Venusian Habitable Climate Scenarios: Modeling Venus through time and
applications to slowly rotating Venus-Like Exoplanets
Abstract
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. This leads to the conclusion that Venus’
present state is the result of a runaway greenhouse from a gradually
warming sun since its formation. 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 (~100 million year) primordial magma ocean
with a CO2 and steam atmosphere because of its closer proximity to the
Sun. This would differ from Earth’s evolution since Earth’s magma ocean
lifetime was probably around 1 million years. Venus’ long lived steam
atmosphere would allow sufficient time to dissociate most of the water
vapor, allow significant hydrogen escape and oxidize the magma ocean.
Maybe the high deuterium to hydrogen ratio is from this period. However,
a dearth of observational data due to a lack of missions to Venus makes
it impossible to choose between these two scenarios. Using a general
circulation model we demonstrate the viability of the optimistic
scenario using the few observational constraints available. 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.”