Polar Motion Dynamics on Slow-Rotating Venus: Signatures of Mantle Flow

Abstract

With its 1 day lasting 243 days on Earth, Venus is the slowest-spinning planet in the Solar System and its rotational bulge is anomalously small. A rotational bulge stabilizes the orientation of planets. Having only a tiny stabilizer, the rotational pole of Venus has been expected to separate from the figure pole in response to mantle flow, which has been used to explain why both poles are observed to be 0.5 degrees apart. Here, we couple 3D mantle-convection simulations and polar motion dynamics to explore how mantle flow, and in particular surface mobilization, drives Venus's polar motion. We provide a predictive framework for polar motion on slow rotators and show that the spin/figure pole separation (or offset) follows a simple law: it scales with the figure-axis drift rate times the planet's Chandler period. Contrary to prior expectations, stronger internal loading does not amplify the offset, and the mantle-driven polar motion is smooth rather than wobbly, more similar to that of fast rotators. In models matching Venus's geoid, figure-axis drift rates reach only up to a few degrees/Myr, too slow compared to ca. 60 degrees/Myr that is needed to match the observed offset. We therefore exclude mantle convection as the cause of Venus' spin and figure poles separation, and suggest that atmospheric and solid tides are not balanced instead.