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

摘要

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 similar to 5.5 h to similar to 76 h; the fastest observed spin rate is much slower than the disruption rotation barrier of asteroids at similar to 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 a > 60 degrees. While most lightcurve amplitudes from observations obtained at a < 30 degrees are < 0.5 mag, most at a > 50 are > 0.5 mag. This campaign marks the first utilization of an interplanetary spacecraft for a systematic photometric survey of irregular moons.