The Cassini State of the Moon's inner core

Abstract

We present a model of the precession dynamics of the Moon that comprises a fluid outer core and a solid inner core. We show that three Cassini states associated with the inner core exist. The tilt angle of the inner core in each of these states is determined by the ratio between the free inner core nutation frequency ($\omega_{ficn}$) and the precession frequency $\Omega_p = 2\pi/18.6$ yr $^{-1}$. All three Cassini states are possible if $|\omega_{ficn}| > 2\pi/16.4$ yr $^{-1}$, but only one is possible otherwise. Assuming that the lowest energy state is favoured, this transition marks a discontinuity in the tilt angle of the inner core, transiting from $-33^\circ$ to $17^\circ$ as measured with respect to the mantle figure axis, where negative angles indicate a tilt towards the orbit normal. Possible Lunar interior density structures cover a range of $\omega_{ficn}$, from approximately half to twice as large as $\Omega_p$, so the precise tilt angle of the inner core remains unknown, though it is likely large because $\Omega_p$ is within the resonant band of $\omega_{ficn}$. Adopting one specific density model, we suggest an inner core tilt of approximately $-17^\circ$. Viscoelastic deformations within the inner core and melt and growth at the surface of a tilted inner core, both neglected in our model, should reduce this amplitude. If the inner core is larger than approximately 200 km, it may contribute by as much as a few thousandths of a degree on the observed mantle precession angle of $1.543^\circ$.