Venus's Polar Drift as a Probe of its Interior Structure

Gravity data from Magellan revealed Venus's excited spin state: the planet does not rotate around its axis of symmetry (principal axis of inertia), but instead, the spin axis is offset from this principal axis by 0.48° ^[1]. As a consequence, Venus's spin pole is expected to move across the planet's surface (polar motion), but this motion has not yet been detected, since currently available images of Venus's surface lack the resolution necessary to resolve it.

Anticipating the exploration of Venus by ESA's EnVision and NASA's VERITAS, we show in this study how a future measurement of polar motion, combined with a refined measurement of the precession (the spin axis's motion in inertial space), could improve our knowledge of Venus's interior structure by helping to reveal essential properties such as the state and size of its core.

We used a distribution of plausible interior density profiles^[2] and GCM-simulated atmospheric dynamics^[3] to derive Venus's expected polar motion. At a timescale of a few years, the spin pole is expected to drift across the surface at a rate of 21.7 (±2.6) meters per year^[4]. This rate depends largely on the moment of inertia of Venus (for models with a fully solidified core) or that of the mantle only (for models with a liquid core).

This polar drift is the short-term expression of a slow wobble (called the Chandler wobble) of the spin axis around the principal axis, with a period of 13,000-19,000 years. We characterized the wobble's damping, caused by dissipation in Venus's solid tides (pole tide and solar tide). From a range of plausible tidal responses^[5], the damping timescale ranges from 0.8 to 13 million years. Combined with the Chandler frequency acting here as a resonant frequency, we derived the transfer function characterizing how Venus's polar motion responds to an excitation, thus providing a basis for further investigations of long-term excitation processes that could explain the currently observed excited spin state.

^[1] Konopliv et al. (1999), Icarus, doi:10.1006/icar.1999.6086
^[2] Shah et al. (2022), The Astrophysical Journal, doi:10.3847/1538-4357/ac410d
^[3] Lai et al. (2024), JGR Planets, doi:10.1029/2023je008253
^[4] Phan and Rambaux (2025), Astronomy & Astrophysics, doi:10.1051/0004-6361/202553658
^[5] Musseau et al. (2024), Icarus, doi:10.1016/j.icarus.2024.116245