Abiotic Oxygen and the Limits of Earth Analogy in Exoplanet Biosignature Interpretation

Alexandre Vaillancourt

The detection of atmospheric oxygen on a rocky exoplanet is widely regarded as a strong indicator of biological activity. This expectation derives from a single observational precedent: Earth. We argue that this extrapolation conflates two epistemologically distinct categories of knowledge — universal physical laws, which apply to any rocky planet under any conditions, and contingent terrestrial chemistry, which reflects the specific geochemical boundary conditions of one planet around one star. Through a process-by-process flux analysis of four independently documented abiotic O2 production mechanisms on Archean Earth — UV-C photolysis coupled to hydrodynamic H2 escape, SO2 photodissociation (directly evidenced by the sulfur mass-independent fractionation record), silicate mechanochemistry, and terrestrial plasma electrolysis — we demonstrate that their combined production (~6×1011–6×1012 mol O2 yr−1) is of comparable magnitude to the total Archean reductive sink capacity (~5.1×1012 mol O2-eq. yr−1). This near-equivalence means that the boundary between abiotic and biological oxygen cannot be resolved from Earth's own geochemical record with current data — an epistemological result with direct consequences for exoplanet life detection. We propose a formal framework distinguishing physically universal constraints from geochemically contingent parameters, and argue that biosignature interpretation must be grounded in the former before observational data from JWST and future missions can be evaluated without circular reasoning.