Quantifying Uncertainty in Sustainable Biomass and Production of Biotic Carbon in Enceladus' Notional Methanogenic Biosphere.

Beneath Enceladus' ice crust lies an ocean which might host habitable conditions. Here, the scale and productivity of a notional Enceladean methanogenic biosphere are computed as a function of the core‐to‐ocean flux of hydrogen and the ratio between abiotic and biotic methane in Enceladus' space plume. Habitats with an ocean‐top pH range of 8–9 have up to 40%–60% probability of being energy‐limited. Those at pH > 9 are increasingly uninhabitable, and those <8.5 are increasingly likely to host exponential growth, possibly leading to compositional inconsistencies between the ocean and Cassini gas observations. In those cases, energy‐based habitability models cannot infer an inhabited Enceladus consistent with both Earth life and Cassini measurements without including additional microbial growth limiters such as nutrient limitation, toxicity, or spatial constraints. If methanogens are isolated to a 350 K seafloor habitat and consume 10 mol s−1 of H2, the most probable biomass is 103, 103.7 kg C with ocean‐top pH 8,9, respectively. Biomass production consistent with space plume fluxes is 104–106 kgC yr−1—milligrams of cellular carbon per kilogram of H2O ejected—but requires that >50% of the space plume methane is biotic. Alternative scenarios are presented, and biomass is generally lower when habitat temperature is higher. Ocean biomass density cannot yet be reliably estimated owing to uncertainties in the scale and physicochemical properties of Enceladus' putative habitats. Evaluating abiotic to biotic ratios in plume methane and organic material could help identify false negative results from life detection missions and constrain the scale of an underlying biosphere. Plain Language Summary: The Cassini spacecraft observed energy and nutrient sources for life erupting from a water ocean below the icy exterior of Saturn's moon Enceladus. Future spacecraft planned for Enceladus, Europa and Titan will obtain similar measurements in greater detail. In this work, we calculate the probability that different suites of environments inside Enceladus' ocean are habitable for methanogens, an ancient type of organism which may be able to survive there. Three scenarios are possible: uninhabitable, habitable, and energy‐saturated. Next, we extend this calculation to show that the possible biosphere is small but productive. Microbial waste, generated at the seafloor, could be ejected from the ocean and consequently snow down onto Enceladus' surface. Mass spectrometers on future spacecraft could constrain these possibilities further and may be critical to inferring biosignatures when cell densities are too low to detect. Key Points: Spatial habitability on Enceladus is not guaranteed, and additional modeling parameters are needed to evaluate possible vacant habitatsCell turnover is important for life detection missions and can be constrained more tightly than total biomassAbiotic to biotic ratios of space plume carbon are critical as biosignatures and for assessing the scale of Enceladus' notional biosphere [ABSTRACT FROM AUTHOR]