Ages and activity
- Ganymede intermediate in crater density
- some regions have been re-surfaced
- young regions comparable in age to Lunar Maria and Martian Plains
- Europa crater density low, comparable to Earth
- surface recently renewed
- water “volcanoes”?
- liquid water below solid surface
- flows through cracks, gaps, and freezes on surface
- Io crater density extremely low -- in fact none at all seen!
- Youngest surface in the solar system
- 10 active volcanoes seen by Voyager
- powered by sulphur and sulphur compounds (i.e. SO2)
- sulphur plays the role of water which powers volcanic eruptions on the Earth
- sulphur lighter than rock and floats to the surface