  Ancient Venus and Earth may have been similar in crucial ways for the
development of life, such as liquid water oceans, land-ocean interfaces,
favorable chemical ingredients and energy pathways. If life ever developed on,
or was transported to, early Venus from elsewhere, it might have thrived,
expanded and then survived the changes that have led to an inhospitable surface
on Venus today. The Venus cloud layer may provide a refugium for extant life
that persisted from an earlier more habitable surface environment. We introduce
the Venus Life Equation - a theory and evidence-based approach to calculate the
probability of extant life on Venus, L, using three primary factors of life:
Origination, Robustness, and Continuity, or L = O x R x C. We evaluate each of
these factors using our current understanding of Earth and Venus environmental
conditions from the Archaean to the present. We find that the probability of
origination of life on Venus would be similar to that of the Earth and argue
that the other factors should be nonzero, comparable to other promising
astrobiological targets in the solar system. The Venus Life Equation also
identifies poorly understood aspects of Venus that can be addressed by direct
observations with future exploration missions.
