Changes in the Structure of Comet Nuclei Due to Radioactive Heating

The initial structure of a comet nucleus is most probably a homogeneous, porous, fine-grained mixture of dust and ices, predominantly water. The water ice is presumably amorphous and includes considerable fractions of occluded gases. This structure undergoes significant changes during the early evolution of the nucleus at large heliocentric distances, due to internal radiogenic heating. Structural changes occur mainly as a result of gas flow through the porous medium: the gas pressure that builds up in the interior is capable of breaking the fragile structure and altering the pore sizes and porosity. These effects are modeled and followed numerically, testing a large number of parameters. This revised version was published online in June 2006 with corrections to the Cover Date.