Quo Vadis, the Spin Pole of Venus?

Reorientation of Mars due to volcanic activity in the Tharsis region, adopted from Bouley, S., Baratoux, D., Matsuyama, I. et al.: Late Tharsis formation and implications for early Mars. Nature 531, 344–347 (2016). https://doi.org/10.1038/nature17171

For example, Mars was reoriented by 20 degrees when Olympus Mons, the largest volcano in the Solar System, was born. Or at least the orientation of valleys carved into the Martian surface by rivers in the Noachian period indicates so. Today, the 22 km high shield volcano is near the equator, far from the rotational axis where its position is stable. Venus seemed to be an odd bird in this respect. It spins around its axis in the opposite direction than other planets, and does so only slowly, one turn lasting 243 days. Due to the extremely slow rotation, Venus practically does not have an equatorial bulge – flattening that other planets have, which stabilizes their rotation (on Earth, the polar and equatorial radii differ by 21 km). It was thus believed that, unlike other planets, Venus “mega-wobbles” – that it changes its orientation cyclically while the rotational pole wanders along big circles on Venus’ surface.

However, a new scientific study, pursued in cooperation between Prague’s Charles University and German Aerospace Center (DLR), shows that this is unlikely. The results stem from numerical simulations in which, for the first time, 3D models of mantle convection are coupled with non-linear polar motion dynamics. “On geological time scales, Venus should reorient quite similarly to fast rotators such as Earth or Mars. But it is difficult to confirm. While on Earth we have precise measurements of polar motion from over a century, getting the data for Venus is complicated,” says Vojtěch Patočka, lead author of the study, published in the flagship journal of the American Geophysical Union, AGU Advances.

Numerical simulation of mantle convection, with indicated position of the rotation axis (ω) and the principal axis of inertia – the figure axis of Venus (MIA). The observed angular offset of both axes is approximately 0.5 degrees, several orders of magnitude larger than on Earth.

The performed simulations also helped to determine a scaling law (equation) for the angular offset between the rotational and figure axes of the planet. The observed value of this angular separation is anomalously large on Venus and was typically associated with the hypothesis of the aforementioned mega-wobble – a theory not supported by the present study. So what actually causes the angular offset of both axes then?

“The derived law predicts that mantle convection on Venus would have to be really atypical to force both axes apart from each other to the extent at least comparable with the observations,” says Ana-Catalina Plesa, mantle convection specialist from DLR. “Although the surface of Venus shows signs of tectonics that are unique in the Solar System, and hence its mantle flow may be full of surprises, the explanation for the angular offset probably lies elsewhere,” concludes Plesa.

One possibility is to look into the thick and hot atmosphere of Venus. The atmospheric pressure on Venus is comparable to that one kilometer below the surface of Earth's oceans, which is why no rovers drive there like on Mars. This hot and dense mass flows around the surface of Venus and can affect both its rotation and reorientation, a hypothesis the research team wants to address in future work.

Contact details:
Mgr. RNDr. Vojtěch Patočka, Ph.D.
vojtech.patocka@matfyz.cuni.cz

References:
Patočka, V., Maia, J., Plesa, A. C. (2025): Polar motion dynamics on slow-rotating Venus: signatures of mantle flow, AGU Advances
https://doi.org/10.1029/2025AV001976