The spin and inertia of Venus

One of the most remarkable rotational features of Venus is the presence of a relatively large offset between spin and maximum inertia axis. According to the most recent gravity field models, this offset may amount to ∼0.5°, considerably larger than the one which characterizes the Earth's Chandler wobble (∼0.3″). At present, it is not clear whether the offset of Venus is the result of the limited resolution of the gravity models or, alternatively, it may be regarded as an evidence in support to the existence of a wobble. Purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time‐scales of mantle convection, due to the peculiar rotational characteristics of Venus. We also show that the inertia perturbation associated to the rising of hot thermal plumes from the core‐mantle boundary to the surface is large enough to maintain offsets ranging between 0.1 and 1°. Of course, this does not preclude that other processes, such as Venusquakes or periodic atmospheric variabilities, may contribute significantly to the observed offset on shorter time‐scales.