There are two easily discernible types of regions on the surface of Enceladus, one smooth, the other heavily cratered.  Because craters created by meteor impacts are ex- pected on a moon, the observed smoothness suggests that Enceladus’s internal heat has melted part of the icy surface, eras- ing craters in the area. Like Jupi- ter’s moon Io, Enceladus may be heated by a gravitational tug-of-war between Saturn and other moons. 
    Enceladus may be geologically active today. This photograph shows unusual parallel lines on Enceladus’s surface, similar to those on Jupiter’s moon Ganymede. The lines form a system of surface faults extending around the moon, which is only 510 kilometers in diameter.