Compositional mapping of Jupiter's satellite Io utilizing high speed multifilter photometry during mutual satellite occultations, 1990-1991

We observed selected mutual occultations of Jupiter's satellites in 1990-1991 at four wavelengths and we used the occultation profiles to constrain the areal distribution of selected spectrally active compounds on Io's surface. We performed high speed photometry of Io at four wavelengths while it was being occulted by another Galilean satellite. The wavelengths of the filters that we used for this work were 0.305, 0.345, 0.45, and 0.56 [[micro]meter]. These wavelengths were selected to constrain the areal distribution of sulfur and/or sulfur-bearing compounds and sulfur dioxide (S[O.sub.2]) frost, believed to be the two major surface constituents on Io. During a typical occultation, a Galilean satellite whose spectral properties are well defined passed in front of Io, gradually blocking the sunlight reflected by Io from reaching an Earth-based observer. In the observations reported here the occulting satellite was always Europa. Europa's contribution to the combined signal was removed and the remaining Io occultation brightness variations were compared to curves produced by synthesized occultations that would be expected assuming a two component surface, where one component is an unknown mixture of sulfur bearing materials and the other is S[O.sub.2] frost. We utilized the results of previous IUE derived ultraviolet longitudinal phase angle variation studies (Nelson, R. M., A. L. Lane, D. L. Matson, F. P. Fanale, D. B. Nash, and T. V. Johnson 1980. Science 210, 784-786) which localized the distribution of S[O.sub.2] on Io in longitude. Thus, we were able to vary the distribution of S[O.sub.2] in latitude and produce a synthetic lightcurve based on the reflectance properties of solid S[O.sub.2] and elemental sulfur. This was done by simulating different distributions of sulfur bearing materials and S[O.sub.2] frost on Io's surface and minimizing the difference between the synthetic lightcurve and the observed lightcurve. On Io's trailing hemisphere, we find that the region north of the equator between latitudes 0 [degrees] and 60 [degrees] and between 180 [degrees] and 270 [degrees] longitude to be highest in concentration of S[O.sub.2] frost. It also coincides with the highly reflective 'white areas' seen in the Voyager spacecraft television images. All of the observations reported are of Io's trailing hemisphere.