Time variability in the radio brightness distribution of Saturn

We present images of Saturn at wavelengths of 0.35, 2.0, 3.6, and 6.1 cm ta ken in 1990-1995. These include the first radio images of the planet's enti re southern hemisphere, which is shown to be approximate to 5% brighter tha n the northern at 6.1 and 2.0 cm, and possibly at 0.35 cm. The latitudinal brightness distribution varies substantially over time. The bright band at latitude 30 degrees N seen throughout the 1980s at wavelengths 6.1 cm and l onger by A. W Grossman, D. O. Muhleman, and G. L. Berge (1989, Science 245, 1211-1215), by I. de Pater and J. R. Dickel (1991, Icarus 94, 474-492), an d in our 1990 6.1-cm image is absent in our 6.1-cm image from 1995. Instead , this image shows a bright band around latitude approximate to 40 degrees S and a dark zone around the equator. An image at 2.0 cm from 1994 shows a bright band around latitudes approximate to 40 degrees N and another one ar ound approximate to 17 degrees N which displays substructure. This contrast s with the flat 2.0-cm brightness distribution observed throughout the 1980 s. We model the changes in Saturn's brightness at radio wavelengths caused by supersaturation and humidity effects in the NH4SH and NH3-ice clouds, as we ll as by variations in the temperature structure of the upper troposphere. It is found that each of these processes is by itself able to change the pl anet's radio brightness, but that a multi-wavelength study can disentangle their effects. The 3.6- and 6.1-cm observations from 1990 can be reproduced by supersaturation of the NH4SH cloud, while humidity effects and supersat uration of NH3 ice are ruled out. Detailed modeling of the data from 1990 s hows that at northern midlatitudes, NH4SH condensed at the thermochemical e quilibrium temperatue of 235.5 K, while over most of the planet, condensati on did not occur until T = (190 +/- 5) K. Supersaturation may also cause th e dark equatorial region seen in 1995 at 6.1 cm. Observations of the rings show that the west (dusk) ansa is brighter than t he east (dawn) ansa by factors of up to 2. The polarization characteristics are as expected in the case of single scattering of Saturn's thermal emiss ion. The magnitude of the asymmetry increases with increasing wavelength an d with decreasing distance to the planet, implying the effect arises in the scattered planetary emission rather than in the rings' thermal emission. W e show that the east-west asymmetry may be due to multiple scattering in gr avitational (Julian-Toomre) wakes, although more detailed models are needed to assess this possibility. The measured brightness of the A and inner B rings as a function of scatter ing angle agrees to within approximate to 30% with model calculations by J. N. Cuzzi, J. B. Pollack, and A. L. Summers (1980, Icarus 44, 683-705) of s cattering of Saturn's thermal emission off ice particles with N(r) similar to r(-3) between r = 0.1 and 100 cm. In particular, the predicted strong fo rward peak of the scattering is clearly seen in the data. The brightness of both ansae in the outer B ring is a factor of 2 lower than that of the mod el and than the brightness at intermediate scattering angles, suggesting an excess of large (radius greater than or similar to 100 cm)particles in thi s ring. (C) 1999 Academic Press.