The Irregular Satellites of Saturn

Abstract

With 38 known members, the outer or irregular moons constitute the largest group of satellites in the saturnian system. All but exceptionally big Phoebe were discovered between the years 2000 and 2007. Observations from the ground and from near-Earth space constrained the orbits and revealed their approximate sizes (~4 to ~40 km), low visible albedos (likely below ~0.1), and large variety of colors (slightly bluish to medium-reddish). These findings suggest the existence of satellite dynamical families, indicative of collisional evolution and common progenitors. Observations with the Cassini spacecraft allowed lightcurves to be obtained that helped determine rotational periods, coarse shape models, pole-axis orientations, possible global color variations over their surfaces, and other basic properties of the irregulars. Among the 25 measured moons, the fastest period is 5.45 h. This is much slower than the disruption rotation barrier of asteroids, indicating that the outer moons may have rather low densities, possibly as low as comets. Likely non-random correlations were found between the ranges to Saturn, orbit directions, object sizes, and rotation periods. While the orbit stability is higher for retrograde objects than for progrades very far away from Saturn, a compelling physical cause for size and spin relations to orbital elements is not yet known. The large moon Phoebe was resolved by Cassini during a close flyby in June 2004, showing numerous craters of all sizes on a surface composed of water ice and amorphous carbon. While the origin of the irregulars is still debated, capture of comets via three-body interactions during giant planet Encounters do the best job thus far at reproducing the observed orbits. This chapter gives a summary of our knowledge of Saturn’s irregular moons as of the end of 2017.