Secular variation of the mean motion ratio of Ariel-Umbriel (violet curve) and Umbriel-Titania (green curve) in the two scenarios considered in Sect. 4, obtained using t_α,2 = 4.2 Gyr and t_α,4 = 80 Gyr (left) or 10.4 Gyr (right). The plots are computed as in Sect. 3, similarly to that in Fig. 1. The two evolutions assume that the two inner moons were locked into the 2:1 mean motion resonance and the crossing of the three-body resonant chain 4:2:1 (left) or 3:2:1 (right) with Titania succeeded in breaking the resonance. The violet and green dots indicate the current values of n₂/n₃ and n₃/n₄, respectively, while the violet and green dashed lines are the continuation of the evolution given by resonance locking without capture into the 2:1 resonance.