Saturn's moon Hyperion is known for its non-synchronous rotation, unlike the other regular moons of Saturn, but for changing its rotation state over a matter of orbits. The Voyager flyby showed a moon in non-principal axis rotation, with a spin axis near the moon's long axis and a rate of over four times the orbital rate.Dynamical studies by Wisdom et al. (1984) predicted that the strongly-varying solid-body torques on Hyperion from Saturn would result in a moon with no low-order, stable spin-resonant states, with chaos over several orbits (about 50 days). Using models constructed with ground-based and Voyager data, Klavetter (1989) and Black et al. (1995) confirmed the chaos and concluded their models were consistent with a uniform-density Hyperion. In 2005, Cassini made three close passes of Hyperion. Changes in the spin rate of the moon are observable between fly-bys, but Hyperion appears to be in a similar quasi-periodic state to the one observed by Voyager. While Hyperion is likely too small to be differentiated, the large porosity inferred from its bulk density (Thomas et al., 2007) allows for an uneven distribution of voidspace not apparent from surface features. We modeled Hyperion's rotation during 2005, which allowed estimation of moments of inertia, giving insight on its interior in a way that can be done with few other moons of its size. We conclude that the moment of inertia ratios of Hyperion do not agree well with those predicted from Thomas's shape model.  Furthermore, models that place a principal axis nearer to the spin axis than the shape implies fit the data much better than simply varying the moments of inertia and fixing the principal axes to the shape.  This indicates that Hyperion may have large-scale inhomogeneities within its interior.
AAS/Division for Planetary Sciences Meeting Abstracts #42
- Pub Date:
- October 2010