The Obliquity of Venus

Solar gravitational tides tend to reduce the obliquity of Venus from its present value near 1800 whether or not Venus is locked in the synodic spin resonance with the Earth. Both a thermally driven atmospheric tidal torque and dissipation of energy at the boundary between a rigid mantle and a differentially rotating liquid core are possible mechanisms for maintaining the retrograde spin. The latter mechanism, where precession of the spin vector about the orbit normal induces the differential rotation, is almost certaintly capable of driving the obliquity to 1800 from values greater than 900 for a wide range of reasonable core viscosities and spin angular velocities.