Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old.

Two units of smooth plains were also observed by Voyager 2.

A small part of the surface is covered by smooth plains that may be cryovolcanic in origin.

A smaller part of the surface is represented by the smooth plains on the trailing hemisphere.

One other possibility is that the very smooth plains are volcanic.

They walked across a great smooth plain of plastic turf.

They turned their back on the crater and peered across the broad smooth plains to the south.

The entire district is situated on a smooth plain.

Both of these features should be apparent in the upper half of Figure 7.5, which shows a smooth plain.

You have seen that the smooth plains may well be of non-volcanic origin.