Biosignatures in the atmospheres of ocean worlds: methodological challenges in distinguishing biotic from abioticsources of methane and phosphine

The detection of a putative biosignature gas is only as strong as the abiotic explanationsit can exclude, and recent history — above all the phosphine controversy at Venus — shows howquickly a claimed sign of life can dissolve into instrument artifact or unremarkable geochemistry.This article addresses the methodological problem directly for the two reduced gases most oftenproposed as biosignatures on ocean worlds and habitable-zone exoplanets: methane and phosphine. I argue that prevailing frameworks err by asking a gas-first question — is methane, orphosphine, a biosignature? — and by combining lines of evidence through weighted averagesthat let a strong showing on one axis mask an unaddressed weakness on another. In their place Idevelop a provenance-first Biotic Provenance framework that scores any candidate detection onthree logically independent abiotic-rejection axes: thermodynamic-disequilibrium context, theabiotic-flux ceiling, and spectral and conceptual robustness. Crucially, the framework combinesthese axes under a weakest-link rule — the Biotic Provenance Index is the minimum of the threeaxis scores, not their average — on the argument that a false positive is governed by the leastexcluded abiotic alternative rather than by the average strength of the case. Applied to threetouchstone cases, the framework localizes precisely where each fails: Venus phosphine is limitedby spectral robustness, with the detection itself contested; K2-18b methane and dimethyl sulfideare limited by the same axis, resting on marginal spectral features; and Enceladus methane islimited instead by the abiotic-flux ceiling, because serpentinization can supply the observed abundance. None of the three currently clears the threshold for biotic provenance, but each fails fora different, identifiable reason — which is exactly the diagnostic the field needs, and which aweighted-average confidence ladder obscures.