The moons Titania and Oberon, the largest and most distant satellites of the planet Uranus, show different signs of cratering.
On Titania, the craters are small, which probably means that the original large craters were eroded by some kind of geological force.
Astronomers believe that large surface cracks, over 600 miles (960 km) long and 3 miles (5 km) deep, exuded slushy ice after large meteors hit the surface.
After the period of bombardment, Titania’s interior cooled to solid ice, leaving the cracks above visible.
Oberon has no cracks, but it does display a geologic time line of craters, from early, large impacts to later, smaller ones.
While the surface seems to have remained mostly unchanged, melted matter from its interior has apparently leaked out to cover the floors of some craters.
Both of these moons are rockier than expected.
The ice under their surfaces is probably not pure water, since the moons do not get warm enough to melt water ice.
Methane clathrate, a mixture of methane and water, melts at a lower temperature, and is likely the composition of the moons’ ice.