Letter to the Editor

Orbital evolution of Saturn’s satellites due to the interaction between the moons and the massive rings

¹ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
e-mail: nakajima.a.ah@m.titech.ac.jp
² Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan
e-mail: ida@elsi.ac.jp
³ Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan
⁴ ISAS, JAXA, Kanagawa, Japan
e-mail: y.ishigaki@stp.isas.jaxa.jp

Received: 25 June 2020
Accepted: 20 July 2020

Abstract

Context. Saturn’s mid-sized moons (satellites) have a puzzling orbital configuration with trapping in mean-motion resonances with every-other pairs (Mimas-Tethys 4:2 and Enceladus-Dione 2:1). To reproduce their current orbital configuration on the basis of a recent model of satellite formation from a hypothetical ancient massive ring, adjacent pairs must pass first-order mean-motion resonances without being trapped.

Aims. The trapping could be avoided by fast orbital migration and/or excitation of the satellite’s eccentricity caused by gravitational interactions between the satellites and the rings (the disk), which are still unknown. In our research we investigate the satellite orbital evolution due to interactions with the disk through full N-body simulations.

Methods. We performed global high-resolution N-body simulations of a self-gravitating particle disk interacting with a single satellite. We used N ∼ 10⁵ particles for the disk. Gravitational forces of all the particles and their inelastic collisions are taken into account.

Results. Dense short-wavelength wake structure is created by the disk self-gravity and a few global spiral arms are induced by the satellite. The self-gravity wakes regulate the orbital evolution of the satellite, which has been considered as a disk spreading mechanism, but not as a driver for the orbital evolution.

Conclusions. The self-gravity wake torque to the satellite is so effective that the satellite migration is much faster than was predicted with the spiral arm torque. It provides a possible model to avoid the resonance capture of adjacent satellite pairs and establish the current orbital configuration of Saturn’s mid-sized satellites.