Proto-Biosignatures and Planetary Geochemical Metabolism: A Thermodynamic Screening Model of Prebiotic Geochemical Organization

Astrobiological observations usually return atmospheric, mineralogical, molecular, isotopic, or textural states rather than organisms. The interval between environmental habitability and confirmed life detection is therefore an evidential problem. This article develops planetary geochemical metabolism (PGM) as a scale-explicit description of abiotic water–rock–fluid reactions, including atmospheric or ice-shell boundary conditions where relevant, that sustain redox disequilibria, catalytic mineral interfaces, prebiotic molecular fluxes, and preservable mineral products. Proto-biosignatures are defined as contextual, non-diagnostic signatures of this interval: signals that do not demonstrate life, but increase the plausibility of prebiotic network organization relative to low-organization abiotic chemistry. A nondimensional Geochemical Metabolic Potential, ΦPGM, is formalized as a target-specific screening index describing redox exergy, catalytic-interface density, reaction-network closure, environmental cycling efficacy, and preservation potential. The index is not calibrated as a probability of life. A reproducible Latin-hypercube experiment across Msim=5000 synthetic environments examines internal model behavior rather than ranking planets. Higher ΦPGM values mark hypotheses to be tested under declared scale, uncertainty, and observability constraints. Applications to early Earth, Noachian Mars, ocean worlds, and terrestrial exoplanets illustrate that habitability, prebiotic organization, biological inference, and preservation are distinct quantities.