Deviation of Mercury's spin axis from an exact Cassini state induced by dissipation

TL;DR

This paper predicts how Mercury's spin axis deviates from a Cassini state due to various dissipation mechanisms, with implications for understanding its interior properties and mantle viscosity.

Contribution

It provides a detailed model of Mercury's spin axis deviation caused by tidal, viscous, and electromagnetic dissipation, incorporating elastic deformations and core-mantle boundary effects.

Findings

Maximum phase lead of 0.027 arcsec for small inner core

Maximum phase lag of 0.1 arcsec for large inner core

Mantle viscosity lower limit of ~10^{17} Pa s based on observations

Abstract

We compute predictions of the deviation of Mercury's spin axis from an exact Cassini state caused by tidal dissipation, and viscous and electromagnetic (EM) friction at the core-mantle boundary (CMB) and inner core boundary (ICB). Viscous friction at the CMB generates a phase lead, viscous and EM friction at the ICB produce a phase lag; the magnitude of the deviation depends on the inner core size, kinematic viscosity and magnetic field strength, but cannot exceed an upper bound. For a small inner core, viscous friction at the CMB results in a maximum phase lead of 0.027 arcsec. For a large inner core (radius $>1000$ km), EM friction at the ICB generates the largest phase lag, but it does not exceed 0.1 arcsec. Elastic deformations induced by the misaligned fluid and solid cores play a first order role in the phase lead/lag caused by viscous and EM coupling, and contribute to a perturbation in mantle obliquity on par with that caused by tidal deformations. Tidal dissipation results in a phase lag and its magnitude (in units of arcsec) is given by the empirical relation (80/Q), where Q is the quality factor; Q=80 results in a phase lag of ~1 arcsec. A large inner core with a low viscosity of the order of 10^{17} Pa s or lower can significantly affect $Q$ and thus the resulting phase lag. The limited mantle phase lag suggested by observations (<10 arcsec) implies a lower limit on the bulk mantle viscosity of approximately 10^{17} Pa s.