The link between internal and rotational dynamics of Venus: The amplitude of mantle convection-driven wobble

The spin period of Venus is anomalously large. With one Venusian day being 243 Earth days, the rotational bulge ofVenus has the amplitude of only tens of centimetres, making the Earths hotter twin the least rotationally stable planet inthe Solar System. Being a slow-rotator creates a unique link between internal and rotational dynamics. This is because,on a slow-rotator, convection driven redistribution of mass may produce perturbations of the bodys inertia tensor thatare comparable in amplitude with the inertia of the rotational bulge. Venus thus may respond to mantle convection bywobbling (Spada et al., 1996), and wobbling is detectable when both the rotational and the figure axes are measuredaccurately. The present-day estimate of the angle between the two axes is 0.5, but it is based on gravity models with alimited resolution (Konopliv et al., 1999). Future missions to Venus, namely VERITAS and EnVision, are likely to providea more robust measurement. The geodynamic regime of Venus mantle remains enigmatic. Observational data does not support the existence ofcontinuous plate tectonics on its surface, but some recent evidence of ongoing tectonic and volcanic activity (e.g. Herrickand Hensley, 2023) and crater statistics analyses (e.g. O'Rourke et al., 2014) indicate that the planet is unlikely to be in astagnant lid regime (see also Rolf et al., 2022). Here we perform 3D spherical mantle convection simulations of the differentpossible tectonic scenarios and compute the resulting reorientation of Venus. The reorientation is accompanied by a wobblewhose average amplitude we evaluate and compare to the present day estimate of 0.5 (Konopliv et al., 1999). Since thedifferent convective regimes predict vastly different rotational dynamics, the comparison provides a useful constraint onthe interior dynamics of Venus. This work was supported by the Czech Science Foundation through project No. 22-20388S. ReferencesHerrick, R., Hensley, S., 2023. Surface changes observed on a venusian volcano during the magellan mission. Sciencedoi:10.1126/science.abm7735. Konopliv, A., Banerdt, W., Sjogren, W., 1999. Venus gravity: 180th degree and order model. Icarus 139, 318.doi:10.1006/icar.1999.6086. O'Rourke, J.G., Wolf, A.S., Ehlmann, B.L., 2014. Venus: Interpreting the spatial distribution of volcanically modifiedcraters. Geophys. Res. Lett. 41, 82528260. doi:10.1002/2014gl062121. Rolf, T., Weller, M., Gulcher, A., Byrne, P., ORourke, 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, 22532266. doi:10.1029/95JE03222.