If a huge object collided with the planet, the impact would have sent many fragments into orbit around the planet. Eventually, the fragments would have united, forming a satellite. Scientists study a satellite's surface features to learn about its geologic history. For example, the difference in the number of craters in two different areas indicates the relative age of the two areas. The surface with more craters is older.
Many scientists divide satellites into four main groups based on their geologic history.
Io, Jupiter's closest large satellite, is the most active moon in the solar system. Constant eruptions create so much new terrain that Io's surface features consist of lava flows, mountains, and pits rather than craters. The cause of Io's volcanic activity is the gravity of Jupiter and its other satellites. The different gravitational forces pull Io's interior in different directions. This process is known as tidal heating.