  Venus is Earth's sister planet, with similar mass and density but an
uninhabitably hot surface, an atmosphere with a water activity 50-100 times
lower than anywhere on Earths' surface, and clouds believed to be made of
concentrated sulfuric acid. These features have been taken to imply that the
chances of finding life on Venus are vanishingly small, with several authors
describing Venus' clouds as "uninhabitable", and that apparent signs of life
there must therefore be abiotic, or artefactual. In this article, we argue that
although many features of Venus can rule out the possibility that Earth life
could live there, none rule out the possibility of all life based on what we
know of the physical principle of life on Earth. Specifically, there is
abundant energy, the energy requirements for retaining water and capturing
hydrogen atoms to build biomass are not excessive, defenses against sulfuric
acid are conceivable and have terrestrial precedent, and the speculative
possibility that life uses concentrated sulfuric acid as a solvent instead of
water remains. Metals are likely to be available in limited supply, and the
radiation environment is benign. The clouds can support a biomass that could
readily be detectable by future astrobiology-focused space missions from its
impact on the atmosphere. Although we consider the prospects for finding life
on Venus to be speculative, they are not absent. The scientific reward from
finding life in such an un-Earthlike environment justifies considering how
observations and missions should be designed to be capable of detecting life if
it is there.
