Scientists have developed a new quantitative framework to help determine whether potential signs of life on icy ocean worlds can truly be distinguished from non-biological processes.

The study, led by researchers from Harvard University and the University of Toronto, uses Saturn’s moon Enceladus as a case study. Enceladus is considered one of the Solar System’s most promising locations in the search for extraterrestrial life because it contains a global subsurface ocean beneath its icy crust and releases water-rich plumes into space.

However, identifying life is not simply a matter of detecting organic molecules or unusual chemical signatures. Many potential biosignatures can also be produced, altered or erased by abiotic geological and chemical processes. The researchers developed a modular framework that models this “abiotic baseline” before considering possible biological contributions. They tested the approach using two proposed biosignatures: carbon-isotope patterns in methane and carbon dioxide, and the molecular handedness, or chirality, of amino acids. The results show that methane produced by hypothetical microorganisms on Enceladus could have an isotopic composition that substantially overlaps with methane generated through non-biological reactions. This means that measurements of carbon isotopes alone may not provide definitive evidence of life.

The team also found that amino-acid chirality could produce false negative results. Even if biological amino acids initially display a characteristic preference for one molecular form, long ocean transport times and elevated temperatures may gradually erase this signal through racemization before the material reaches the surface or plume.

The researchers conclude that future missions searching for life on Enceladus, Europa, Titan and other ocean worlds should combine direct biosignature measurements with detailed geophysical and geochemical observations. Better constraints on internal temperatures, ocean circulation, rock composition, organic inventories and transport timescales may be essential for distinguishing biological activity from non-biological chemistry and seaching for extraterrestrial life.