The viscosity of Venus’ mantle inferred from its rotational state

Abstract

Venus’ retrograde rotation is the slowest of all planetary objects in the solar system. It is commonly admitted that such a rotation state results from the balance between the torques created by solid and atmospheric tides (Dobrovolskis and Ingersol, 1980; Correia and Laskar, 2001; Correia and Laskar, 2003a; Revol et al. 2023). The internal viscous friction associated with gravitational tides drives the planet into synchronization (i.e. deceleration to a tidally locked rotation) while the bulge due to atmospheric thermal tides tends to accelerate the planet out of this synchronization (Correia and Laskar, 2001; Leconte et al., 2015). The purpose of this work is first to provide an estimate of the viscosity of Venus’ mantle explaining the current balance with atmospheric forcing. A second goal is to quantify the impact of the internal structure and its past evolution on the rotation history of Venus.

Using atmospheric pressure simulations, we first provide an estimate of the atmospheric thermal torque value contrasting with previous estimates (Leconte et al., 2015). Computing the viscoelastic response of the interior to gravitational tides and to atmospheric loading (Dumoulin et al., 2017; Kervazo et al., 2021), we show that the current viscosity of Venus’ lower mantle must range between 2 × 10²⁰ Pa s and 6 × 10²¹ Pa s to explain a rotation in equilibrium. We then investigate the possible past evolution of Venus’ rotation by considering simple viscosity and thermal evolution paths. We show that in absence of additional dissipation processes, viscous friction cannot slow down Venus’ rotation to its current state from an initial rotation period shorter than 1 day.