Other
Scientific paper
Nov 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..142..125v&link_type=abstract
Icarus, Volume 142, Issue Icarus, pp. 125-147.
Other
13
Scientific paper
We present images of Saturn at wavelengths of 0.35, 2.0, 3.6, and 6.1 cm taken in 1990-1995. These include the first radio images of the planet's entire southern hemisphere, which is shown to be ~5% brighter than 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 degN seen throughout the 1980s at wavelengths 6.1 cm and longer 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), and 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 ~40 degS and a dark zone around the equator. An image at 2.0 cm from 1994 shows a bright band around latitudes ~40 degN and another one around ~17 degN which displays substructure. This contrasts with the flat 2.0-cm brightness distribution observed throughout the 1980s. 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 well 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 planet's radio brightness, but that a multiwavelength 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 supersaturation of NH3 ice are ruled out. Detailed modeling of the data from 1990 shows that at northern midlatitudes, NH4SH condensed at the thermochemical equilibrium temperatue of 235.5 K, while over most of the planet, condensation did not occur until T=(190+/-5) K. Supersaturation may also cause the dark equatorial region seen in 1995 at 6.1 cm. Observations of the rings show that the west (dusk) ansa is brighter than the 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 emission. The magnitude of the asymmetry increases with increasing wavelength and with decreasing distance to the planet, implying the effect arises in the scattered planetary emission rather than in the rings' thermal emission. We show that the east-west asymmetry may be due to multiple scattering in gravitational (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 scattering angle agrees to within ~30% with model calculations by J. N. Cuzzi, J. B. Pollack, and A. L. Summers (1980, Icarus 44, 683-705) of scattering of Saturn's thermal emission off ice particles with N(r)~r-3 between r=0.1 and 100 cm. In particular, the predicted strong forward 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 model and than the brightness at intermediate scattering angles, suggesting an excess of large (radius >~100 cm) particles in this ring.
de Pater Imke
Millan Robyn
Silva Adriana
van der Tak Floris
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