Subsolidus convective cooling histories of terrestrial planets

Abstract

The subsolidus convective cooling histories of terrestrial planets evolving from hot initial states are investigated with a simple analytic model which simulates the average heat transport in a vigorously convecting mantle devoid of internal heat sources. The temperature dependence of the effective viscosity of mantle rocks is the single most important factor controlling thermal history. It is responsible for the growth of the rigid lithosphere, a rheological and thermal boundary layer, and serves the function of a thermostat, regulating the rate of cooling by the negative feedback between viscosity and temperature. Except for a relatively short period of time when mantle temperature decreases rapidly during the early stages of cooling, a planet cools mainly by thickening its lithosphere; the underlying mantle temperature decreases relatively slowly. On one-plate planets, the growth of a rigid lithosphere involves an imbalance between the surface heat flux and the heat flow from the mantle; the former is always larger than the latter. Primordial heat can contribute substantially, e.g., as much as about a fourth or a third, to the present surface heat flux of a planet. For both these reasons, the radiogenic heat source content of a planet is likely to be overestimated by inferences from surface heat flow observations.