There are several ways to group the moons of the Solar
System. Some groupings are based on the sequence of the
moons in their orbits around their parent planets; others
are based on the sizes or compositions of the moons. For example,
a few moons are predominantly rocky objects, some
are mostly ices, and most appear to be mixtures of ice and
rock. In this chapter we will group moons by the amount
and history of the moons’ geological activity.
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.
As discussed in Chapter 8, the size and number of
impact craters indicate the relative timing of events such
as volcanism, and this timing enables scientists to gauge
whether and when a moon may have been active in the
past. Older surfaces have more craters. Observations of
erupting volcanoes, which are found on Io and Enceladus,
for example, are direct evidence that some moons are geologically
active today.
Some moons in the Solar System have been frozen in
time since their formation; others are even more geologically
active than Earth. The geological classification
scheme used in this section of the chapter includes four
categories of geological activity: (1) definitely active today,
(2) probably or possibly active today, (3) active in the past
but not today, and (4) apparently not active at any time since
their formation.