The Compositions of Kuiper Belt Objects

Objects in the Kuiper belt are difficult to study in detail, even with the best telescopes available. Therefore, for many years, studies of the compositions of these objects were relegated to collections of moderate-quality spectroscopic and photometric data that remained difficult to interpret. Much early effort was put into simple correlations of surface colors and identifications of spectral features, but connecting these observations to a larger understanding of the region remained elusive. The past decade, however, has seen a blossoming in our understanding, a product of the discoveries of larger-and thus easier to study-objects, continued collection of high-quality photometric and spectroscopic observations, and continued work at the laboratory and theoretical levels. Today, we now know of many processes that affect these objects' surface compositions, including atmospheric loss, differentiation and cryovolcanism, radiation processing, the effects of giant impacts, and the early dynamical excitation of the Kuiper belt. I review the large quantity of data now available and attempt to build a comprehensive framework for understanding the compositions and their causes.