The link between cometary and interstellar dust.

From spectral observations, both interstellar and cometary dust appear to contain silicates, amorphous carbon, hydrocarbons and mantles comprised of complex organic matter (e.g. Tielens and Allamandola, 1987a; Mathis and Whiffen, 1989; Chyba et al., 1989; Greenberg and Hage, 1990). Also, some interplanetary dust particles, which may have a cometary origin, are collections of submicron-sized individual units of silicates, carbonaceous material (hydrocarbons and more complex organic compounds), and amorphous or graphitized carbon, loosely bonded together into a fractal aggregate (Rietmeijer, 1992). Comets are believed to have formed in cold regions far out in the presolar nebula and to retain some interstellar material, stored in fairly pristine conditions since their formation (Mumma et al., 1993). Accordingly, the composition and properties of cometary dust grains should reflect those of grains in the outer part of the protosolar nebula which, at least in part, were inherited from the molecular cloud out of which the solar system formed (Pollack et al., 1994). On the other hand, infrared emission features in comets differ from their interstellar counterparts (Tokunaga and Brooke, 1990; Hanner et al., 1993). These differences imply processing of progenitor interstellar material on its way to incorporation in comets and suggest that the dust components of comets bear evidence of their own formation and evolution as building blocks of planetary material. In this paper we compare cometary and interstellar dust in order to gain a better insight into the properties and composition of dust during its evolution from the interstellar medium (ISM) into the solar system.