COMETARY ACTIVITY AND NUCLEUS MODELS

Models of cometary nuclei are discussed from the viewpoint of their co mpatibility with observed behaviour of cometary activity and related p hysical properties of cometary matter in the bulk of the nucleus and a t its surface. It is shown that comets, in general, have to be covered by an isolating mantle or crust. The conclusion of this paper is simi lar to the result of Kuhrt and Keller (Icarus 109, 121-132, 1994) name ly that cometary nuclei consist of a cohesively bound matrix of relati vely refractory and porous matter, possibly with water ice in the pore s. Cometary activity is explained by the existence of ''debris zones'' between building blocks generated by impacts. These debris zones were formed at the interface of colliding building blocks during the impac t growth of comets. The matrix in these zones has been destroyed by im pacts and, behaves like an ice-dust mixture without any efficient betw een the debris ''dust''.