Thermal evolution of Venus

Abstract

The theory of three-dimensional and finite-amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified Boussinesq fluid assumption. The lateral dependences of the variables are resolved through their spherical harmonic representations, whereas their radial and time dependences are determined by numerical procedures. The theory is then applied to produce thermal evolution models for Venus. The emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet. The main conclusions achieved in this paper are (1) a significant portion of the present temperature in the mantle and heat flux at the surface of Venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation, (2) the effective Rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers, (3) the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer, and (4) a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust. The models presented in this paper suggest that Venus has been highly convective during its history until ∼ 0.5 Ga ago. The vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags, floating on the surface, in to the mantle. The rate of cooling of the planet was so high that its core has solidified. In the last 0.5 Ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer. The tectonic style on Venus has, consequently, changed from the recycling of crustal plates to hot spot volcanics. At the present time the planet is completely solid, except in the upper part of its mantle where partial melting may occur.