A generic frequency dependence for the atmospheric tidal torque of terrestrial planets

Abstract

Thermal atmospheric tides have a strong impact on the rotation of terrestrial planets. They can lock these planets into an asynchronous rotation state of equilibrium. We aim at characterizing the dependence of the tidal torque resulting from the semidiurnal thermal tide on the tidal frequency, the planet orbital radius, and the atmospheric surface pressure. The tidal torque is computed from full 3D simulations of the atmospheric climate and mean flows using a generic version of the LMDZ general circulation model (GCM) in the case of a nitrogen-dominated atmosphere. Numerical results are discussed with the help of an updated linear analytical framework. Power scaling laws governing the evolution of the torque with the planet orbital radius and surface pressure are derived. The tidal torque exhibits i) a thermal peak in the vicinity of synchronization, ii) a resonant peak associated with the excitation of the Lamb mode in the high frequency range, and iii) well defined frequency slopes outside these resonances. These features are well explained by our linear theory. Whatever the star-planet distance and surface pressure, the torque frequency spectrum -- when rescaled with the relevant power laws -- always presents the same behaviour. This allows us to provide a single and easily usable empirical formula describing the atmospheric tidal torque over the whole parameter space. With such a formula, the effect of the atmospheric tidal torque can be implemented in evolutionary models of the rotational dynamics of a planet in a computationally efficient, and yet relatively accurate way.