Photometry of Irregular Satellites of Uranus and Neptune

We present BVR photometric colors of six Uranian and two Neptunian irregular satellites, collected using the Magellan Observatory (Las Campanas, Chile) and the Keck Observatory (Manua Kea, Hawaii). The colors range from neutral to light red, and like the Jovian and the Saturnian irregular satellites (Grav et al.) there is an apparent lack of the extremely red objects found among the Centaurs and Kuiper Belt objects. The Uranian irregular satellites can be divided into three possible dynamical families, but the colors collected show that two of these dynamical families, the Caliban and Sycorax clusters, have heterogeneous colors. Of the third possible family, the 168° cluster containing two objects with similar average inclinations but quite different average semimajor axes, only one object (U XXI Trinculo) was observed. The heterogeneous colors and the large dispersion of the average orbital elements lead us to doubt that they are collisional families. We favor single captures as a more likely scenario. The two Neptunian satellites observed (N II Nereid and S/2002 N1) both have very similar neutral, Sun-like colors. Together with the high collisional probability between these two objects over the age of the solar system (Nesvorný et al.; Holman et al.), this suggests that S/2002 N1 is a fragment of Nereid, broken loose during a collision or cratering event with an undetermined impactor.