The 2.5-5.0 μm spectra of Io: Evidence for H 2S and H 2O frozen in SO 2
Abstract
Infrared spectra of Io in the region 2.5-5.0 μm, including new observational data, are analyzed using detailed laboratory studies of plausible surface ices. Besides the absorption bands attributable to sulfur dioxide frosts, four infrared spectral features of Io are shown to be unidentified. These unidentified features show spatial and temporal band strength variations. One pair is centered around 3.9 μm (3.85 and 3.91 μm) and the second pair is centered around 3.0 μm (2.97 and 3.15 μm). These absorptions fall close to the fundamental stretching modes in H 2S and H 2O, respectively. The infrared absorption spectra of an extensive set of laboratory ices ranging from pure materials, to binary mixtures of H 2S and H 2O (either mixed at different concentrations or layered), to H 2O:H 2S:SO 2 mixtures are discussed. The effects of ultraviolet irradiation (120 and 160 nm) and temperature variation (from 9 to 130 K) on the infrared spectra of the ices are examined. This comparative study of Io reflectance spectra with the laboratory mixed ice transmission data shows the following: (1) Io's surface most likely contains H 2S and H 2O mixed with SO 2. The 3.85- and 3.91-μm bands in the Io spectra can be accounted for by the absorption of the SH stretching vibration ( ν1) in H 2S clusters and isolated molecules in an SO 2-dominated ice. The weak 2.97- and 3.15-μm bands which vary spatially and temporally in the Io spectra coincide with the ν3 and ν1 OH stretching vibrations of clusters of H 2O molecules complexed, through hydrogen bonding and charge transfer interactions, with SO 2. (2) The observations are well matched qualitatively by the transmission spectra of SO 2 ices containing about 3% H 2S and 0.1% H 2O which have been formed by the condensation of a mixture of the gases onto a 100 K surface. (3) No new features are produced in the region 2.5 to 5.0 μm in the spectrum of these ices under prolonged ultraviolet irradiation or temperature variation up to 120 K. (4) Comparison of the Io spectra to transmission spectra of both mixed molecular ices and layered ices indicates that only the former can explain the shifts and splitting of the absorption bands seen in the Io spectrum and additionally can account for the fact that solid H 2S is observed in the surface material of Io at temperature and pressure conditions above the sublimation point of pure H 2S.
- Publication:
-
Icarus
- Pub Date:
- January 1990
- DOI:
- 10.1016/0019-1035(90)90006-U
- Bibcode:
- 1990Icar...83...66S
- Keywords:
-
- Hydrogen Sulfide;
- Icy Satellites;
- Infrared Spectra;
- Io;
- Sulfur Dioxides;
- Water;
- Absorption Spectra;
- Satellite Surfaces;
- Spectral Reflectance;
- JUPITER;
- SATELLITES;
- SPECTRA;
- HYDROGEN SULFIDE;
- WATER;
- SULFUR DIOXIDE;
- IO;
- INFRARED;
- LABORATORY STUDIES;
- ICE;
- SURFACE;
- ABSORPTION;
- FROST;
- IRRADIATION;
- TEMPERATURE;
- COMPARISONS;
- REFLECTANCE;
- COMPOSITION;
- MIXTURES;
- CONDENSATION;
- FORMATION;
- PRESSURE;
- SUBLIMATION;
- PROCEDURE;
- EARTH-BASED OBSERVATIONS;
- ULTRAVIOLET;
- ICY BODIES;
- WAVELENGTHS