Tectonic and Thermal Evolution of Venus and the Role of Volatiles: Implications for Understanding the Terrestrial Planets

Venus is similar to Earth in size and bulk composition. The dramatic differences between the two planets indicate that planetary size alone does not control geologic evolution. Earth's geology is dominated by plate tectonics, or active lid convection. The crater retention age of Venus demonstrates that the planet has been very geologically active in the last b.y., but there is no evidence of active plate tectonics. Instead the surface is dominated by diverse volcanism and tectonism, and highlands interpreted as mantle upwellings and downwellings. Venus' surface is much hotter and drier than Earth's, suggesting that the interior may be drier as well. The volatile content has a major effect on both the rheology of the interior and predicted melt products. The low water content of the interior may create a stronger lithosphere and preclude asthenosphere formation, leading to the development of a stagnant rather than an active lid convective regime. A stable, stagnant lid fosters development of phase changes and other density anomalies that can lead to delamination and associated opportunities for melting. Additionally, the thicker boundary layer formed in a stagnant lid regime may increase the number of expected plumes. Mars also is in a stagnant lid regime, probably due to early heat loss rather than volatile loss. Here we discuss what is known about interior volatile content and explore the implications of a dry interior for volcanism, tectonism, and interior convection as a working hypothesis to explain the profound differences in the evolution of Venus' Earth, and Mars.