The top spectrum (a) was taken by Bernard Schmitt from a laboratory SO2 ice maintained at a temperature of 125 K (-148 C, -234 oF). The other two spectra were taken from Io using the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii, the first (b) from Io's trailing hemisphere and the second (c) from Io's leading hemisphere. The strong similarity in the positions and relative strengths of the absorption bands demonstrate that SO2 covers a significant fraction of Io's surface. (Content copied from Public domain source NASA archive copy at the Wayback Machine)

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For more detailed information on the infrared spectral comparisons done in the laboratory for the purposes of identifying ices within our own Solar System, see the following:

Salama, F. (1998). UV Photochemistry of Ice. In The Solar System Ices, B. Schmitt, C. de Bergh, and M. Festou, eds., Astrophysics and Space Science Library Series, (Kluwer Academic Publishers: Dordrecht), pp. 259-279.

Salama, F., Allamandola, L. J., Sandford, S. A., Bregman, J. D., Witteborn, F. C., & Cruikshank, D. P. (1995). The Detection of a New Strong Band Near 3590 cm-1 (2.79 µm) in the Spectrum of Io. In Airborne Astronomy Symposium on the Galactic Ecosystem: From Gas to Stars to Dust, eds. M.R. Haas, J.A. Davidson, & E.F. Erickson, Astronomical Society of the Pacific Conference Series, Vol. 73, pp. 337-340.

Sandford, S. A., Geballe, T. R., Salama, F., & Goorvitch, D. (1994). New Narrow Infrared Absorption Features in the Spectrum of Io Between 3600 and 3100 cm-1 (2.8-3.2 µm). Icarus 110, 292-302.

Salama, F., Allamandola, L. J., Sandford, S. A., Bregman, J. D., Witteborn, F. C., & Cruikshank, D. P. (1994). Is H2O Present on Io? The Detection of a New Strong Band Near 3590 cm-1 (2.79 µm). Icarus 107, 413-417.

Sandford, S. A., & Allamandola, L. J. (1993). The Condensation and Vaporization Behavior of Ices Containing SO2, H2S, and CO2: Implications for Io. Icarus 106, 478-488.

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