The thermal regime of Venus

Abstract

Models for the thermal evolution of Venus are constructed numerically. The mantle is proposed to consist of two independently convecting layers (upper and lower mantles); the floating crust keeps the temperature of the upper surface of the convecting mantle near 1200°C and the core may solidify at temperatures below the liquidus. Previously, these models were calculated with the usual parameterization of convection based mostly on investigations of constant-viscosity convection. Now this parameterization is modified to take into account new numerical investigations of convection in media with complicated rheology. The thermal evolution of Venus is assumed to have begun 4.6 billion years (b.y.) ago. It can be divided into three periods: (1) adjustment of the upper mantle to the thermal regime of the lower mantle (∼0.5 b.y.); (2) transition of the entire mantle to the asymptotic regime (∼3–4 b.y.); (3) asymptotic regime. Parameters of the convecting planet in the asymptotic regime do not depend on initial conditions (the planet “forgets” its initial state) and are found analytically. The present-day heat flux is equal to ∼50 erg cm −2s −1. The results are close to those obtained previously. Comparison of the present thermal models of Venus and Earth with the recent studies of melting of the Fe FeS system suggest that the absence of a dipole magnetic field is closely connected to the absence of solidification of the Venusian core at present. The position of the iron triple point (γ-ε-liquid) makes this conclusion very probable. The thermal regime of the Venusian crust is also discussed. Convection in the lower part of the crust is shown to play a role in certain regions with specific crustal composition. The prevailing mechanism of heat transfer to the surface is advection by magmas produced by melting the lower layers of the crust.