Venus has not traditionally been considered a promising target for Astrobiological exploration. Yet, Venus should be central to such an exploration program for several reasons. Venus is the only other Earth-sized terrestrial planet that we know of, and certainly the only one we will have the opportunity to explore in the foreseeable future. Understanding the divergence of Earth and Venus is central to understanding the limits of habitability in the inner regions of habitable zones around solar-type stars. Thus, Venus presents us with a unique opportunity for putting the bulk properties, evolution and ongoing geochemical processes of Earth in a wider context. Many geological and meteorological processes otherwise active only on Earth at present are currently active on Venus. For example, active volcanism is most likely responsible for maintaining the global cloud cover. If we think beyond the specifics of a particular chemical system required to build complexity and heredity, we can ask what general properties a planet must possess in order to be considered a possible candidate for life. The answers might include an atmosphere with signs of flagrant chemical disequilibrium and active, internally driven cycling of volatile elements between the surface, atmosphere and interior. At present, the two planets we know of which possess these characteristics are Earth and Venus.

Venus most likely once had warm, habitable oceans. The duration (and even existence) of this phase is poorly understood, but recent modeling results suggest it may have lasted for billions of years. During this time the terrestrial planets were not isolated. Rather, due to frequent impact transport, they represented a continuous environment for early microbial life. Life, once established in the early oceans of Venus, might possibly have migrated to the clouds which, on present day Venus, may represent a habitable niche. Though highly acidic and possessing very low water activity, this aqueous environment enjoys moderate temperatures, surroundings far from chemical equilibrium, and potentially useful radiation fluxes. Observations of unusual chemistry in the clouds, and particle populations that are not well characterized, suggest that this environment must be explored much more fully before biology can be ruled out.  

 I will give a brief history of ideas about habitability on Venus, focusing on evolving understanding of water sources, abundance and escape histories, climate evolution and cloud properties. Then I will discuss the key observations, relevant for astrobiology, to be made by currently planned missions and the modeling opportunities both before and after these observations, which will enrich our understanding of the past and present habitability of Venus and provide valuable perspective for the prospects for life on rocky worlds throughout the universe.