Variations in the rotation rate of Venus due to orbital eccentricity modulation of solar tidal torques

Abstract

The rotational state of Venus is presumably determined by a balance between two different solar tidal torques: an atmospheric thermal tide yields a torque which acts in the same direction as the retrograde rotation of Venus and a gravitational solid body tide yields a torque which opposes the current rotation. The strengths of these opposing torques both depend on the eccentricity of the orbit of Venus but with different sensitivities. Thus variations in the orbital eccentricity cause fluctuations in the rotation rate at which the tidal torques balance. If Venus is considered to be a single rigid body, the rotational velocity will change by amounts equivalent to ∼1 km/yr at the equator, over timescales of 106 years. Allowance for differential rotation between the core, mantle, and atmosphere increases the range over which the solid surface rotation rate varies. For plausible values of the strength of viscous coupling at the atmosphere‐mantle boundary and the core‐mantle boundary the variations in solid surface rotational velocity can be 2–3 times larger than in the rigid coupling case. When these long‐period forced variations in rotation rate are considered, the present rotation rate of Venus is seen to be close enough to a resonant spin‐orbit interaction with the Earth that it may occasionally pass through that resonance.