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 of
Venus has the amplitude of only tens of centimetres, making the Earth’s hotter twin the least rotationally stable planet in
the 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 body’s inertia tensor that
are comparable in amplitude with the inertia of the rotational bulge. Venus thus may respond to mantle convection by
wobbling (Spada et al., 1996), and wobbling is detectable when both the rotational and the figure axes are measured
accurately. The present-day estimate of the angle between the two axes 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 to 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 of Venus. The reorientation is accompanied by a wobble
whose average amplitude we evaluate and compare to the present day estimate of 0.5° (Konopliv et al., 1999). Since the
different convective regimes predict vastly different rotational dynamics, the comparison provides a useful constraint on
the interior dynamics of Venus. This work was supported by the Czech Science Foundation through project No. 22-20388S.

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