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Ring Moon Morphology and Libration Measurements

Presentation #110.06 in the session Many Planets, More Rings Posters (Poster + Lightning Talk)

Published onOct 23, 2023
Ring Moon Morphology and Libration Measurements

During its grand finale stage, the Cassini-Huygens space mission made a series of observations of Saturn’s small inner moons in order to study the shape, geology, and surface composition of this populations of small satellites. These bodies probably originated recently from the rings and are witenesses of the formation of the satellites. At present two scenario are proposed for the origin of the shape of these bodies, the ornamental accretion (Charnoz et al. 2007, Quillen et al. 2021) and the slow-velocity collision (Leleu et al. 2018).

Here, we used the Cassini images to perform limb measurements and the tessellated models of Thomas and Helfenstein (2020) to analyse the shape of these bodies (Rambaux et al. 2022). First, we extract from the tessellated model a spherical harmonics representation of the surface and we determine a confidence interval between 50 and 150 meters using our limb measurements. Then, we describe the topography based on spherical harmonics and the root mean square spectrum. These methods allow us to highlight equatorial ridges for some of these ring satellites. Finally, we deduce fundamental quantities, such as gravity field, and amplitude of the diurnal libration by assuming that the bodies are homogeneous. We obtain good agreement with previously published librational measurements for Epimetheus while we highlight the proximity of the resonance for Epimetheus, Pandora, and Prometheus. Then in Lainey et al. (2023), we used Cassini Imaging Science Subsystem (ISS) astrometric data to extract the mean density and the physical libration amplitudes for Atlas, Prometheus, Pandora, Epimetheus, and Janus. We confirm the large porosity of these five moons and our physical libration determinations are consistent with homogeneous assumption for four satellites Atlas, Prometheus, Epimetheus, and Janus whereas the solution for Pandora shows plausibility for heterogeneous interior.

Similar studies could be carried out on the internal satellites of Jupiter using images from the Europa Clipper (NASA) or JUICE (ESA) missions.

Charnoz, S., Brahic, A., Thomas, P. C., & Porco, C. C. 2007, Science, 318, 1622 Lainey V., Rambaux N., Cooper N., Dahoumane R., Zhang Q., 2023, A&A, 670, L25 Leleu, A., Jutzi, M., & Rubin, M. 2018, Nat. Astron., 2, 555 Quillen, A. C., Zaidouni, F., Nakajima, M., & Wright, E. 2021, Icarus, 357, 39 114260 Rambaux, N., Lainey, V., Cooper, N., Auzemery, L., & Zhang, Q. F. 2022, A&A, 667, A78 Thomas, P., & Helfenstein, P. 2020, Icarus, 344, 113355

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