New constraints on Saturn's interior from Cassini astrometric data

TL;DR

This study uses over a century of astrometric data, including Cassini observations, to precisely determine Saturn's tidal parameters, especially its Love number and dissipation rate, providing new constraints on the planet's interior structure.

Contribution

First determination of Saturn's Love number from moon orbit data, reducing model uncertainties and offering insights into the planet's internal composition and dissipation mechanisms.

Findings

Saturn's Love number $k_2=0.390 1.024$ is higher than traditional models.

High tidal dissipation rate $k_2/Q=(1.59 0.74) imes 10^{-4}$ confirmed.

Potential frequency-dependent dissipation processes suggested.

Abstract

Using astrometric observations spanning more than a century and including a large set of Cassini data, we determine Saturn's tidal parameters through their current effects on the orbits of the eight main and four coorbital moons. We have used the latter to make the first determination of Saturn's Love number, $k_2=0.390 \pm 0.024$, a value larger than the commonly used theoretical value of 0.341 (Gavrilov & Zharkov, 1977), but compatible with more recent models (Helled & Guillot, 2013) for which $k_2$ ranges from 0.355 to 0.382. Depending on the assumed spin for Saturn's interior, the new constraint can lead to a reduction of up to 80% in the number of potential models, offering great opportunities to probe the planet's interior. In addition, significant tidal dissipation within Saturn is confirmed (Lainey et al., 2012) corresponding to a high present-day tidal ratio $k_2/Q=(1.59 \pm 0.74) \times 10^{-4}$ and implying fast orbital expansions of the moons. This high dissipation, with no obvious variations for tidal frequencies corresponding to those of Enceladus and Dione, may be explained by viscous friction in a solid core, implying a core viscosity typically ranging between $10^{14}$ and $10^{16}$ Pa.s (Remus et al., 2012). However, a dissipation increase by one order of magnitude at Rhea's frequency could suggest the existence of an additional, frequency-dependent, dissipation process, possibly from turbulent friction acting on tidal waves in the fluid envelope of Saturn (Ogilvie & Li, 2004). Alternatively, a few of Saturn's moons might themselves experience large tidal dissipation.