Studies of irregular satellites: I. Lightcurves and rotation periods of 25 Saturnian moons from Cassini observations

Abstract

Disk-integrated time-resolved photometric observations have been performed for 25 irregular moons of Saturn with the ISS camera of the Cassini spacecraft in a wide range of solar phase angles. From the derived lightcurves, synodic rotation periods were determined, ranging from 5.5 h to 76 h; the fastest observed spin rate is much slower than the disruption rotation barrier of asteroids at 2 h. The lightcurves show very different structures which indicates that Saturn's irregular moons have a large variety of shapes. All lightcurves except for Phoebe and Paaliaq show 2-maxima/2-minima or 3-maxima/3-minima patterns. The lightcurve amplitudes range between 0.1 mag and > 2 mag, the latter is measured in observations of objects Ymir, Kiviuq, and Siarnaq at phase angles α > 60°. While most lightcurve amplitudes from observations obtained at α < 30° are <0.5 mag, most at α > 50° are >0.5 mag. This campaign marks the first utilization of an interplanetary spacecraft for a systematic photometric survey of irregular moons.