Laboratory measurements of the microwave opacity of gaseous Ammonia (NH 3) under simulated conditions for the Jovian atmosphere

Abstract

Gaseous ammonia (NH 3) has long been recognized as a primary source of microwave opacity in the atmosphere of Jupiter. In order to more accurately infer the abundance and distribution of ammonia from radio emission measurements in the 1 to 20-cm wavelength range and radio occultation measurements at 3.6 and 13 cm, we have made measurements of the microwave opacity from gaseous ammonia under simulated conditions for the Jovian atmosphere. Measurements of ammonia absorptivity were made at five frequencies from 1.62 to 21.7 GHz (wavelengths from 18.5 to 1.38 cm), at temperatures from 178 to 300°K, and at pressures from 1 to 6 atm, in a 90% hydrogen/10% helium atmosphere. The results of these measurements show that in the 1.38- to 18.5-cm wavelength range, the absorption from gaseous ammonia is correctly expressed by the modified Ben-Reuven lineshape as per G.L. Berge and S. Gulkis (1976, Earth-based radio observations of Jupiter: Millimeter to meter wavelengths. In Jupiter (T. Gehrels, Ed.), pp. 621–692, Univ. of Arizona Press, Tucson). When applied to the microwave opacity measured by radio occultation measurements, or the microwave opacity inferred from radio emmission measurements, these results suggest that either an abundance of ammonia 1.5 to 2.0 times greater than the solar abundance must exist at levels below the 1- to 2-bar pressure range, or that some other microwave absorber must be present. We conclude by suggesting further laboratory measurements of other potential microwave-absorbing constituents and additional investigation of the microwave absorption from ammonia in the 10- to 20-cm wavelength range and at wavelengths shortward of 1 cm.