Azimuthal brightness variations in Saturn's rings were first noted by Camichel (1958, Ann. d'Astroph. 21, 231), and Thompson et al. (1981, Icarus 46, 187) showed that the amplitude of the variations varies with tilt angle, reaching +/- 20% in the brighter parts of the A ring. The A ring's asymmetry is even more pronounced in Voyager images of the unlit face of the rings (Franklin et al. 1987, Icarus 69, 280). In low-phase, lit-face images, Dones et al. 1993 (Icarus 105, 184) found that the asymmetry's amplitude shows a narrow peak in the mid-A ring. The most promising explanation for the asymmetry invokes the spontaneous formation of transient gravitational ``wakes'' (e.g., Colombo et al. 1976, Nature 264, 344; Salo et al. 2000, this meeting), which can form more easily further from Saturn due to the weaker tidal forces there. Taking advantage of the remarkable resolution and photometric accuracy of the Hubble Space Telescope's WFPC2, we have accurately measured this asymmetry as the rings have changed from their nearly edge-on aspect to their current more open configuration. Our observations span the full range of phase angles accessible from the Earth (α <6o), with densest sampling near opposition, at wavelengths from 255 nm to nearly 1 μ m, including the standard UBVRI filter set. The amplitude, phase, and shape of the brightness asymmetry vary substantially with ring orbital radius, sub-solar longitude, solar phase angle, and ring opening angle as seen from the Earth (B) and the Sun (B'). The asymmetry varies only slightly with wavelength, due perhaps to the relative unimportance of multiple scattering over a range of ring albedos. We compare the measured amplitude, location of minimum brightness, and detailed shape of the azimuthal variations with profiles predicted using N-body simulations of gravitational instabilities (Salo et al. 2000, this meeting). The agreement is remarkably good, both confirming the underlying physical explanation for the asymmetry and providing strong constraints on ring particle properties. This work was supported in part by NASA Grant GO-06806.01-95A.
AAS/Division of Dynamical Astronomy Meeting
- Pub Date:
- May 2000