As with the terrestrial planets and Earth’s Moon, surface
features provide critical clues to a moon’s geological
history. For example, water
ice is a common surface
material among the
moons of the outer Solar
System, and the freshness
of that ice indicates the
age of those surfaces. Meteorite dust darkens the icy surfaces
of moons just as dirt darkens snow late in the season
in urban areas. A bright surface often (but not always)
means a fresh surface.