Microorganisms in extreme environments with a view to astrobiology in the outer Solar System

We review the various manifestations of the evolution of life in extreme environments. We review those aspects of extremophiles that are most relevant for astrobiology. We are aware that geothermal energy triggering sources of heat in oceanic environments are not unique to our planet, a fact that was exposed by the Voyager mission images of volcanic activity on Io, the Jovian moon. Such activity exceeded by far what was known form terrestrial geology. The science of astrobiology has considered the possible presence of several moon oceans in the vicinity of both giant gas and icy planets. These watery environments include, not only Europa (strongly suggested by data from the Galileo mission), but the Voyager flybys exposed, not only the unusual geothermal activity on Io, but also the possible presence of subsurface oceans and some geothermal activity on the Neptune's moon Triton. More recently, calculations of Hussmann and coworkers with available data do not exclude that even Uranus moons may be candidates for bearing subsurface oceans. These possibilities invite a challenge that we gladly welcome, of preliminary discussions of habitability of extremophiles in so far novel environments for the science of astrobiology. Nevertheless, such exploration is currently believed to be feasible with the new generations of missions suggested for the time window of 2030 - 2040, or even earlier. We are envisaging, not only the current exploration of the moons of Saturn, but in the coming years we expect to go beyond to Uranus and Neptune to include dwarf planets and trans-neptunian worlds. Consequently, it is necessary to begin questioning whether the Europa-like conditions for the evolution of microorganisms are repeatable elsewhere. At present three new missions are in the process of being formulated, including the selection of payloads that will be necessary for the exploration of the various so far unexplored moons.