Plate tectonics on rocky exoplanets: Influence of initial conditions and mantle rheology

Abstract

Several numerical studies have been published in the past years speculating about the existence of plate tectonics on large exoplanets. They focus on aspects like the mass of a planet, the interior heating rate and the occurrence of water in the mantle. Different trends in the propensity for plate tectonics have been observed in particular when varying the planetary mass: with increasing mass the surface mobilization is found to be either more, equally or less likely than on Earth. These studies and their implications are, however, difficult to compare as they assume different initial conditions and parameter sets, and either neglect the pressure effect on the viscosity or assume a rather small influence of the pressure on the rheology. Furthermore, the thermal evolution of the planets (i.e. cooling of core and decrease in radioactive heat sources with time) is typically neglected.In our study, we use a 2D finite volume code and apply a pseudo-plastic rheology. We investigate how a strong pressure-dependence of the viscosity influences not only the convective regime in the lower mantle, but also in the upper mantle and hence the likelihood to obtain plate tectonics. We examine how our results change when either assuming a wet or a dry rheology or when employing different initial conditions, focussing on the initial temperature in the lower mantle and at the core-mantle boundary. We find that the initial temperature conditions have a first-order influence on the likelihood of plate tectonics on large exoplanets. For standard literature values for initial temperatures of super-Earths, surface mobilization is less likely than on Earth, for warm initial temperature the result is vice versa.Simulations that neglect the time-dependence of internal heat sources on the other hand tend to lead to an increasing likelihood of plate tectonics with increasing mass. Finally, our investigations suggest that a wet rheology does not necessarily favor plate tectonics, but – depending on the reference viscosity – may rather lead to a stagnant-lid regime.