Investigating the atmospheric thermal tides on Venus and their consequences for its rotation state.

Abstract

Venus’ rotation is the slowest of all planetary objects in the solar system and is in the retrograde direction. It is commonly admitted that such a rotation state results from the balance between the torques created by solid and atmospheric tides effects (Dobrovolskis et Ingersoll, 1980; Correia et Laskar, 2001, 2003; Revol et al., 2023). The internal viscous friction associated with gravitational tides drives the planet into synchronization (i.e. deceleration toward a tidally locked rotation) while the bulge due to atmospheric thermal tides tends to accelerate the planet out of this synchronization (Correia et Laskar, 2001; Leconte et al., 2015).The atmospheric thermal perturbations arise from the contrast in atmospheric temperature distribution caused by the day-night cycle. This results in a transfer of energy toward cooler regions through atmospheric circulation, leading to variations in atmospheric pressure with high pressures (or density) concentrated in the cooler regions. Because the heat peak of the atmosphere created by the solar insolation occurs in the early afternoon, the atmospheric pressure bulge forms with a delay between its main axis and the Venus-Sun direction (Gold et Soter, 1969; Dobrovolskis et Ingersoll, 1980). This lag creates an atmospheric thermal torque, due to the gravitational attraction of the Sun, which tends to push Venus’ rotation out of synchronization.Using atmospheric pressure simulations, we showed in a previous study (Musseau et al., submitted) that ignoring the topography when evaluating the thermal tides (which was assumed in previous studies (Leconte et al., 2015; Auclair-Desrotour et al., 2017; Revol et al., 2023)) significantly underestimates the amplitude of the atmospheric torque and creates less pronounced variations throughout a Venusian day. Moreover, from the balance between the atmospheric torque and the solid interior torque, we assess that the viscosity of Venus’ deep mantle should range between 3×1020 and 6×1021 Pa.s, about one order of magnitude smaller than Earth’s deep mantle. This estimate relies on the atmospheric torque which has been computed for present-day topography and rotation state.As shown by Correia et Laskar (2001) and, more recently, by Revol et al. (2023), the rotation state (obliquity and rotation rate) may have changed in a recent past, and may still be evolving. Any change in rotation state may affect the strength of the atmospheric tides and hence in return affect the rotation rate. Here, we propose to evaluate the strength of the atmospheric tides using GCM simulations performed with different rotation configurations. First GCM simulations and implications for the rotation evolution will be presented and discussed during the conference.