Life on Venus

A fundamental question in exobiology remains the degree to which habitats on Venus, past and present, were, or are suitable for life. This has relevance for assessing the exobiological potential of extrasolar Venus-like greenhouse planets. In this paper the parameters of the Venusian surface and atmosphere are considered and the biochemical adaptations required to survive them are explored in the light of new information on microbial adaptations to extreme environments. Neither the pressure (9.5 MPa) nor the high carbon dioxide concentrations (97%) represent a critical constraint to the evolution of life on the surface or in the atmosphere. The most significant constraints to life on the surface are the lack of liquid water and the temperature (464°C). In the lower and middle cloud layers of Venus, temperatures drop and water availability increases, generating a more biologically favorable environment. However, acidity and the problem of osmoregulation in hygroscopic sulfuric acid clouds become extreme and probably life-limiting. If it is assumed that these constraints can be overcome, considerations on the survival of acidophilic sulfate-reducing chemoautotrophs suspended as aerosols in such an environment show that Venus does come close to possessing a habitable niche. Conditions on the surface and in the atmosphere may have been greatly ameliorated on early Venus and may also be ameliorated on extrasolar planets with early Venus-like characteristics where temperatures are less extreme and liquid water is available.