  One popular view of Venus' climate history describes a world that has spent
much of its life with surface liquid water, plate tectonics, and a stable
temperate climate. Part of the basis for this optimistic scenario is the high
deuterium to hydrogen ratio from the Pioneer Venus mission that was interpreted
to imply Venus had a shallow ocean's worth of water throughout much of its
history. Another view is that Venus had a long lived (approximately 100 million
year) primordial magma ocean with a CO2 and steam atmosphere. Venus' long lived
steam atmosphere would sufficient time to dissociate most of the water vapor,
allow significant hydrogen escape and oxidize the magma ocean. A third scenario
is that Venus had surface water and habitable conditions early in its history
for a short period of time (< 1 Gyr), but that a moist/runaway greenhouse took
effect because of a gradually warming sun, leaving the planet desiccated ever
since. Using a general circulation model we demonstrate the viability of the
first scenario using the few observational constraints available. We further
speculate that Large Igneous Provinces and the global resurfacing 100s of
millions of years ago played key roles in ending the clement period in its
history and presenting the Venus we see today. The results have implications
for what astronomers term "the habitable zone," and if Venus-like exoplanets
exist with clement conditions akin to modern Earth we propose to place them in
what we term the "optimistic Venus zone."
