Effect of the inclination in the passage through the 5/3 mean motion resonance between Ariel and Umbriel

Abstract

The orbits of the main satellites of Uranus are expected to slowly drift away owing to tides raised in the planet. As a result, the 5/3 mean motion resonance between Ariel and Umbriel was likely encountered in the past. Previous studies have shown that, in order to prevent entrapment in this resonance, the eccentricities of the satellites must be larger than ~0.01 at the epoch, which is hard to explain. On the other hand, if the satellites experience some temporary capture and then escape, the inclinations rise to high values that are not observed today. We have revisited this problem both analytically and numerically focussing on the inclination, using a secular two-satellite model with circular orbits. We show that if the inclination of Umbriel was around 0.15° at the time of the 5/3 resonance encounter, capture can be avoided in about 60% of the cases. Moreover, after the resonance crossing, the inclination of Umbriel drops to a mean value around 0.08°, which is close to the presently observed one. The final inclination of Ariel is distributed between 0.01° and 0.25° with a nearly equal probability, which includes the present mean value of 0.02°.