Radio observations of Saturn as a probe of its atmosphere and cloud structure

Abstract

Centimeter and decimeter radio observations probe atmospheric depths of the giant planets that are inaccessible by other techniques. Presented here are results of theoretical modeling of the NH 3H 2SH 2O cloud decks of Saturn and their influence on the radio opacity of the planet's atmosphere. The study is motivated by the acquisition of a new radio data set spanning the wavelength range 1.3 to 70 cm. Disk-averaged brightness temperatures are 132.5 ± 1.3 K at 1.33 cm, 136.1 ± 3.2 K at 1.99 cm, 173.6 ± 0.8 K at 6.14 cm, 219.0 ± 3.5 K at 20.73 cm, and 352 ± 42 K at 69.72 cm. These temperatures are in general agreement with those previously reported in the literature, but have smaller uncertainty, which improves the discrimination between models. Within the text of a detailed atmospheric model, we found that satisfactory fit to the radio brightness spectrum can be obtained if (1) the ammonia mixing ratio deep in the atmosphere (∼ 25 bar) takes on a value in the range 4 to 6 × 10 −4, which is ∼ 3 times that inferred from solar abundances of the elements, and (2) H 2S is present in an amount ∼ 10 times the solar value so that a substantial NH 4SH cloud forms, leading to an ammonia mixing ratio of 0.7 to 1.1 × 10 −4 in the vicinity of the 2-bar level in the atmosphere. There is an apparent increase of microwave absorption as lines of sight cross the NH 4SH cloud (at ∼ 5 bar). Water does not strongly influence the microwave spectrum either as a microwave absorber or in creating cloud decks that remove ammonia, although precise radiometry at decimeter to meter wavelengths may ultimately be able to place limits on the deep atmospheric water abundance.