Linking internal and rotational dynamics: the amplitude of mantle convection driven wobble of Venus

With one Venusian day being 243 Earth days, the rotational bulge of Venus has the thickness of a few tens of centimetres only, making the Earth’s hotter twin the least rotationally stable planet in the Solar System. There could be a unique link between internal and rotational dynamics on such slowly rotating bodies. This is because the redistribution of mass driven by mantle convection produces perturbations of the body’s inertia tensor that are comparable in amplitude with those associated with the rotational flattening. In effect, Venus may respond to mantle convection by large-amplitude wobbling (Spada et al., 1996), that is, the orientation of Venus with respect to its rotation axis may cyclically change. Wobbling is detectable when both the rotational and the figure axes are measured accurately. The present-day estimate for the angle between the two axes, i.e. the wobble amplitude, is 0.5°, but it is based on gravity models with a limited resolution (Konopliv et al., 1999). Future missions to Venus, namely VERITAS and EnVision, are likely provide a more robust measurement.

The geodynamic regime of Venus’ mantle remains enigmatic. Observational data does not support the existence of continuous plate tectonics on its surface, but some recent evidence of ongoing tectonic and volcanic activity (e.g. Herrick and Hensley, 2023) and crater statistics analyses (e.g. O'Rourke et al., 2014) indicate that the planet is unlikely to be in a stagnant lid regime (see also Rolf et al., 2022).

Here we perform 3D spherical mantle convection simulations of the different possible tectonic scenarios and compute the resulting reorientation (or true polar wander, TPW) of Venus. The TPW path is accompanied with a wobble whose average amplitude we evaluate and compare to the present day estimate of 0.5° (Konopliv et al., 1999). We show that it is unlikely that the present-day wobble of Venus is triggered by mantle convection. For most simulated scenarios, the convection-induced wobble has at least one order of magnitude smaller amplitude when compared to the observed value.
The wobble amplitude is proportional to the rate at which the main inertia direction of mantle convection (MID-MC) changes – the largest wobble is thus obtained in cases with rapid surface mobilization. In simulations with a catastrophic resurfacing, the MID-MC rate reaches its maximum during the lithospheric overturn, and the convection-induced wobble gets closer to the observed value. In a few millions of years after the resurfacing event, however, the wobble amplitude drops again.

References
Herrick, R., Hensley, S., 2023. Surface changes observed on a venusian volcano during the magellan mission. Science
doi:10.1126/science.abm7735.

Konopliv, A., Banerdt, W., Sjogren, W., 1999. Venus gravity: 180th degree and order model. Icarus 139, 3–18. doi:10.1006/icar.1999.6086.

O'Rourke, J.G., Wolf, A.S., Ehlmann, B.L., 2014. Venus: Interpreting the spatial distribution of volcanically modified craters. Geophys. Res. Lett. 41, 8252–8260. doi:10.1002/2014gl062121.

Rolf, T., Weller, M., G ̈ulcher, A., Byrne, P., O’Rourke, J.G., Herrick, R., Bjonnes, E., Davaille, A., Ghail, R., Gillmann, C., Plesa, A.C., Smrekar, S., 2022. Dynamics and evolution of venus’ mantle through time. Space Science Reviews 218, 70. doi:10.1007/s11214-022-00937-9.

Spada, G., Sabadini, R., Boschi, E., 1996. Long-term rotation and mantle dynamics of the Earth, Mars, and Venus. J. Geophys. Res. Planets 101, 2253–2266. doi:10.1029/95JE03222.