Venus' rotation and atmospheric tides

Abstract

VENUS rotates with a period of 243.00 ± 0.04 d retrograde¹. The obliquity, or angle between the spin vector and orbit vector is 178 ± 1° (ref. 1). Both the long period and the near 180° obliquity suggest that the spin has evolved under the influence of tidal torques. Tides raised by the Sun in the body of Venus would de-spin the planet in ∼10⁸ yr if no other torques were acting². The final state would be one of synchronous rotation (one side always facing the Sun). So either we are living during the final stages of Venus' tidal evolution, an unlikely circumstance, or else Venus has already reached a stable equilibrium in which other influences balance the solar body tide. It is possible that the Earth has ‘captured’ Venus into a resonance in which the spin period is 243.165 d. However, for this resonance to be a stable equilibrium, either Venus must have a gravitational field that is ∼10 times ‘rougher’ than the Earth's², a possibility that is largely ruled out by direct observation³, or a third torque must balance the body torque due to the Sun. Tides in the atmosphere driven by periodic solar heating could supply the necessary third torque^4–6, but no quantitative theory has previously been published. Such a theory⁷ is presented here, in which we argue that the current rotation is a stable balance between atmospheric and solar body tides.