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 irregulars (Grav et al. 2003)
there is an apparent lack of the extremely red objects found among the Centaurs
and Kuiper belt objects.
The Uranian irregulars 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-degree cluster containing two objects with similar average inclinations
but quite different average semi-major axis, only one object (U XXI Trinculo)
was observed. The heterogeneous colors and the large dispersion of the average
orbital elements leads us to doubt that they are collisional families. We favor
single captures as a more likely scenario. The two neptunians 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 (Nesvorny et al. 2003, Holman et al. 2004), this suggests that
S/2002 N1 be a fragment of Nereid, broken loose during a collision or cratering
event with an undetermined impactor.Comment: 13 pages (including 3 figures and 2 tables). Submitted to ApJ Letter
