Irregular-Moons Science Today and in Cassini's Final Three Years
Abstract:The outer or irregular moons belong to the by far most numerous, but least investigated group of moons in the Saturnian system. The group is comprised of at least 38, mainly small objects which orbit at quite large distances to the planet, the rings, the inner regular moon system, and the Cassini spacecraft. As seen from Earth, their apparent visual magnitudes mainly range from ~20 to >25, the phase angles from 0 to 6°. From Cassini, some irregular moons can reach <12th mag, while most are at least occasionally brighter than ~16.5 mag. Their phase angles can assume any value from 0 to 180°.
The Cassini ISS camera regularly observed the irregular moons over the last few years at various phase angles, with this research program now also being part of an accepted Participating Scientist proposal. Because of the large distances, the objects appear as unresolved point sources to the ISS. By means of photometric time series, rotational periods and minimum equatorial-axis ratios (a/b)min for 21 out of the 38 known objects could be determined. The rotation periods range from ~5½ hours for Hati to ~3 days for Tarqeq (Fig. top row). The largest (a/b)min are 2.9 for Kiviuq and 1.7 for Erriapus and Bestla (Fig. middle row).
Many objects are subject of repeated observations under varying illumination and viewing geometries, with the aim of determining pole-axis directions, convex-hull shapes, and sidereal periods. So far, lightcurve inversion of Ymir revealed a convex shape reminiscent to a triangular prism and a spin-axis direction close to the South Ecliptic pole. The figure shows the lightcurve at 64° phase (bottom left panel), and four equatorial and two polar views of the derived shape model are shown at bottom right.
Within Cassini's final three years in Saturn orbit, further observations offer the potential of pole and shape determinations for up to 15 irregular moons and up to 30 rotational periods total. These data may reveal (or already do so) hemispherical color variations, limits for object densities and sizes, pole-direction patterns, correlations of spin frequencies to orbit parameters, and hints for contact binaries or binary natures of the moons. Such information, besides its value per se, provides constraints for the investigation on the formation and evolutionary processes within the Saturnian satellite system.