Cryogenic Mineralisation and Capture of Microorganisms in Simulated Enceladan Fluids

The Saturnian moon Enceladus exhibits large cryovolcanic plumes that contain evidence of subsurface liquid water, hydrothermal activity and endogenic organic chemistry. Because of these factors, Enceladus has come to be viewed as one of the most promising bodies in the solar system to harbour extraterrestrial life. Particles in the plumes have been observed to contain salts, silica and macromolecular organics. If a microbial biosphere is currently active in the subsurface of Enceladus, it is likely that biomass is entrained in these particles and surface materials, and thus accessible to future spacecraft. Understanding how mineralisation occurs and how microorganisms become partitioned under different freezing regimes is therefore crucial to the success of future life-detection missions at Enceladus and other bodies with active cryovolcanism, such as Europa. However, the mechanisms by which cells or other biological material can become captured by cryovolcanic processes are poorly understood. We investigated microbial mineralisation in simulated Enceladus ocean fluid, under both flash-freezing (at approx. 80 K) and gradual-freezing (at approx. 240 K) regimes. We show that mineral phase partitioning occurs at both freezing rates, and that ice crystal templating of cryoprecipitated salts and cryogenic opal-A ensues even when fluids are flash-frozen. Using both a pure strain of methanogenic archaea, and a mixed microbial community selectively enriched on our Enceladus ocean simulant, we demonstrate how microorganisms become captured in complex mineral assemblages during hypothetical end-member freezing scenarios.