Instantaneous Habitable Windows in the Parameter Space of Enceladus' Ocean

In recent years, Enceladus' subsurface ocean has become a tantalizing case study for potentially habitable conditions in an extraterrestrial ocean world. However, we still know very little about its subsurface conditions. Its oceanic composition is difficult to characterize with current data and estimates are highly dependent on model-based interpretations which are also not yet tightly constrained. In light of these uncertainties, we consider a wide selection of the inferred parameter spaces to quantify the energy available to putative hydrogenotrophic methanogens on Enceladus in the bulk ocean at cool and elevated temperatures. We estimate the instantaneous power supply their metabolism could provide in these conditions and compare it to the expected power demands of life on Earth. To be habitable for methanogens a 273 K ocean with relatively high salt content must have pH < 10, and a relatively low salt ocean must have pH < 8 at 273 K, or pH < 9 when heated to >360 K. Some combinations meet the power demands of exponential growth, but large swathes of the parameter space appear energetically uninhabitable. The habitability of the Enceladus ocean for methanogens appears to be a delicate balance between its temperature, pH, salinity, and concentrations of carbonates, nutrients, and dissolved gases (particularly H2). Many of these parameters are co-dependent; variation in any one of them could tip the balance into uninhabitable conditions. Further constraining these variables should be a priority for future missions to ocean worlds to enable definitive assessments of their habitability.

Automated summary

Research on the energy supply for potential hydrogenotrophic methanogens in Enceladus' subsurface ocean suggests that habitability depends on a delicate balance of factors including temperature, pH, and salinity. Further study of these parameters is needed to definitively assess habitability.