Dusty Rings and Circumplanetary Dust: Observations and Simple Physics

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. Jupiter’s dusty rings (as discovered by the Voyager and Galileo spacecraft) have three components: i) a radially confined and vertically extended halo which rises abruptly, probably due to an electromagnetic resonance; ii) a 6500-km-wide flattened main ring that shows patchiness and whose outer edge is bounded by the orbit of the satellite Adrastea; and iii) a pair of exterior 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 to interplanetary space. 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 Uranian rings, forming a sheet that is punctuated by narrow bands and gaps. 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 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.