A search for volatile ices on the surfaces of cold classical Kuiper Belt Objects

The surprisingly complex dynamical distribution of small bodies among and beyond the orbits of the planets has changed our understanding of Solar System evolution and planetary migration. Compositional information about the small bodies in the Solar System provides constraints for models of Solar System formation. According to most models, the Kuiper Belt population known as the cold classicals formed at distances far enough from the Sun for these objects to be composed of an appreciable fraction of volatile ices of diverse composition (H₂O, CO₂, CH₄, light hydrocarbons, e.g. CH₃OH) and their orbits have remained stable. Cold classical objects should still be volatile rich. Broadband data from the Spitzer Space Telescope Infrared Array Camera (IRAC) can detect and distinguish between absorptions of relevant ices in the 3-5 um [micron] infrared region. Of the 46 cold classical Kuiper Belt Objects in this study, 78% of their surfaces exhibit absorptions from ices or organics in IRAC channel 1 (3.6 um [micron]). The combination of data from IRAC channels 1 and 2 (4.5 um [micron]) provides gross surface composition for the six objects with secure observations in both channels. These six objects are observed to have ices or organics on their surfaces; this is the first detection of ices on four of these objects. The surface of 20000 Varuna contains organic material. The surface of 50000 Quaoar is confirmed to be rich in water ice. The surface composition of 19521 Chaos is mixed ice and organics. Mixed ices, with a high fraction of water ice, and other components are on the surface of 119951 2002 KX14. The surface of 66652 Borasisi is methane rich. Methanol or light hydrocarbons are on the surface of 138537 2000 OK67. Cold classical objects are found to be volatile rich and of diverse surface composition. The presence of ices and organics indicate these objects formed far from the Sun.