What causes a Planet's surface to change its appearance?

This is an image of the Martian dunefields. Click on image for full size version (40K GIF) Image from: NASA

Over the course of time there are many things which can cause the surface of a planet to change its appearance.

• winds, as shown in the example from the Martian surface
  • Monument Valley on Earth is an example
• weather & water, which cause erosion
• volcanism, which pours out a new surface
  • The Moon is an example.
• continental drift
• slow forces of deformation like those which cause mountains to form.
  • The crust of Ganymede is an example.
• slumping of craters, mountains and volcanoes.
  • The surface of Callisto is an example.

In their earliest histories, every planet & moon was bombarded with the remains of the material which formed them. If a planet's surface does not show many craters, it means that the surface is new, and the planet has been resurfaced, perhaps by one of the processes above. If the planet's surface still shows the many craters left over from it's formation, then that surface is very old, and has not been changed by any activity.