Volcanic Resurfacing of Io: Post-Repair HST Imaging

J. Spencer, A. McEwen, M. McGrath, P. Sartoretti, D. Nash, K. Noll, and D. Gilmore

Submitted to Icarus July 20th 1996.

Revised December 2nd 1996, accepted December 12th 1996.

The following preprint is no longer available: my apologies.
Revised version now available in compressed postscript form, including figures (one in color) from here, or if that doesn't work, try anonymous ftp to ftp.lowell.edu, pub/spencer, and get file iohstrev.ps.Z. File size is 2.6 Mbytes


In March 1994, we used the newly-refurbished Hubble Space Telescope (HST) to obtain global imaging of Io at five wavelengths between 0.34 and 1.02 microns, with a spatial resolution of 160 km. The images provided the clearest view of Io since Voyager and the first systematic observations in the 0.7-1.0 micron wavelength range. We have produced absolutely-calibrated global mosaics of Io's reflectance in all our five wavelengths. The near-infrared images reveal that the 0.55-0.7 micron absorption edge seen in Io's disk-integrated spectrum has a very different spatial distribution from the better-known 0.40-0.50 micron absorption edge studied by Voyager, and must be generated by a different chemical species. The 0.55-0.7 micron absorption edge is strongly concentrated in the pyroclastic ejecta blanket of the volcano Pele, at a few much smaller discrete spots, and probably also in the polar regions. The Pele ejecta spectrum is consistent with the idea that S2O, partially decomposed to S4 (and probably S3), may be the species responsible for the 0.55-0.7 micron absorption edge at Pele and elsewhere on Io, though S4 generated by other processes may also be a possibility. S2O can be produced by high-temperature decomposition of SO2 gas, and the high temperature of the Pele volcano may account for its concentration there. Spectral anomalies of comparable size and prominence are not seen around the other ``Pele type'' volcanos Surt and Aten (McEwen and Soderblom 1983), suggesting that these volcanos, if chemically similar to Pele, are much less active. The spectrum of high-latitude regions is similar to that of quenched red sulfur glass, and if this identification is not coincidental, the glass may be preserved here by the low polar surface temperatures. Alternatively, the low polar temperatures may preserve sulfur that has been reddened by radiation. There are many changes in albedo patterns in the 15 years between Voyager and these HST observations, but these are generally subtle at HST resolution and are not strongly concentrated in longitude. However there was a major brightening of a 400 km diameter region centered on Ra Patera between March 1994 and repeat HST observations in July 1995, which was a larger albedo change than any seen in the previous 15 years. This was presumably due to a large eruption at Ra Patera, as confirmed by Galileo images. Long-exposure eclipse images of Io at 1.02 microns on March 6th 1994 place strong limits on the area of exposed silicate magma on Io at the time of the observations.
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