Jupiter and its moons are often compared to the Sun and its planets. One reason for this is that tidal forces have played an important role in the amount of geological activity on the moons. The volcanic activity on Jupiter's moon Io occurs because It is geologically young and has not yet cooled completely. The interior is heated by radioactive decays. It is tidally heated by Jupiter.
Meteorites break open the surface exposing the hot interior.
The surfaces of the main moons of Jupiter seem to have different ages because the moons themselves were formed at different times. The most abundant gas in the atmosphere is nitrogen. The Sun's ultraviolet light causes complex molecules to form. The surface is covered with geysers spewing molten sulphur. It may have an ocean of liquid ethane.