PLUTO AND CHARON: Formation, Seasons, Composition

■ Abstract Pluto and Charon, once thought to be a singular system in an odd orbit at the edge of the solar system, are now known as members of a vast population of icy bodies beyond Neptune. Models for the occurrence of the odd orbit and formation of these bodies in the context of the total population are reviewed. Pluto’s orbital characteristics, coupled with the existence of volatiles on the surface, suggest that large-scale seasonal change should occur on the surface. Models of seasonal variability are discussed, past and current observations are examined for evidence of variability, and a straw-man model of seasonal changes is proposed. Finally, recent observations of the surface composition of Charon are discussed and compared with observations of other similarly sized icy bodies in the outer Solar System.