Cupid Is Not Doomed Yet: On the Stability of the Inner Moons of Uranus

TL;DR

This study reevaluates the stability of Uranus's inner moons, revealing that the Belinda group can be stable for hundreds of millions of years due to orbital resonance, challenging previous predictions of rapid instability.

Contribution

It demonstrates that the Belinda group of Uranus's moons can remain stable over 10^8 years because of an orbital resonance, using updated orbital data and considering tidal effects.

Findings

Belinda-Perdita resonance contributes to long-term stability.

Tidal dissipation alone cannot form the resonance.

The Portia group may be destabilized by convergent migration.

Abstract

Some of the small inner moons of Uranus have very closely-spaced orbits. Multiple numerical studies have found that the moons Cressida and Desdemona, within the Portia sub-group, are likely to collide in less than 100 Myr. The subsequent discovery of three new moons (Cupid, Perdita, and Mab) made the system even more crowded. In particular, it has been suggested that the Belinda group (Cupid, Belinda, and Perdita) will become unstable in as little as 10$^5$ years. Here we revisit the issue of the stability of the inner moons of Uranus using updated orbital elements and considering tidal dissipation. We find that the Belinda group can be stable on $10^8$-year timescales due to an orbital resonance between Belinda and Perdita. We find that tidal evolution cannot form the Belinda-Perdita resonance, but convergent migration could contribute to the long-term instability of the Portia group. We propose that Belinda captured Perdita into the resonance during the last episode of disruption and re-accretion among the inner moons, possibly hundreds of Myr ago.