We have investigated the radial motion of moonlets in the vicinity of a density depletion (gap) basing on the perturbation theory of the orbital motion (Goldreich and Tremaine 1980; Petit and Henon 1986; 1987). It is shown that a moonlet will be attracted by a gap owing the angular momentum exchange between the moonlet and the ring-material. Furthermore the radial orbital evolution of two moonlets has been integrated numerically giving that the stationary positions of the moonlets inside the gap depend on the difference of the surface mass density and on the mass ratio of the moonlets, respectively. These results have been applied to six moonlet-candidates (Spahn and Sponholz 1989) which are located in a density depletion in the middle of the B-ring of Saturn (4 events), in the Cassini-division (Huygens-ringlet) and also to the Encke- gap-ringlet. In the case of the latter either ringlets their actual positions in the gaps (Cassini; Encke) have been well confirmed in the frame of our approach and also features of the surface mass density field - double peaks, density wakes - observed by Voyager point to an origin by the action of a moonlet. © 1992.
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