  Present-day Venus is an inhospitable place with surface temperatures
approaching 750K and an atmosphere over 90 times as thick as present day
Earth's. Billions of years ago the picture may have been very different. We
have created a suite of 3D climate simulations using topographic data from the
Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion
years ago, present day Venus orbital parameters, an ocean volume consistent
with current theory and measurements, and an atmospheric composition estimated
for early Venus. Using these parameters we find that such a world could have
had moderate temperatures if Venus had a rotation period slower than about 16
Earth days, despite an incident solar flux 46-70% higher than modern Earth
receives. At its current rotation period of 243 days, Venus's climate could
have remained habitable until at least 715 million years ago if it hosted a
shallow primordial ocean. These results demonstrate the vital role that
rotation and topography play in understanding the climatic history of
exoplanetary Venus-like worlds being discovered in the present epoch.
