LifeTracer is therefore an interesting tool for future space missions. When probes return samples from Mars, its moons, or the oceans of Europa and Enceladus, the organic mixtures will likely come from multiple sources. This method will help estimate whether the overall chemical landscape resembles biology or fortuitous geochemistry more closely, thus complementing techniques already in use.