>> Brian Skiff would know better....
Not sure what it is I'm supposed to know, but a look at the Almanac
suggests a simple albedo difference would account for Callisto being dark:
body albedo
Jupiter 0.52
Io 0.63
Europa 0.67
Ganymede 0.44
Callisto 0.20
The Almanac also lists Jupiter's surface brightness as around 5.4/square
arcsec. You could also take the magnitudes of the moons and their
angular sizes and do the arithmetic to find their mean surface brightnesses
as well (mean opposition mangitudes are 5.0,5.3,4.6,5.7 for Io, Europa,
Ganymede, Callisto, respectively). Presumably you could get accurate
values for angular diameter and magnitude for any particular night and
use those numbers for the calculations.
\Brian
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