Abstract
Near-infrared observations of Europa's disk-integrated opposition
surge by Cassini VIMS, first published in Fig. 4 of Brown et al.
(2003, Icarus, 164, 461), have now been modeled with the commonly
used Hapke photometric function. The VIMS data set emphasizes observations
at 16 solar phase angles from 0.4degrees to 0.6degreesthe first time
the < 1degrees phase "heart" of Europa's opposition surge has been
observed this well in the near-IR. This data set also provides a
unique opportunity to examine how the surge is affected by changes
in wavelength and albedo: at VIMS wavelengths of 0.91, 1.73, and
2.25 mum, the geometric albedo of Europa is 0.81, 0.33, and 0.18,
respectively. Despite this factor-of-four albedo range, however,
the slope of Europa's phase curve at < 1degrees phase is similar
at all three wavelengths (to within the error bars) and this common
slope is similar to the phase coefficient seen in visible-light observations
of Europa. The two components of the opposition surge-involving different
models of the physical cause of the surge-are the Shadow Hiding Opposition
Effect (SHOE) and the Coherent Backscatter Opposition Effect (CBOE).
Because of sparse VIMS phase coverage, it is not possible to constrain
all the surge parameters at once in a Hapke function that has both
SHOE and CBOE; accordingly, we performed separate Hapke fits for
SHOE-only and CBOE-only surges. At 2.25 mum, where VIMS data are
somewhat noisy, both types of surges can mimic the slope of the VIMS
phase curve at < 1degrees phase. At 0.91 and 1.73 mum, however-where
VIMS data are "cleaner"-CBOE does a noticeably poorer job than SHOE
of matching the VIMS phase coefficient at < 1degrees phase; in particular,
the best CBOE fit insists on having a steeper phase-curve slope than
the data. This discrepancy suggests that Europa's near-IR opposition
surge cannot be explained by CBOE alone and must have a significant
SHOE component, even at wavelengths where Europa is bright. (C) 2004
Elsevier Inc. All rights reserved.
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