Testing the Habitability of Saturn’s Icy Moon Enceladus in an Interdisciplinary Attempt

Autoren:Taubner, Ruth-Sophie; Pappenreiter, Patricia (Johannes Kepler Universität Linz); Pruckner, Christian (Universität Wien); Zwicker, Jennifer; Smrzka, Daniel; Bernacchi, Sebastian (Krajete GmbH); Paulik, Christian (Johannes Kepler Universität Linz); Schleper, Christa; Firneis, Maria Gertrude; Rittmann, Simon Karl-Maria Rasso
Abstrakt:The indirect proof of subsurface water oceans in icy moon like Jupiter’s Europa or Saturn’s Enceladus extended the number of potential habitats in the Solar System. The latter is one of the hot spots in the Solar System due to its erupting water plumes, which most likely origin in a global subsurface liquid water ocean. The detection of silicon-rich, nanometre-sized dust particles and the abundance of various salt components in a certain population of E-ring grains suggests that his subsurface aquifer is (or at least was) in direct contact with the underlying rocky core, which might imply water-rock interactions like low temperature serpentinization. Therefore, the most promising area on Enceladus where life may exist is at the seafloor of this subsurface water reservoir. If a subsurface ocean on Enceladus would host life, the organisms would have to be chemoautotroph, have to be independent of products directly or indirectly produced by photosynthesis, and have to be anaerobes. Organisms which fit this profile are for example methanogens. Regarding the plume composition, there may be a huge variety of potential substrates and even products of methanogens on Enceladus, like CO2, H2, and CH4. Therefore, methanogens are the most likely known terrestrial life form that could thrive under Enceladus-like conditions.We performed numerical experiments using the PHREEQC code to test the possibility of low temperature serpentinization on Enceladus. For that, we assume the core composition to be similar to chondritic chondrites, i.e. to be composed of various combinations of olivine and pyroxenes. We show that the amount of produced H2 during serpentinization is sufficient to serve as a potential substrate for methanogens.Among other methanogens we tested the hydrogenotrophic methanogenic strain Methanothermococcus okinawensis in respect to its tolerance towards potential inhibitors of biological methanogenesis detected in Enceladus’ plume, e.g., high ammonia levels, formaldehyde (H2CO), ethene (C2H4), and methanol (CH2O). Based on the results of these experiments, we performed high pressure closed batch experiments in the range of 10 to 90 bar, i.e. the pressure range expected in the subsurface ocean of Enceladus. The closed batch experiments were performed in a 0.7L stirred reactor at 64°C. Our experiments showed that biological methanogenesis could be feasible under Enceladus-like conditions, however abiotic methanogenesis is rather not likely to occur.Our study combines studies of various scientific fields and introduces novel aspects on potential habitats for microbial life in the Solar System, especially concerning icy moons.