A framework for evaluating biosignature potential against the abiotic baseline on ocean worlds

Ocean worlds are considered as targets for life detection missions because they meet several key requirements for habitability. However, identifying potential life on other worlds requires observing clear and unambiguous biosignature signals above the existing abiotic baseline. Consequently, this necessitates evaluating uncertainty and variability in the abiotic baseline, including processes that can overlap, attenuate, or obfuscate biosignatures before they are observed. This article develops a quantitative framework for holistically evaluating abiotic baselines on ocean worlds to guide life detection strategies. Using Enceladus as an example, we assess the potential of using: i) CH$_{4}$ isotopes and their relationship with CO$_{2}$, and ii) amino acid chirality as biosignatures, demonstrating that uncertainties in abiotic processes currently prevent hypothetical future ${\delta}^{13}$C$_{\mathrm{CO2}}$ and ${\delta}^{13}$C$_{\mathrm{CH4}}$ measurements from definitively inferring a biosphere on Enceladus. Additionally, our results quantitatively show that neglecting the abiotic baseline risks false negative life detection claims for both isotopic and chiral biosignatures. Interpreting these and other alternative biosignatures on Enceladus, Europa, Titan, and similar planetary bodies therefore requires complimentary geophysical observations such as constraining internal temperatures to within $\sim$10-100$^{\circ}$C, and improving characterisation of the target's rheology, lithology, initial abiotic organic inventory and ocean transport timescales.