Observations of Saturn's Irregular Moons

The irregular moons of Saturn [1] were observed with the ISS camera of the Cassini spacecraft at a large phase-angle range to obtain lightcurves [2]. About one-quarter of the observations has been executed at α < 30°, while about one-third at α > 70°. The measured amplitudes vary between 0.1 and 2.6 mag and generally increase from low to high phase angles. For α < 30°, most amplitudes of Saturn's irregulars were found to be below 0.5 mag. On the other hand, for α > 50°, there are only very few lightcurves left with amplitudes smaller than 0.5 mag. For high phase angles above 80°, almost all amplitudes are >1 mag. The shapes of the lightcurves vary considerably between different objects and even between observations of the same object, indicating that these moons are not just sort of "fluid equilibrium figures" (like the small inner Saturnian moons Methone and Pallene), but very individual worlds with irregularly-shaped surfaces. With respect to extrema, almost all show either a 2-maxima/2-minima or a 3-maxima/3-minima pattern, indicating that they are likely shape-dominated. The periods range from 5.5 h to 76 h. The results approximately triple the number of known spin rates of irregular moons in the Solar System. The average spin rate for 22 irregular moons of Saturn (all except Phoebe, Loge, Fornjot) is 2.10 d^-1 (rotation period: 11.4 h). Remarkably, it appears that there exist dependencies between spin rates, object sizes, distances to Saturn, and orbit directions. The retrograde irregular moons of Saturn are on average rather small, far away from the planet, and have fast rotations, while the prograde satellites are mostly large, closer to Saturn, and rotate more slowly. The presentation will report the current status of our work on irregular-moon physical properties from Cassini imaging data. [1] Denk, T., Mottola, S., Tosi, F., Bottke, W.F., Hamilton, D.P. (2018): The Irregular Satellites of Saturn. In: Enceladus and the Icy Moons of Saturn (Schenk, P. et al., eds.), Space Science Series, The University of Arizona Press, 26 pp. [2] Denk, T., Mottola, S. (2018): Studies of Irregular Satellites: I. Lightcurves and Rotational Periods of 25 Saturnian Moons from Cassini Observations. Submitted to Icarus.