Microbial-Based Proxies for Europa's Habitability and Biosignature Detection

Del Moral, Alvaro (2024). Microbial-Based Proxies for Europa's Habitability and Biosignature Detection. PhD thesis The Open University.

Abstract

Europa is one of Jupiter's icy moons, and below its icy surface, there is a tidally heated, liquid water ocean. Chemically similar environments on Earth can support microbial communities. However, whether the physical and chemical conditions of Europa's ocean are conducive to life is currently unknown, which is critical for understanding the potential habitability of Europa. This thesis identifies Earth analogue environments for Europa's putative habitable sub-surface ocean. For this, a machine learning (ML) algorithm was developed that selected terrestrial analogues with a high degree of similarity to the physicochemical environment proposed for Europa, reducing the amount of bias introduced by humans. The work focused on several models for Europa’s ocean composition (Kargel et al., 2000; McCollom, 1999; Melwani Daswani et al., 2021; Zolotov & Kargel, 2009). A multivariate database of terrestrial aqueous environments was grouped using a clustering solutions based on their physicochemical characteristics. A Random Forest (RF) algorithm was used to assign these groups to modelled Europa ocean chemistries. This approach identified several putative analogue environments from which Basque Lakes, Canada, was selected. This thesis presents the characterisation of the Basque Lake microbial community that grew under simulated Europa ocean conditions. Microbes were grown in simulated ocean fluid based on the carbonate-chloride rich model of Melwani Daswani et al. (2021), with a 2 bar of 80%-20% H2/CO2 gas headspace. Using a bespoke environmental simulation chamber, the pressure was then increased using N2 gas to 10, 20 and 30 MPa to simulate different regions in and below Europa’s ice shell. Sulfate-reducing bacteria dominated the community when grown in the simulated ocean fluids and at high pressure. Cells produced extracellular polymeric substances in response to an increase in pressure. To our knowledge, this is the first study to experimentally test the habitability of Europa’s ocean and obtain a proxy microbial community. This community can provide information about the survival mechanisms microbial life might use in Europa, the regions of habitability within the moon, and the biosignatures that could be found on the surface. Focusing on the proxy microbial community obtained from the high-pressure experiments, this thesis characterises the biosignatures that could be used as evidence of life within the ice shell of Europa. Simulating different transport processes by subjecting the cells to different freezing rates, microbes were shown to affect the precipitation of minerals in Europan-simulated ice. A fraction of the microbial colony survived the freezing process and was found to be associated with minerals in the brine network of ice. The results extend our knowledge of putative biosignatures and regions of habitability, informing the targets for future life-seeking missions to the icy moon.

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