Article,
Galileo observations of Europa's opposition effect
Icarus, 135 (1): 41--63 (1998)
Abstract
During Galileo's G7 orbit, the Solid State Imaging (SSI) camera acquired
pictures of the spacecraft shadow point on Europa's surface as well
as a comparison set of images showing the same geographic region
at phase angle alpha = 5 degrees. Coverage, obtained at three spectral
bandpasses (VLT, 0.41 mu m, GRN, 0.56 mu m; and 1MC, 0.99 mu m) at
a spatial resolution of 404 m/ pixel, shows a 162 x 220-km region
of Europa's surface located at 30 degrees N, 162 degrees W, We have
used these images to measure the near-opposition spectrophotometric
behavior of four primary Europan terrain materials: IR-bright icy
material, IR-dark icy material, dark lineament material, and dark
spot material. The high spatial resolution of the G7 images reveal
low-albedo materials in dark spots that are among the darkest features
(17% albedo at 0.56 mu m and 5 degrees phase) yet found on icy Galilean
satellites, While material of comparable albedo is found on Ganymede
and Callisto, low-albedo Europan materials are much redder, All Europan
surface materials exhibit an opposition effect; however, the strength
of the effect, as measured by the total increase in reflectance as
phase angle decreases from alpha = 5 degrees to alpha = 0 degrees,
varies among terrains, The opposition effects of IR-bright icy and
IR-dark icy materials which dominate Europa's surface are about 1.5
times larger than predicted from pre-Galileo studies, Low-albedo
materials in dark spots exhibit unusually intense opposition effects
(up to four times larger than bright icy Europan terrains), consistent
with the presence of a strong shadow-hiding opposition surge. The
strengths of the opposition surges among average Europan terrains
systematically vary with terrain albedo and can be explained in terms
of the simultaneous contributions of shadow-hiding and coherent-backscatter
to the total opposition effect. Coherent backscatter introduces a
narrow angular contribution (<0.2 degrees wide) to all Europan terrains
while the presence of a shadow-hiding contribution is revealed by
the fact that the opposition surge is especially strong in low-albedo
terrain materials. Stratigraphically young ridges with relatively
high topographic relief exhibit anomalously weak opposition surges
that could be caused by the presence of relatively coarse-grained
regolith, highly compacted particulates, exposures of solid ice,
or some combination of these. Very-high resolution images (22 m/pixel)
of young-appearing ridges suggest that downslope wasting of particulate
cover exposes more lithified ice along ridge crests and wall escarpments.
We propose that over time, erosion of ridge topography accompanied
by accumulation of detritus and mantling by regolith cover results
in more mature ridge surfaces of increasingly lower (mature) porosity.
We interpret the dark, reddish color of dark ridges and lineaments
as a particulate coating of low-albedo materials vented from (or
collected adjacent to) reactivated ridge-margins and within lenticulae.
(C) 1998 Academic Press.
