Thermal evolution and differentiation of Edgeworth-Kuiper belt objects

The region beyond Neptune's orbit is populated by numerous bodies with semimajor axes from 31 to 48 AU. This region, known as the Kuiper belt, should contain primitive bodies, perhaps among the most primitive objects in the solar system. These bodies could be remnants of the solar system formation. They seem to be dark, volatile-rich objects showing a strong relationship with comets: the Kuiper belt is probably the source of most short-period comets and Centaurs. The Kuiper belt objects (KBOs) could still contain ices and organic compounds in unaltered proportions with respect to those of their formation. Thermal models of bodies moving in Kuiper belt orbits have been developed to follow their evolution and differentiation and to better understand the relations between them and the short-period comets and Centaurs. In these models, we assume that KBOs are porous bodies composed of ices and dust. The solar energy is very low, between 30 and 50 AU, and radiogenic heating becomes a nonnegligible source of energy for differentiation. The radioactive elements, if they exist in sufficient quantity, may modify the original composition of cometary nuclei. In the models reported here, we have assumed that the radiogenic elements stored in the refractory component are K-40, Th-232,U-235, and U-238, in meteoritic proportions. In some models, we have also included the short-lived radio nuclide Al-26. The aim of this work is to see how an undifferentiated Kuiper belt body can change its internal structure under the combined effect of radiogenic heating and solar irradiation. Moderate heating can permit the sublimation of the most volatile ices both from the interior and from the surface, depending on the dominant heat source. The main result is that Kuiper belt objects can be strongly volatile depleted. From the surface down to several hundred meters below the surface, the most volatile ices (like CO) can be completely absent.