Satellites of Uranus and Neptune, and the Pluto-Charon system.

Recent work on the Uranian satellites has revealed many of their basic physical properties. Radiometric measurements have shown that Ariel, Umbriel, Titania and Oberon have diameters which range from 1630 to 1110 km and albedos which range from 0.30 to 0.18. Spectrophotometric observations of Miranda suggest that it may have the highest albedo of the known Uranian satellites and a diameter of about 500 km. All five known satellites of Uranus have surfaces composed of water ice contaminated with small amounts of dark material. Of the two confirmed satellites of Neptune, only Triton is sufficiently bright to permit studies of its physical characteristics. Spectroscopy has indicated methane and possibly nitrogen on Triton's surface. An atmosphere must surround Triton. Pluto and Charon constitute a unique planet-satellite pair in the solar system. Pluto is sublunar in size and has a surface with solid methane distributed irregularly upon it. A tenuous atmosphere consisting of methane and perhaps other gases envelopes Pluto.