Bacterial spore morphology remains highly recognizable after exposure to simulated Enceladus and Europa surface conditions

Future astrobiology missions have a great interest in sampling material from the subsurface oceans of Europa and Enceladus to look for evidence of life. Realistically, the first lander missions will have to rely on sampling material near the surface that is suspected to have originated at depth because of the technological challenge of direct ocean access. A key question then is whether indicators of life, or biosignatures, derived from putative cells deposited at the surface would persist long enough to be detected, given that the extremely harsh near-surface conditions would tend to degrade them. Previous studies assessing the viability of Bacillus subtilis spores under conditions representative of Europa and Enceladus surface environments have shown that they are unlikely to be viable at relevant surface exposure ages. However, little work has been done to determine how similar exposures to the radiation, vacuum, and temperature conditions of Ocean World surface environments would affect the likelihood of detecting biosignatures left behind by these organisms. The goal of this work was to experimentally determine how long morphological and spectroscopic biosignatures derived from spores persist under simulated Ocean World surface environments over geologically relevant timescales. Our results show that spore structure is highly resilient in the face of extreme conditions long after spores have been inactivated, suggesting that methods targeting cell morphology would be valuable components in a suite of life detection strategies used in future missions to Ocean Worlds.