Identification of Three Absorption Bands in the 2-μm Spectrum of Io
Abstract
Spectroscopic observations of the trailing face of Io at a resolution of 1.02 cm-1 performed with the FTS on the 3.6-m Canada-France-Hawaii telescope on Mauna Kea confirm the presence of two weak absorption features in the 2-μm region. The first feature occurs at 4704.9 ± 0.2 cm-1 (2.12545 ± 0.00010 μm); it is about 6.8% deep and 3 cm -twide (FWHM). The second feature is at 5047.1 ±1 cm-1 (1.98135 ± 0.0004 μm) with a broad central core; its depth is about 8% and its width approximately 7 cm-1. A laboratory investigation of spectra of solid SO2 with relatively thick samples, as well as of mixtures of SO2 with CO2 and H2S, indicates that the two features at 4705 cm-1 and 5047 cm-1 are best explained by the 3v1 + v3 and v1 + 3v3 modes of solid SO2 around 130 K, respectively. Previous work tentatively proposing CO2 clusters as an explanation for the first feature and condensed H2S for the second can no longer be supported. In addition, a careful a posteriori look at our Io spectrum shows the presence of a band at 3933 ± 1 cm-1 (2.5426 ± 0.0007 μm) (depth ≈ 30%, FWHM ≈ 8 cm-1) due to the 3 v3 band of solid SO2, as predicted by our laboratory experiments. The positions and widths of these bands indicate that a temperature gradient may exist between the surface of the frost and several centimeters below and suggest that some SO2 may be mixed at the molecular level with some neutral component. Three different models are proposed to explain the apparent discrepancy between the large variability with 1ongitude of the strong 2457 cm-1 (4.07 μm) band and the nearly constant depth of the weak 4705 cm-1 (2.1254 μm) band. All three models lead to mean grain sizes from a few hundreds of micrometers to about 1 mm. The first model invokes a large variation in grain sizes (factor 2-3) over lo's hemispheres. This model correctly fits the observed depths of the 2. and 4.07-μm bands but not that of the 3.78-μm (2645 cm-1) band. The second model assumes a considerable variation in thermal flux with Io's longitude (at 2457 cm-1, up to 20% of the reflected continuum flux). This model can reconcile the band strengths in the whole 2- to 4-μm range with about the same grain size at all longitudes and with moderate variations in frost coverage, but does not explain the albedo variability in the UV range. The third model assumes an uniform layer of coarse grained SO2 frost (a few hundred micrometers in size) with a variable longitudinal coverage of a thin layer of very fine frost grains (less than a few millimeters of micrometer-sized grains). This model qualitatively explains the observed IR and UV variations. Using the optical constants of solid SO2 measured in the laboratory, we predict the position and depth (≥-1%) of about 25 additional absorption features of SO2 frost, in the 2800-5000 cm-1 (2.0-3.6 μm) range, that should be observed in future high quality spectra of Io.
- Publication:
-
Icarus
- Pub Date:
- September 1994
- DOI:
- 10.1006/icar.1994.1135
- Bibcode:
- 1994Icar..111...79S
- Keywords:
-
- Absorption Spectra;
- Infrared Spectra;
- Io;
- Near Infrared Radiation;
- Satellite Surfaces;
- Spectroscopic Analysis;
- Sulfur Dioxides;
- Bandwidth;
- Carbon Dioxide;
- Fourier Transformation;
- Hydrogen Sulfide;
- Infrared Spectrometers;
- Mathematical Models;
- Prediction Analysis Techniques;
- Telescopes