Hyperion: Rotation, Shape, and Geology from Voyager Images

Abstract

Voyager 2 images have been used to measure the instantaneous rotation state of Hyperion during the period of the spacecraft's 1981 flyby of Saturn, to define the satellite's shape, and to map geological features. For the 38 hr of image coverage at resolutions better than 17 km/pixel, control point calculations yield a rotation rate of 72°/day, with a pole direction of RA = 226 ± 4°, Dec = 35 ± 4°. Rotation was essentially about the long axis of Hyperion. A shape model with 5° × 5° spacing is developed using stereo, limb, and terminator data. About 95% of the surface of Hyperion is constrained within 20 km by the control point and lower resolution images. An ellipsoidal fit to this model gives principal axes (radii) of 164, 130, and 107 km. The shape model has a mean radius of 133 ± 8 km; the model moments of inertia (I/MR 2) are A = 0.32 ± 0.02, B = 0.46 ± 0.02, and C = 0.52 ± 0.02. G. J. Black et al . (1995, Icarus) have used the shape model and the instantaneous rotation state to calculate the expected rotation of Hyperion throughout the Voyager 2 encounter. They predict significant changes in the rotation pole orientation, but only minor changes in rotation rate during the 18 days of low-resolution, disk-resolved imaging of Hyperion. We compare the Voyager-observed lightcurve of Hyperion to lightcurves predicted from the dynamical model. During this 18-day period the dynamical model matches the observed lightcurve very well; the best lightcurve fit is for a rotation rate of 72 + 3 - 4°/day (identical to the control point solution) at the time of the first of the high-resolution images. Much of the Hyperion lightcurve amplitude derives from wobble rather than from the spin. The good agreement of the dynamical model with observed behavior suggests that Hyperion's moments of inertia are well modeled by the shape and that it has no substantial internal density contrasts. The overall shape of Hyperion is typical for icy objects smaller than 200 km mean radius. Small icy satellites appear to have less structural control of their shapes than do rocky satellites and asteroids. A 300-km-long arcuate scarp and a more subdued, nearly parallel scarp suggest spallation of several kilometers of material from nearly 1/4 the area of the satellite due to the formation of a 140-km-diameter crater.