Rotation and tides of the large moons of Uranus

The Uranus Orbiter and Probe (UOP) has been proposed as the next Flagship-class mission by the 2023-2033 Planetary Science and Astrobiology Decadal Survey [1]. During its 4-year tour, the mission will address important questions regarding the five large icy moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon): What are their rock-ice ratios? Do they have internal oceans?

During the orbital motion, the rotation of the synchronous moons is modified by the time-varying gravitational torque of Uranus on their flattened shape. The non-zero orbital eccentricity causes diurnal librations (periodic variations in rotation), whereas the orbital precession forces the obliquity (angle between the spin axis and the normal to the orbit) to be non-zero. The gravitational field of Uranus also induces tidal periodic deformations of the moons which in turn cause periodic variations in their own potential, proportional to the tidal Love number k2. These deformations also modify the rotational response of the moons.

Here, we assess how tidal and rotation measurements could inform us on the interior of the icy moons. We first build a set of interior models with a global ocean between an ice shell and a rocky core, following [2-4]. Rotation and tidal observables are then calculated from the models in [5-7]. The difference with results for oceanless moons indicates the measurement precision required to detect the presence of an internal ocean.

Acknowledgments: This work was financially supported by the French Community of Belgium under FRIA grants and through the Belgian PRODEX program managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office.

References:
[1] National Academies of Sciences, Engineering, and Medicine, Origins, Worlds, and Life, 2023, DOI:10.17226/26522
[2] Hussmann et al. (2006), Icarus, 185.
[3] Bierson and Nimmo (2022), Icarus, 373.
[4] Castillo-Rogez et al. (2023), JGR planets, 128.
[5] Van Hoolst et al. (2013), Icarus, 226.
[6] Baland et al. (2019), CMDA, 131.
[7] Sabadini and Vermeersen, (2004).