Viability of bacterial spores under icy-world surface conditions

Abstract:

Icy-ocean worlds like Europa and Enceladus are our best targets to find non-Earth life in our Solar System, owing to the presence of an ocean under their ice shell. Microorganisms formed there can potentially migrate to the surface via plumes, ice shell tectonics, or in chaos regions, and be accessible to an eventual landed mission. Laboratory studies are needed to better understand microbial degradation under the harsh conditions encountered on Icy World surfaces to both help with their detection and for planetary protection purposes. Furthermore, such studies are of high interest for planetary protection purposes in order to understand the risk of contamination by terrestrial life of outer Solar System landing targets.

To accomplish this, we study the viability of Bacillus subtilis bacterial spores, a microorganism known for its resilience under extreme conditions found on Earth. Bacillus subtilis spores are irradiated in the laboratory under high vacuum by UV photons using a mini-arc Ar lamp or an electron gun to simulate the low pressure and energetic conditions encountered on most icy bodies. They are cryogenically cooled as low as 10~K and in some instances, molecular films are deposited by condensing gaseous mixtures onto the cooled spore samples. Viability is quantified by retrieving the microorganisms after irradiation, culturing them onto agar plates, and calculating the ratio of colony-forming units between irradiated and control samples.

Here we present viability fractions of Bacillus subtilis spores irradiated by UV photons under a large range of temperatures, as well as preliminary results on their viability upon electron irradiation. Inactivation decreases with temperature and follows an exponential behavior with a slight leveling off at higher fluence, likely due to complex DNA inactivation/repair mechanisms and the potential presence of sub-populations. Fitting the kinetics allows us to extrapolate the inactivation fraction at various fluence and temperatures. From this quantification, we expect Bacillus subtilis spores to be deactivated on sub-hour timescales at the surface of Europa and Enceladus, taking into account their distance to the Sun and average surface temperature.