[Previous] | [Session 48] | [Next]
P.D. Nicholson, D.B. Campbell (Cornell), R.G. French (Wellesley), G.J. Black (NRAO), J-L. Margot, M. Nolan (NAIC)
In October 1999 we obtained the first radar images of Saturn's rings, using the Arecibo telescope operating at a wavelength of 12.6~cm. The opening angle of the rings, B, was -19.9\circ, and dual-circular polarization data were collected over a period of 5 days, at ~2~min of receive time per day. We used a frequency-stepped technique similar to that used by Ostro et al. [Icarus, 49, 367 (1982)]. A train of sixteen 100~msec pulses was transmitted every 2.2~sec, an interval slightly longer than the delay depth of the rings, providing a range resolution of 15,000~km. The return echoes were processed to a frequency resolution of about 2~kHz, corresponding to a spatial resolution at the ring ansae of ~000~km. In order to avoid return pulse overlap, the transmitter frequency was stepped by 800~kHz between pulses. Our observations yield a circular polarization ratio, \muC = 0.70 in the A ring and 0.78 in the B ring, intermediate between values obtained in the 1970s at B=-24.4\circ and -11.7\circ [Ostro & Pettengill IAU Colloq. No. 75 (1984)]. In agreement with earlier results [Ostro et al. (1982)], we find no detectable echo from the C ring. At the 3\sigma level, we can set an upper limit of 0.06 on the ratio of its radar cross-section to that of the B Ring.
The most striking feature of the new images is a very pronounced quadrupole azimuthal asymmetry in the brightness of the A Ring, seen in both circular polarizations, which is reminiscent of that seen in ground-based and Voyager images and more recently in HST images [French et al., BAAS 32, 864 (2000)]. By generating synthetic delay-Doppler images of the individual ring components and subtracting these piecewise from the Arecibo images, we find that the azimuthal asymmetry is concentrated in the A Ring, as in visible images, though there is also a suggestion of a much weaker asymmetry in the B ring. Fits of a dynamical model of gravitational wakes [Salo & Karjalainen BAAS 31, 1160 (1999)] to the data show that the asymmetry in the radar maps is over three times that measured in HST images obtained at 439~nm and the same opening angle. This work was supported by NASA's Planetary Geology & Geophysics Program.