NEW NARROW INFRARED-ABSORPTION FEATURES IN THE SPECTRUM OF IO BETWEEN3600 AND 3100 CM(-1) (2.8-3.2-MU-M)

We report the discovery of a series of infrared absorption bands betwe en 3600 and 3100 cm-1 (2.8-3.2 mum) in the spectrum of Io. Individual narrow bands are detected at 3553, 3514.5, 3438, 3423, 3411.5, and 340 1 cm-1 (2.815, 2.845, 2.909, 2.921, 2.931, and 2.940 mum, respectively ). The positions and relative strengths of these bands, and the differ ence of their absolute strengths between the leading and trailing face s of Io, indicate that they are due to SO2. The band at 3438 cm-1 (2.9 09 mum) could potentially have a contribution from an additional molec ular species. The existence of these bands in the spectrum of Io indic ates that a substantial fraction of the SO2 on Io must reside in trans parent ices having relatively large crystal sizes. The decrease in the continuum observed at the high frequency ends of the spectra is proba bly due to the low frequency side of the recently detected, strong 359 0 cm-1 (2.79 mum) feature. This band is likely due to the combination of a moderately strong SO2 band and an additional absorption from anot her molecular species, perhaps H2O isolated in SO2 at low concentratio ns. A broad (FWHM almost-equal-to 40-60 cm-1), weak band is seen near 3160 cm-1 (3.16 mum) and is consistent with the presence of small quan tities of H2O isolated in SO2-rich ices. There is no evidence in the s pectra for the presence of H2O vapor on Io. Thus, the spectra presente d here neither provide unequivocal evidence for the presence of H2O on Io nor preclude it at the low concentrations suggested by past studie s. (C) 1994 Academic Press, Inc.