Long-term evolution of objects in the Kuiper Belt zone - Effects of insolation and radiogenic heating

The Kuiper Belt zone is unique insofar as the major heat sources of objects a few tens of kilometers in size-solar radiation on the one hand and radioactive decay on the other-have comparable power. This leads to unique evolutionary patterns, with heat waves propagating inward from the irradiated surface and outward from the radioactively heated interior. A major radioactive source that is considered in this study is Al-26. The long-term evolution of several models with characteristics typical of Kuiper Belt objects is followed by means of a I-D numerical code that solves the heat and mass balance equations on a spherically symmetric grid. The free parameters considered are radius (10-500 km), heliocentric distance (30-120 AU), and initial Al-26 content (0-5 x 10(-8) by mass). The initial composition assumed is a porous mixture of ices (H2O, CO, and CO2) and dust. Gases released in the interior are allowed to escape to the surface. It is shown that, depending on parameters, the interior may reach quite high temperatures (up to 180 K). The models suggest that Kuiper Belt objects are likely to lose the ices of very volatile species during early evolution; ices of less volatile species are retained in a surface layer, about 1 km thick. The models indicate that the amorphous ice crystallizes in the interior, and hence some objects may also lose part of the volatiles trapped in amorphous ice. Generally, the outer layers are far less affected than the inner part, resulting in a stratified composition and altered porosity distribution. These changes in structure and composition should have significant consequences for the short-period comets, which are believed to be descendants of Kuiper Belt objects. (C) 2002 Elsevier Science (USA).