Laboratory measurements of the microwave and millimeter-wave opacity of gaseous sulfur dioxide (SO2) under simulated conditions for the Venus atmosphere

Abstract

Gaseous sulfur dioxide (SO2) has long been recognized as one of the primary absorbers in the Venus atmosphere at microwave frequencies (3 < f < 30GHz). However, the effects of gaseous SO2 on the millimeter-wave emission of Venus are not fully understood. This is mainly due to the lack of measurements of opacity under Venus-like conditions at millimeter wavelengths (λ < 1cm). As a result, we have made laboratory measurements of the opacity of gaseous SO2 in a CO2 atmosphere at 2.24 GHz (13.3 cm), 21.7 GHz (1.32 cm), and 94.1 GHz (0.32 cm). The results of our measurements show a close agreement with the absorptivity predicted from a Van Vleck-Weisskopf formalism at the two shortest wavelengths but not at the longest wavelength. In addition, our results show a frequency dependence that is slower than the f2 dependence of absorptivity proposed by Janssen and Poynter (Icarus 46, 51–57, 1981) and Steffes and Eshleman (Icarus 48, 180–187, 1981). Our results are incorporated into a radiative transfer model to infer a new abundance profile for gaseous SO2 in the middle atmosphere of Venus. Finally, the developed model is used to determine the effects of a SO2/CO2 gaseous mixture on the millimeter-wavelength spectrum of Venus.