A numerical model of the Uranian dust rings

Abstract

The tenuous dust rings of Uranus ( τ ≈ 10 −5) discovered by Voyager 2 are not readily associated with any of the other observed components of the Uranus ring system. Their short lifetime due to exospheric drag and other disruptive processes requires continuous production of the dust particles. Our numerical simulations of the Uranus rings show that the dust rings could be the visible component of low optical depth moonlet belts. These belts would be made up of objects from 10 to 10 5 cm which serve as sources of the dust via micrometeoroid bombardment and collisional release of ejecta. Dust is created within the main rings of Uranus through the same processes. The dust bands cannot be the transient remnants of large impacts onto unseen moons because the impacts occur too infrequently since the dust band lifetimes are very short (≤100 years). We propose that the dust bands are the result of continuous ejection and reabsorption of micron-sized regolith material within hypothesized moonlet belts ( τ MB ≤ 10 −3). Some of the ejecta is removed from the moonlet belt giving rise to a continuous sheet of micron-sized dust particles. Numerical simulations of such a system in a Markov chain formalism reproduce the observed characteristics of the Uranian dust rings. The broad sheet of dust which extends approximately from the δ ring to Cordelia could be dust ejected from Cordelia by continuous meteoroid erosion. This dust would then radially evolve until accreting onto δ ring particles. Our simulations show that the surface of Cordelia would have to be relatively hard, or regolith-free, for enough dust to be excavated to account for the observed optical depth. In addition, we would expect a similar sheet between Ophelia and the ε ring, and none is observed. We therefore propose that this sheet is also the visible component of one or more particularly broad moonlet belts, with Cordelia the largest member of the moonlet belt distribution.