IUE Observations of Spatial and Temporal Variations in Io's Surface Composition

Astronomy and Astrophysics – Astronomy

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Scientific paper

The visible wavelength disk-integrated rotational light curve for Io has remained constant within 4-5% for the past 50 years; implying that the overall surface composition has remained constant despite known volcanic activity (Morrison et al. 1979). Earth-based telescopes have measured Io's spectra in an effort to map surface compositional variations. Based on IUE UV observations, Nelson et al. (1980) mapped the fractional abundance of sulfur dioxide and sulfur as a function of longitude. They found that the greatest fractional abundance of sulfur dioxide was between 70 and 140 degrees longitude, whereas the greatest fractional abundance of sulfur was between 250 and 320 degrees longitude. IR observations by Howell et al. (1984) confirmed their findings. Neither Nelson et al. (1980) or Howell et al. (1984) observed temporal changes in their datasets, which spanned only a few years. Since the work of Nelson et al. (1980), the IUE project has improved and updated their calibration software, and have reprocessed the observations between 1984 and 1986. A decade later, we aquired new observations of Io with IUE between 1995 and 1996. The earlier 1980's dataset is contemporary with the Voyager mission, while this new dataset is contemporary with the current Galileo mission. We re-examined the Voyager epoch dataset for longitudinal variations in composition and found that the data support the findings of Nelson et al. (1980) and Howell et al. (1984). The newer Galileo epoch dataset also demonstrates a leading/trailing hemisphere dichotomy in composition. On a decade time scale, the IUE data also demonstrate temporal changes in Io's surface composition. While the composition of the trailing side appears to have remained constant, the composition of the leading side has changed. The sulfur dioxide dominance on the leading side seen by Nelson et al. (1980) and Howell et al. (1984) has diminished by 20-30%, and shows evidence for a higher sulfur component than observed in the Voyager era.

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