of 4.1 
and 0.5 , 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.
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 of 4.1
and 0.5 , 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 = 0.5 than at
= 4.1 . 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 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 N, 190 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 
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 S, 180 W.