The largest moons in our Solar System aren't very different in terms of mass and geophysics from the diminutive planet Mercury. While they are controlled by the same geophysics we see in studying the terrestrial worlds, they also have some truly unique attributes. For example, Io, the sulfur-dominated satellite of Jupiter, has the most active volcanoes of any body in the solar system. Another moon, Europa, is covered with smooth, mysteriously banded ice with no land at all. And Saturn's moon Titan possesses a nitrogen atmosphere denser than the Earth's rains methane across a terrain with rivers and deltas. All this diversity makes the satellites of the outer Solar System exciting to explore.
Jupiter and its moons Callisto and Europa. Click here for original source URL
Most of the satellites in the outer Solar System are made of an ice-rock mixtures, and have highly reflective (white), icy surfaces or darker, tan-gray surfaces where older ice has reacted with cosmic rays. While many of the moons could be mistaken for pure ice Kuiper Belt objects or comet nuclei, the largest moons also have rock cores.
Composite image of Titan with haze of the atmosphere removed. Click here for original source URL.
To understand such satellites, we can consider some basic principles of comparative planetology. For instance, surfaces with fewer impact craters are younger. Thus, whenever we see an uncratered region on a satellite (or in the case of Europa a relatively uncratered entire surface) we know that some geological force has been at work to modify and renew its surface. Many different possibilities exist. Several of the icy moons are either known to posses (i.e. Enceladus) or are suspected to possess geysers or cryovolcanos. Material falling back from these geologic features can resurface icy moons just as lava from erupting volcanoes can resurface with planets (and Io).
Another tenet of comparative planetology is that worlds with more internal heat have more geological activity. While most satellites are just cratered ice balls with no internal activity, tidal effects such as a gravitational kneading of a moon can transfer energy to that moon and drive geological processes. This effect is seen on Jupiter's moons Europa and Io. These moons have slightly elliptical orbits, and as their distances from Jupiter (and other moons as well) alter, they cyclicly flex and heat. This drives icy plate tectonics on Europa, and volcanism on Io.
Most satellites in the outer Solar System are actually geologically inert chunks of ices and trace amounts of organic material. These compounds start evenly mixed in highly reflective ices. As cosmic rays and interactions with the solar wind weather these surfaces, the chemistry can evolve, and black organic compounds will form. These black, carbon-rich organic compounds are called carbonaceous materials. Even a small percentage of the material can stain ice black. This material builds up over periods of years, but if even modest heating occurs, the underlying icy component can melt and erupt on the surface, leaving a fresh white surface. Satellites (and Kuiper Belt objects) often show either dark surfaces or bright, icy surfaces, depending on their histories. The interacting roles of black carbonaceous material and white ice add to the many unexpected properties of these diverse worlds.
Together, planets, asteroids, Kuiper Belt objects, and planetary moons form a continuum of geological environments. By studying all of them, nature allows us to see how varying different characteristics effects the observed geologic processes. This can be used to limit our theories, and motivate further exploration of the Solar System.