SATELLITE MASSES IN THE URANUS AND NEPTUNE SYSTEMS.

The present knowledge of satellite masses, and how they are derived, is reviewed with emphasis on implications for bulk densities and albedos. In general for the Uranian system, the inner satellites have lower densities and/or higher albedos than the outer ones. However, uncertainties are great enough that all five satellites may have nearly equal densities. In that case, albedo would generally (but not monotonically) decrease with semi-major axis. A more severe constraint than previously published is here placed on Miranda's mass, and hence on its density and albedo. The recent radiometric value for Triton's diameter, combined with now-rather-old mass determinations, yields a density greater than 4 gm/cm**3, but systematic errors are possible in both mass and diameter.