Europa The Search for Life on Europa: Limiting Environmental Factors, Potential Habitats, and Earth Analogues

The putative ocean of Europa has focused considerable attention on the potential habitats for life on Europa. By generally clement Earth standards, these Europan habitats are likely to be extreme environments. The objectives of this paper were to examine: (1) the limits for biolog-ical activity on Earth with respect to temperature, salinity, acidity, desiccation, radiation, pres-sure, and time; (2) potential habitats for life on Europa; and (3) Earth analogues and their lim-itations for Europa. Based on empirical evidence, the limits for biological activity on Earth are: (1) the temperature range is from 253 to 394 K; (2) the salinity range is aH2O 5 0.6–1.0; (3) the desiccation range is from 60 % to 100 % relative humidity; (4) the acidity range is from pH 0 to 13; (5) microbes such as Deinococcus are roughly 4,000 times more resistant to ionizing radia-tion than humans; (6) the range for hydrostatic pressure is from 0 to 1,100 bars; and (7) the max-imum time for organisms to survive in the dormant state may be as long as 250 million years. The potential habitats for life on Europa are the ice layer, the brine ocean, and the seafloor en-vironment. The dual stresses of lethal radiation and low temperatures on or near the icy sur-face of Europa preclude the possibility of biological activity anywhere near the surface. Onl