Weighing Uranus’ Moon Cressida with the η Ring

Abstract

The η ring is one of the narrow rings of Uranus, consisting of a dense core that is 1-2 km wide and a diffuse outer sheet spanning about 40 km. Its dense core lies just exterior to the 3:2 Inner Lindblad Resonance of the small moon Cressida. We fit the η ring radius residuals and longitudes from a complete set of both ground-based and Voyager stellar and radio occultations of the Uranian rings spanning 1977-2002. We find variations in the radial position of the η ring that are likely generated by this resonance, and take the form of a 3-lobed structure rotating at an angular rate equal to the mean motion of the moon Cressida. The amplitude of these radial oscillations is 0.667±0.113 km, which is consistent with the expected shape due to the perturbations from Cressida. The magnitude of these variations provides the first measurement of the mass and density of the moon Cressida (m = 2.5 ± 0.4 × 10 17 kg and ρ = 0.86 ± 0.16 g/cm 3 ) or, indeed, any of Uranus' small inner moons. A better grasp of inner Uranian satellite masses will provide another clue to the composition, dynamical stability, and history of Uranus' tightly packed system of small moons.

A Pluto–Charon Sonata: Dynamical Limits on the Masses of the Small Satellites

“…The densities of Phobos (1.9 g cm −3 ) and Deimos (1.5 g cm −3 ) are much larger than those of the somewhat larger Prometheus (0.5 g cm −3 ) and Pandora (0.5 g cm −3 ; Jacobson & French 2004;Renner et al 2005;Jacobson 2010;Pätzold et al 2014). The density of Uranus' moon Cressida (0.9 g cm −3 ) is roughly midway between the two Martian satellite and Saturn's satellites orbiting close to or within the ring system (Chancia et al 2017). Estimates for the densities of other small satellites 2 rely on estimates instead of direct measurement of their masses.…”

Uranus' hidden narrow rings

“…This could correspond to the 50-km wide low-optical depth extension to this ringlet seen in the Voyager radio occultations (Gresh et al 1989;French et al 1991). There is also a distinct narrow peak at 47,270 km in the NAC profile, which we desig-2 Note that the location of this ringlet only varies by a couple of kilometers within the various occultations (Chancia et al 2017). nate U47.27.…”

“…For all the other rings the connections with satellite resonances are less clear-cut. The η-ring's shape is clearly affected by the nearby 3:2 resonance with Cressida (Chancia et al 2017) , but the resonance falls 5 km interior to the actual location of that ring so it is not clear whether the resonance plays any direct role in confining it. On the other hand, the brightness of the γ ring is affected by a 6:5 resonance with Ophelia (Showal-ter 2011), but the overall shape of the ring is dominated by patterns with different azimuthal wavenumbers (French et al 1991), so it is still unclear how the resonance is influencing the dynamics of this ring.…”

Seasonal structures in Saturn's dusty Roche Division correspond to periodicities of the planet's magnetosphere

“…Satellite resonances can also influence the entirety of narrow dense rings. This is the case for Uranus' 2 km wide η ring which has a 3-lobed oscillation in its radius produced by the ring's proximity to Cressida's 3:2 ILR (Chancia et al 2017).…”