An interpretation of the near-ultraviolet absorption spectrum of SO 2: Implications for Venus, Io, and laboratory measurements

Abstract

The character of the line spectrum of SO 2 between 2000 and 3200 Å is described, and a crude band model for its absorption characteristics longward of 2280 Å is formulated. The model predicts planetary spectra that are substantially different from those predicted using a continuum model (Beer's law) for SO 2 absorption, the latter model being widely used to interpret the IUE spectrum of Io and ground-based and Pioneer orbiter spectra of Venus. As a result, the SO 2 scale height and mixing ratio in the Venus stratosphere could be substantially different from the values that have been previously supposed, thereby changing the range of acceptable photochemical models. Also, it is found that the absence of the signature of SO 2 vapor absorption in Io's spectrum can be treated as a physically reasonable lower limit to the mean column abundance of SO 2 on the satellite as well as an upper limit. Thus a substantial SO 2 atmosphere may yet be found on Io. Laboratory measurements to establish accurate parameters for the band model are urgently required.