Mimas

Mimas has a very low density, which indicates that the planet is composed mostly of water and ice, with a small amount of rock.

The surface of Mimas is dominated by an impact crater 130 km across, known as Herschel; it's almost 1/3 of the diameter of the entire moon. Herschel's walls are approximately 5 km high, parts of its floor measure 10 km deep, and its central peak rises 6 km above the crater floor. The impact that made this crater must have nearly disrupted Mimas. Fractures can be seen on the opposite side of Mimas that may be due to the same impact.

   The surface is saturated with impact craters. But no others are nearly as large as Herschel. This suggests that early in its history, Mimas was probably impacted by even larger bodies than the one that created Herschel which completely disrupted the new moon (wiping out the evidence of earlier large impacts) but that the impact debris then coalesced again to form present-day Mimas. 

Enceladus

At least five different types of terrain have been identified on Enceladus. In addition to craters there are smooth plains and extensive linear cracks and ridges. At least some of the surface is relatively young, probably less than 100 million years.

   This means that Enceladus must have been active until very recently (and perhaps is still active today). Perhaps some sort of "water volcanism" is at work.

   Enceladus is much too small to still be heated by the decay of radioactive material in its interior (the heat would have all dissipated long ago).

Enceladus may be the source of the material in Saturn's tenuous E-ring. And since the material cannot persist in the ring for more than a few thousand years, it may be due to very recent activity on Enceladus. Another possibility, though, is that the rings are maintained by high-velocity collisions between dust particles and the various moons.