A new class of absorption feature in Io's near-infrared spectrum

We report our discovery of an absorption feature in the infrared spectrum of Io centered at 2.1253 micrometers (4705.2 cm-1). This band is marginally resolved at resolving power 1200 with a deconvolved full width at half-maximum (FWHM) of about 4 cm-1. This contrasts with the 30- to 50-cm-1 widths of the broad absorption features previously detected at longer wavelengths which arise from mixtures of SO2 with H2S and H2O. This newly discovered feature is relatively weak, having a core only 5% below the continuum at this resolving power. Our survey from 1.98 to 2.46 micrometers (5050-4065 cm-1) at this same resolving power revealed no other feature greater than 1% of the continuum level shortward of 2.35 micrometers, and 3% elsewhere. The feature does not correspond to any gas- or solid-phase absorption that might be expected from previously identified constituents of Io's surface. No temporal or longitudinal variation has been detected in the course of 18 nights of observation over the past year and no significant variation in the strength of the feature was seen during an emergence from eclipse. These observations indicate that the source material of the feature is reasonably stable, and is more uniformly distributed in longitude than Io's hot spots. These characteristics all indicate that the feature belongs to a class different from those characterizing other known absorption features in Io's spectrum. Consequently, it should reveal important new information about Io's atmosphere-surface composition and interaction. A series of laboratory experiments of plausible surface ices indicates that (i) the band does not arise from overtones or combinations of any of the molecular vibrations associated with species already identified on Io (SO2, H2S, H2O) or from chemical complexes of these molecules, (ii) the band does not arise from H2 trapped in SO2, and (iii) the band may arise from the 2 nu3 mode of CO2. If the band arises from CO2, it is clear from its detailed shape and position that the molecules are not embedded in an SO2 matrix, as are H2S and H2O, but may be present as multimers or "clusters."