Discovery of a second narrow absorption feature in the near-infrared spectrum of Io

Abstract

Extension of our high resolution survey of the near-infrared reflectance spectrum of Io to shorter wavelengths has revealed a second relatively sharp absorption feature that, in strength, width, and constancy, appears similar to the one discovered in 1990. The new feature is centered at vacuum wavelength 1.9820 ± 0.0005 μm (5045 ± 1 cm −1) and has an equivalent width of about 0.5 cm −1 (5% deep in our spectra). This new feature is not present in the laboratory spectra of dilute CO 2 in a matrix, and therefore CO clusters, which have been proposed as an identification for the carrier of the original narrow absorption feature at 2.125 μm, do not appear to be responsible for the second feature. The spectrum of cold H 2S ice crystals shows considerable structure near this wavelength. The difference in width between this feature and that of the much broader Ionian H 2S ice features at longer wavelengths may be the result of different phases of ice, at different temperatures, that are emphasized in the different spectral regions. Alternatively, the trapping of H 2S in an SO 2 matrix may account for the small discrepancy in observed wavelength. Possible tests for the H 2S identification of the carrier are discussed.