36th DPS Meeting, 8-12 November 2004
Session 19 Rings
Poster I, Tuesday, November 9, 2004, 4:00-7:00pm, Exhibition Hall 1A

## [19.06] HST Observations of Spokes in Saturn's B Ring

C. A. McGhee, R. G. French (Wellesley College), L Dones (SWRI), J. N. Cuzzi (NASA/Ames), H. J. Salo (U. of Oulu), R. Danos (UCLA)

From several hundred high resolution HST images obtained between 1994--2004 with WFPC2, we have measured the photometric properties of spokes in Saturn's B ring. We determined the spoke particle size distribution by fitting the wavelength--dependent extinction efficiency of a prominent, isolated spoke, using a Mie scattering model. We assumed that the spoke particles were sub--micron size spheres of pure water ice, with a Hansen--Hovenier size distribution. The WFPC2 wavelength coverage is broader than that of the Voyager data, resulting in tighter constraints on the nature of spoke particles. The effective particle size was r\rm eff=0.57±0.05}\ \mum, and the size distribution was quite narrow with a variance of b=0.09±0.03}. In all, we identified 36 spokes or spoke complexes, predominantly on the morning (east) ansa. The photometric contrast of the spokes is strongly dependent on effective ring opening angle, B\rm eff. Spokes were clearly visible on the north face of the rings in 1994, just prior to the most recent ring plane crossing (RPX) epoch, and on the south face shortly after RPX. However, spokes were both less abundant and fainter as the rings opened up, and no spokes were detected after 18 Oct.~1998 (B\rm eff=-15.43\circ), when a single faint spoke was seen on the morning ansa. The detection limit is \leq 1% in fractional brightness contrast for spokes for the post--1998 observations. We compare the observed trend of spoke contrast with B\rm eff to several radiative transfer calculations. In the first, the spoke haze" is uniformly mixed with macroscopic B ring particles. No variation in spoke contrast is predicted for single--scattering, in this case, and for multiple scattering, only a modest decrease in contrast with B\rm eff is predicted. When the spoke dust occupies an extended layer that is thicker than, or simply lies above, the B ring, multiple--scattering Monte Carlo calculations match the trend of spoke contrast exceptionally well. We predict that spokes should be detectable during the Cassini mission when the rings are viewed in either forward-- or backscatter at modest (|B|\leq{10}\circ) ring opening angles.