Influence of topography on atmospheric thermal tides and rotational evolution of Venus

With a period of 243 days, Venus’ rotation is the slowest of all the planets in the solar system and is in the retrograde direction. Such a rotational state results directly from several tidal effects that impact both the atmosphere and the solid part of the planet. Therefore, to fully characterize the rotational dynamic and evolution of Venus, it is mandatory to have a comprehensive understanding of the factors that influence the different tidal effects controlling the rotation (Dobrovolskis et Ingersoll, 1980; Correia et Laskar, 2003; Revol et al., 2023, Musseau et al., 2024), and particularly the atmospheric thermal tides.

Topography is known to play a crucial role in modulating surface pressures and temperatures, as well as influencing atmospheric circulation, and is therefore also expected to influence the amplitude and temporal variability of atmospheric thermal tides. This aspect is particularly relevant for studies of the evolution of Venus' rotation because the topography may have changed significantly over a short geological timescale (< 1 Gyr) through resurfacing events or true polar wander (TPW). Using the Venus Planetary Climate Model (Lebonnois et al., 2016), we investigated how the distribution of the topography affects the atmospheric thermal tides and, consequently, the rotational state of the planet. Because the past topography of Venus is unknown, we focused our study on the effect of a true polar wander acting on the present-day topography. In this framework, the current topography is preserved, but its position relative to the rotation axis is modified. This approach allows us to isolate the effect of large-scale reorientation without introducing speculative topographies.

This work reveals the link between topography and thermal tides, showing that the variations of the atmospheric thermal torque over a solar day are mainly controlled by the near-equatorial large-scale topography. Moreover, we show that changes in the topography not only introduce strong diurnal variations in the torque but also change its daily average value and impact the overall balance between tides. Therefore, these results show that topographic distribution is a key element in the study of the rotational state of the planet. In addition, we update the empirical law that links atmospheric thermal torque to the forcing frequency from Leconte et al. (2015) and show that the presence of topography affects this relationship and may have influenced Venus' rotational evolution.

These results highlight that the present-day rotation of Venus is likely the outcome of a complex interplay between internal and external interactions. A comprehensive understanding of the rotational history of planets and exoplanets therefore requires better constraints on the coupling between internal dynamics, surface properties, and atmospheric processes, as well as their combined influence on tides. Therefore, this study brings out the necessity to precisely model the dynamics of atmospheric thermal tides and to include all the elements that can affect them. In addition, our results highlight the crucial importance of understanding how Venus' topography has evolved throughout the planet's history, which, in turn, is directly related to the planet's resurfacing history, tectonics, and surface rock composition. For this purpose, the upcoming mission EnVision (ESA) aims, in part, to improve our knowledge of the atmosphere, the surface, and the internal properties of Venus and better constrain the different tidal effects.

References:

Correia, A. C. et J. Laskar (2003). “Different Tidal Torques on a Planet with a Dense Atmosphere and Consequences to the Spin Dynamics”. Journal of Geophysical Research: Planets 108(E11), 2003JE002059.

Dobrovolskis, A. R. et A. P. Ingersoll (1980). “Atmospheric Tides and the Rotation of Venus I. Tidal Theory and the Balance of Torques”. Icarus 41(1), 1–17.

Lebonnois, S., N. Sugimoto et G. Gilli (2016). “Wave Analysis in the Atmosphere of Venus below 100-Km Altitude, Simulated by the LMD Venus GCM”. Icarus 278, 38–51.

Leconte, J., H. Wu, K. Menou et N. Murray (2015). “Asynchronous Rotation of Earth-mass Planets in the Habitable Zone of Lower-Mass Stars”. Science 347(6222), 632–635.

Musseau, Y., Tobie, G., Dumoulin, C., Gillmann, Revol, A., Bolmont, E. (2024). “The viscosity of Venus’ mantle inferred from its rotational state”.  Icarus, 422, 116245

Revol, A., E. Bolmont, G. Tobie, C. Dumoulin, Y. Musseau, S. Mathis, A. Strugarek et A. Brun (2023). “Spin Evolution of Venus-like Planets Subjected to Gravitational and Thermal Tides”. Astronomy & Astrophysics