POSSIBLE MICROWAVE-ABSORPTION BY H2S GAS IN URANUS AND NEPTUNE ATMOSPHERES

We present new VLA observations of Neptune at 3.55 and 20.1 cm. The disk-averaged brightness temperatures are 191.2 ± 6 K at 3.55 cm, and 276.4 ± 10 K at 20.1 cm. These values are consistent with thermal spectra of the planet. We further present a comparison between Uranus and Neptune's spectrum, using improved atmospheric models for both planets. We improved the old models by including microwave absorption by H2S. The rotational lines of this gas, which are all at (sub)millimeter wavelengths, are pressure broadened to such an extent that considerable opacity at centimeter wavelengths is expected. We estimate from our calculations that the H2S mixing ratio on Uranus and Neptune is likely enhanced by a factor of 10-30 above the solar sulfur elemental ratio, and that the S/N ratio must exceed 5 × the solar ratio. Our calculations suggest the width of the H2S line to be similar to or less than that of water. There is more microwave opacity in Neptune's atmosphere than in that of Uranus, which may be caused by the presence of some NH3 gas above the NH4SH cloud layer. From a comparison with the radio occultation data, we suggest that NH3 is likely supersaturated in Neptune's atmosphere at levels where T ≲ 210-225 K, or P ≲ 20-25 bar. © 1991.