An accretion disk is formed when matter with angular momentum is flowing on a gravitating object (as e.g. a white dwarf, a neutron star, a young stellar object, or a black bole). It radiates because the transport of angular momentum (required for the matter to reach the central object) necessarily implies the conversion of potential energy into a form of energy that corresponds to higher entropy. Many aspects of the physics (as e.g. the mechanism for the heat generation) are not yet well understood but they are presently one of the centers of astronomical interest (see e.g. the books by Frank, King, and Raine, 1992, or by Wheeler, 1993).
Shoemaker and Terrile have shown that several small satellites can be the parents of Saturn's rings and have suggested these as the origin of the broad systematic differences in the photometric properties of the rings. Dramatic variations on the smallest scales appear in the images obtained from Voyager. The variations of the surface of Enceladus suggest the importance of differentiation of satellites heated by tidal action maintained by resonances, as pointed out by C.M. Yoder. Subsequent break-up of such satellites, either under a large impact or under increasing tidal action of Saturn as its mass increased, would have yielded a mixture of different crustal and interior materials with different spectra of sizes and masses. We shall discuss the possible role of radial differentiation of particles in terms of their masses initiated by the combined activity of instabilities discussed long ago by Maxwell and by Cook and Franklin with those proposed recently by Ward and by Lin and Bodenheimer. In this way, we may reach an explanation of variations on the smallest scales. Comparison will be made with the expected behaviour of a primary accretion disk composed of a uniform distribution of material.
The Role of Opacities in Accretion Disks