Burns, J.A., D.P. Hamilton, and M.S. Showalter 2001. Dusty rings and
circumplanetary dust. In Interplanetary Dust , (E. Gr\"un,
B.A.S. Gustafson, S.F. Dermott and H. Fechtig, Eds.), {Springer
Verlag}, Berlin, pp. 641-725.
Each giant planet is encircled by planetary rings, usually composed of
particles centimeters to meters in radius, but each system also
contains regions where much smaller dust grains predominate. This
chapter summarizes the techniques used to determine the properties of
circumplanetary material, and then gives a precis of the known
characteristics of circumplanetary rings (with emphasis on tenuous
structures) and dust grains, before describing some of the physics and
orbital dynamics relevant to them. {\it Jupiter}'s dusty rings (as
discovered by the Voyager and Galileo spacecraft) have three
components: i) a radially confined and vertically extended {\it halo}
which rises abruptly, probably due to an electromagnetic resonance;
ii) a 6500-km-wide flattened {\it main ring} that shows patchiness and
whose outer edge is bounded by the orbit of the satellite Adrastea;
and iii) a pair of exterior {\it gossamer rings} that seem to be
derived from the satellites Amalthea and Thebe whose orbits
circumscribe these rings. In addition, small particles are strewn
throughout the inner Jovian magnetosphere, especially near the paths
of the Galilean moons, and the jovian system seems to eject very tiny
particles at hypervelocities t o interplanetary space. {\it Saturn}'s
circumplanetary dust is unusual in the size distribution of its
various rings: the broad and diffuse E~ring seems to be mainly
1-micron grains whereas the narrow F and G rings have quite steep size
distributions, indicating the predominance of very small
grains. Surprisingly little dust resides in the main Saturnian rings,
except in the localized spokes. Dust is interspersed between the
narrow classical {\it Uranian} rings, forming a sheet that is
punctuated by narrow bands and gaps. {\it Neptune}'s system contains
at least some grains that lie well off the planet's equatorial plane,
perhaps as a result of Neptune's highly tilted and offset magnetic
field. The debris lost off the small moons Phobos and Deimos is
believed to produce very tenuous {\it dust tori around Mars}. Complex
orbital histories for circumplanetary grains result from conservative
and non-conservative forces (gravity, radiation pressure and
electromagnetism); the latter become most important for smaller
particles and may even lead to ejection or planetary impact. Orbital
resonance phenomena, several of which are unique to circumplanetary
dust, seem to govern the distribution of grains orbiting planets.
Circumplanetary dust is short-lived in a cosmic sense, owing to
erosion through sputtering by the surrounding magnetospheric plasma
and orbital loss due to various evolution mechanisms. These brief
lifetimes imply continual regeneration to supply new
material. Circumplanetary dust is often found in intimate relation
with embedded small moonlets since it can be generated through
energetic impacts into such bodies but is also absorbed by them.
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