title: Constraints on Mimas' interior from Cassini ISS libration measurements.
abstract: Like our Moon, the majority of the solar system's satellites are locked in a 1:1 spin-orbit resonance; on average, these satellites show the same face toward the planet at a constant rotation rate equal to the satellite's orbital rate. In addition to the uniform rotational motion, physical librations (oscillations about an equilibrium) also occur. The librations may contain signatures of the satellite's internal properties. Using stereophotogrammetry on Cassini Image Science Subsystem (ISS) images, we measured longitudinal physical forced librations of Saturn's moon Mimas. Our measurements confirm all the libration amplitudes calculated from the orbital dynamics, with one exception. This amplitude depends mainly on Mimas' internal structure and has an observed value of twice the predicted one, assuming hydrostatic equilibrium. After considering various possible interior models of Mimas, we argue that the satellite has either a large nonhydrostatic interior, or a hydrostatic one with an internal ocean beneath a thick icy shell.
authors.affiliation: Department of Astronomy, Cornell University, Ithaca, NY 14853, USA. IMCCE-Observatoire de Paris, UMR 8028 du CNRS, UPMC, Université Lille 1, 77 Av. Denfert-Rochereau, 75014 Paris, France. Université Pierre et Marie Curie, UPMC - Paris VI, 4 Place Jussieu, 75005 Paris, France. tajeddine@astro.cornell.edu.
authors.forename: R
authors.initials: R
authors.lastname: Tajeddine
authors.affiliation: IMCCE-Observatoire de Paris, UMR 8028 du CNRS, UPMC, Université Lille 1, 77 Av. Denfert-Rochereau, 75014 Paris, France. Université Pierre et Marie Curie, UPMC - Paris VI, 4 Place Jussieu, 75005 Paris, France.
authors.forename: N
authors.initials: N
authors.lastname: Rambaux
authors.affiliation: IMCCE-Observatoire de Paris, UMR 8028 du CNRS, UPMC, Université Lille 1, 77 Av. Denfert-Rochereau, 75014 Paris, France.
authors.forename: V
authors.initials: V
authors.lastname: Lainey
authors.affiliation: Laboratoire AIM, UMR 7158, Université Paris Diderot/CEA IRFU/CNRS, Centre de l'Orme les Merisiers, 91191 Gif-sur-Yvette Cedex, France.
authors.forename: S
authors.initials: S
authors.lastname: Charnoz
authors.affiliation: IMCCE-Observatoire de Paris, UMR 8028 du CNRS, UPMC, Université Lille 1, 77 Av. Denfert-Rochereau, 75014 Paris, France.
authors.forename: A
authors.initials: A
authors.lastname: Richard
authors.affiliation: Royal Observatory of Belgium, Avenue Circulaire 3, B-1180 Brussels, Belgium.
authors.forename: A
authors.initials: A
authors.lastname: Rivoldini
authors.affiliation: Department of Mathematics and Namur Center for Complex Systems, Université de Namur, 5000 Namur, Belgium.
authors.forename: B
authors.initials: B
authors.lastname: Noyelles