Characterising the interior of five inner Saturnian moons using Cassini ISS data

Abstract

Context. The Cassini-Huygens space mission provided a huge amount of data for the Saturnian system. While the mission ended in September 2017, there is still much information to be retrieved from the data. Aims. Here, from their orbital motion, we infer the interior of the five inner Saturnian moons, Atlas, Prometheus, Pandora, Janus, and Epimetheus. Our results benefit from a recent study of the shape of these moons, which provides a much better estimation of their 3D shape and volume. Methods. Using Cassini Imaging Science Subsystem (ISS) astrometric data, we again reduced the Cassini ISS images with an improved shape modelling. We then integrated the orbital motion of the inner Saturnian moons and solved for a large set of physical parameters for this system, including the masses and physical libration amplitudes of the moons. Results. We obtain a density value (in g cm−3 with a 1σ error bar based on the digital terrain models and spherical harmonics available today) of 0.3935 ± 0.0033, 0.4873 ± 0.0026, 0.5045 ± 0.0017, 0.6233 ± 0.0015, and 0.6413 ± 0.0002 for Atlas, Prometheus, Pandora, Epimetheus, and Janus, respectively. We were able to determine the physical libration amplitudes of Prometheus, Pandora, Epimetheus, and Janus to be 0.98 ± 0.08, −5.2 ± 0.7, −6.0 ± 0.6, and −0.39 ± 0.16 (degrees and 1σ error bar), respectively. Our solutions for Epimetheus and Janus are in full agreement with a former estimation performed directly from the measurement of the rotation of these two moons. Conclusions. We confirm the large porosity of these five moons and their increase in density as a function of their size. Our estimations of the physical librations of Prometheus, Pandora, Janus, and Epimetheus are consistent with a homogeneous interior, but for Pandora a heterogeneous interior is also plausible.