Oort-Cloud and Kuiper-Belt comets

This is a study of the magnitude variation of 93 Kuiper-Belt (KB) comets of short period and 77 Oort-Cloud (OC) comets, including 14 Halley-type. Two criteria of their activity show consistently that the Kuiper-Belt comets are more active at large solar distances than the Oort-Cloud comets. This result is consistent with the expectation that the KB comets were formed at lower temperatures, farther from the Sun, than the OC comets, and contain more ices that can freeze only at lower temperatures. The result is supported by the fact that the comets in the oldest KB age class act like the OC comets, suggesting that the cores of very old KB comets have been heated by radioactivity and lost their more volatile ices, most of the remaining ice being water ice. The degree to which spectroscopic data by AʹHearn et al. (1995. Icarus 118, 223–270) support the result of this study is discussed.