Porosity Variations on Io: Galileo SSI Observations of the Satellite's Opposition Surge

D. P. Simonelli, J. Veverka (Cornell), A. S. McEwen (LPL), Galileo Imaging Team

Sequences of Galileo SSI images obtained on orbits G2 (9/96) and E4 (12/96) are designed to map region-to-region variations in the opposition surge, and hence likely regolith porosity, of Io's surface. These two sequences view the anti-Jupiter face of the satellite (resolution 5-12 km/pixel) with violet, green, and near-infrared filters at solar phase angles tex2html_wrap_inline18 of 4.1 tex2html_wrap_inline20 and 0.5 tex2html_wrap_inline20 , respectively. The Galileo images complement Voyager low-phase coverage of the opposite hemisphere of Io and are an improvement over Voyager because they reach a lower phase angle and have better spatial resolution.

Disk-integrated results: In SSI's green filter, the average reflectance over Io's disk is 13% larger at tex2html_wrap_inline18 = 0.5 tex2html_wrap_inline20 than at tex2html_wrap_inline18 = 4.1 tex2html_wrap_inline20 . This is a smaller brightness increase than is predicted for these same phase angles and wavelength by the Io Hapke model of McEwen et al. (telescopic V filter; a 20% increase in reflectance; Icarus 75, 450, 1988). The discrepancy is in the wrong direction to be explained by rotational-lightcurve effects associated with the 40 tex2html_wrap_inline20 change in subspacecraft longitude between G2 and E4. Thus the side of Io seen by Galileo may have a weaker opposition surge (and a less-porous, more-compact regolith?) than indicated by McEwen et al.'s global Hapke model.

Local variations in the opposition surge: Preliminary image-ratio analysis indicates that the ``knob" of bright material extending north from Colchis Regio (35 tex2html_wrap_inline20 N, 190 tex2html_wrap_inline20 W) has a weaker opposition surge, and thus a less-porous regolith, than average Io. This observation is consistent with trends in the Voyager data, and supports the possibility that such bright areas are ``cold traps" that enhance the condensation of SO tex2html_wrap_inline38 frost in the regolith pore space (e.g., Simonelli and Veverka, Icarus 68, 503, 1986). However, the Galileo images also demonstrate that there are exceptions to the correlation between bright material and a weak opposition surge; for example, relatively weak surges are also seen in an intermediate-albedo area immediately north of the Colchis ``knob" and in dark material at 60 tex2html_wrap_inline20 S, 180 tex2html_wrap_inline20 W.