Investigating the influence of topography and rotation rate of Venus on atmopsheric thermal tides

Venus’ rotation is the slowest of all the planets 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 (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 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 higher surface pressure anomalies concentrated in the cooler regions and lower surface pressure anomalies in the warmer regions. Because the atmospheric heat peak 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 Global Climate Model (GCM) numerical simulations, we showed in a previous study (Musseau et al., 2024) that ignoring the topography when evaluating the thermal tides (like 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 its variations throughout a Venusian day. Quantifying the effect of topography is mandatory to correctly estimate both past Venus’ rotational evolution and current dynamical signatures of atmopsheric tides. To better understand the coupling between thermal tides and topography, we performed a series of atmospheric simulations using the Venus Planetary Climate Model (VPCM) (Lebonnois et al., 2016), exploring various configurations of the topography. Our results highlight the link between topography and thermal tides, showing that the variations of the torque over a day are mainly controlled by the near equator altitude at the subsolar longitude.

As shown by Correia et Laskar (2001) and, more recently, Revol et al. (2023), the rotation state (obliquity and rotation rate) may have changed in a recent past, and may be still evolving. Any change in rotation state may affect the strength of the atmospheric tides and hence in return affect the rotation rate (Leconte et al., 2015). Here, we evaluate the strength of the atmospheric tides using VPCM simulations performed with different rotation periods. First GCM simulations and their implications for the rotation evolution will be presented and discussed during the conference.