Ellipsoidal equilibrium figure and Cassini states of rotating planets and satellites deformed by a tidal potential in the spatial case

TL;DR

This paper derives the ellipsoidal equilibrium figure of tidally deformed rotating bodies with arbitrary spin orientation, revealing effects on precession, nutation, and Cassini states, and clarifying classical assumptions in planetary dynamics.

Contribution

It provides a general analytical model for the equilibrium shape and Cassini states of tidally deformed bodies with arbitrary spin orientation, extending previous models limited to planar cases.

Findings

The equilibrium shape depends on the angle between spin and radius vector.

The equilibrium ellipsoid's vertex does not point toward the companion, affecting torque calculations.

Tidally deformed bodies share Cassini states with rigid bodies, with no secular effects at first order.

Abstract

The equilibrium figure of an inviscid tidally deformed body is the starting point for the construction of many tidal theories such as Darwinian tidal theories or the hydrodynamical Creep tide theory. This paper presents the ellipsoidal equilibrium figure when the spin rate vector of the deformed body is not perpendicular to the plane of motion of the companion. We obtain the equatorial and the polar flattenings as functions of the Jeans and the Maclaurin flattenings, and of the angle $\theta$ between the spin rate vector and the radius vector. The equatorial vertex of the equilibrium ellipsoid does not point toward the companion, which produces a torque perpendicular to the rotation vector, which introduces terms of precession and nutation. We find that the direction of spin may differ significantly from the direction of the principal axis of inertia $C$, so the classical approximation $\mathsf{I}\vec{\omega}\approx C\vec{\omega}$ only makes sense in the neighborhood of the planar problem. We also study the so-called Cassini states. Neglecting the short-period terms in the differential equation for the spin direction and assuming a uniform precession of the line of the orbital ascending node, we obtain the same differential equation as that found by Colombo (1966). That is, a tidally deformed inviscid body has exactly the same Cassini states as a rotating axisymmetric rigid body, the tidal bulge having no secular effect at first order.