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Last Updated: 03/10/05

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The Identification of Ices in the Solar System

The composition of ices on other bodies in our Solar System can be determined by comparing the infrared spectra of laboratory ices with the infrared spectra of these objects taken using telescopes. An example of how this is done is provided below.

Io is one of the Galilean moons of the planet Jupiter is perhaps most famous as the Solar System's most volcanically active body. The picture below shows the volcanic plume of one such eruption as it was caught in the act by NASA's Galileo spacecraft (click on the image for a larger version). Also shown are several infrared spectra that demonstrate that much of the surface of Io is covered by SO₂ ice. 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.

Detailed comparison of the laboratory and telescopic data can be used to determine more than just the identity of the ices present. For example, the details of the spectra shown above also tell us that the SO₂ is about 30% more abundant on Io's leading side than on its trailing side and that the ices are relatively well crystallized, i.e., the SO₂ is more in the form of ic cubes than in the form of a frost. This is to be expected on the basis of the prevailing temperature conditions on Io.

Another example of spectral matching for the identification of materials on Io's surface is provided below, where our combined laboratory and telescopic work has provided indications that Io's surface ices may contain trace amounts of H₂O. In this case, the comparison is between the infrared spectra of laboratory SO₂ ices that contain a few percent H₂O and a spectrum of Io taken using the Kuiper Airborne Observatory (KAO). Both show a strong absorption feature near 3600 cm^-1. Similar comparisons have also suggested that these ices contain traces of H₂S, the poisonous compound that gives rotten eggs their distinctively bad odor.

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

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.

Salama, F., Allamandola, L. J., Witteborn, F. C., Cruikshank, D. P., Sandford, S. A., & Bregman, J. D. (1990). The 2.5-5.0 µm spectra of Io: Evidence for H₂S and H₂O frozen in SO₂. Icarus 83, 66-82.

Bohn, R. B., Sandford, S. A., Allamandola, L. J., & Cruikshank, D. P. (1994). Infrared Spectroscopy of Triton and Pluto Ice Analogs: The Case for Saturated Hydrocarbons. Icarus 111, 151-173.

All in all, these comparisons suggest that Io will not be a particularly pleasant place to visit. For a science fiction tale about what it might be like to live on Io, consider reading the story "Stinker Station" written by Astrochemistry Laboratory member Scott Sandford that appeared in the June 1991 issue of Analog Magazine.