Physical Properties of Trans-Neptunian Object (20000) Varuna

Abstract

In 1992, the first body beyond Neptune since the discovery of Pluto in 1930 was found. Since then, nearly a thousand solid bodies, including some of planetary size, have been discovered in the outer solar system, largely beyond Neptune. Observational studies of an expanding number of these objects with space- and groundbased telescopes are revealing an unexpected diversity in their physical characteristics. Their colors range from neutral to very red, revealing diversity in their intrinsic surface compositions and/or different degrees of processing that they have endured. While some show no diagnostic spectral bands, others have surface deposits of ices of H2O, CH4, and N2, sharing these properties with Pluto and Triton. Thermal emission spectra of some suggest the presence of silicate minerals. Measurements of thermal emission allow determinations of the dimensions and surface albedos of the larger (diameter > ~75 km) members of the known population; geometricalbedos range widely from 2.5% to >60%. Some 22 transneptunianobjects (including Pluto) are multiple systems. Pluto has threesatellites, while 21 other bodies, representing about 11% of the sample investigated, are binary systems. In one binary system where both the mass and radius are reliably known, the mean density of the primary is ~500 kg/m3, comparable to some comets (e.g., Comet 1P/Halley).