Search parameters for the remote detection of extraterrestrial life

Direct consequences of biological activity (biosignatures) and alterations of the geological environment due to biological processes (geosignatures) are currently known only for the planet Earth. However, geoindicators remotely detectable by robotic technology have revealed a number of sites in the solar system where conditions compatible with the support of life may exist. By focusing on a search for energy gradients, complex chemistry, liquids that may act as solvents, atmospheres, and indicators of geological differentiation, robotic exploration of the solar system and beyond should lead to fruitful targets in the search for extraterrestrial life. An analysis of all major solar system bodies for these parameters suggests that Mars, Titan, and the Galilean satellites should be given the highest priority in the search for extraterrestrial life in our solar system. Extending them to other bodies in the solar system, however, draws attention to Io, Triton, Titania, Enceladus, and Iapetus, among others, as worthy of greater attention.