Cometary activity and nucleus models

Models of cometary nuclei are discussed from the viewpoint of their compatibility with observed behaviour of cometary activity and related physical properties of cometary matter in the bulk of the nucleus and at its surface. It is shown that comets, in general, have to be covered by an isolating mantle or crust. nclusion of this paper is similar to the result of Kührt and Keller (Icarus 109, 121–132, 1994) namely that cometary nuclei consist of a cohesively bound matrix of relatively refractory and porous matter, possibly with water ice in the pores. 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 impact growth of comets. The matrix in these zones has been destroyed by impacts and behaves like an ice-dust mixture without any efficient bond between the debris “dust”.