Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.1904c&link_type=abstract
American Astronomical Society, DPS meeting #42, #19.04; Bulletin of the American Astronomical Society, Vol. 42, p.981
Astronomy and Astrophysics
Astronomy
Scientific paper
A sharp boundary in Saturn's B ring is located at 99,000 km. At this distance, the optical depth (measured by radio and ultraviolet occultations), the depth of the 3.6 micron water band (measured by the Cassini/VIMS instrument), and the abundance of small particles (inferred from Cassini/VIMS spectra and radio occultations) all increase. This boundary was also noted in Voyager occultation data and attributed to a charged dust dynamics by Northrop and Hill (1982.) While this process does remove sub-micron particles from the inner B ring, it does not result in a large-scale loss of ring material. The same principle, applied to ions produced from the ring atmosphere, shows that ions trajectories are unstable and ions precipitate into Saturn's atmosphere, if produced inside (2/3)1/3 of synchronous orbit (i.e. 99,000 km.) This results in a net loss of water from the surface of ring particles inside this distance. The estimated loss rate, 10-80 microns per millions years, could deplete the ice abundance in the top few IR wavelengths of the ring particles’ surface and account for the weaker water bands observer in VIMS spectra. In addition, the rapid loss of ions inside 99,000 km should result in lower plasma densities inside and outside this distance. Since dust particles charging depends on the local plasma conditions, the surface charge of ring particles would also differ in these two regions. A 2 V surface potential, typical of charging by photoelectrons in a low density plasma, produces sufficient electrostatic repulsion to prevent 65 micron ring particles from colliding. This may be the case in the B1 region (i.e. inside 99,000 km.) A denser, colder plasma in the B2 region could cause particles to charge to a much lower potential. As a result, the abundance of small particles in the B1 and B2 regions would differ.
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