Diagram showing the interior of the Earth's Moon. The outermost layer, the crust, is about 45 miles (70 km) thick. This is thicker than the Earth's crust, which cooled down at a much slower rate. Beneath the crust is a thick silicate mantle, then a zone of partial melt with a radius of 480 km. This is probably where moonquakes occur. Current thinking, based on a re-examination of Apollo lunar seismometer data, is that the core, once thought solid, is now composed of a liquid outer component (330 km radius) and a solid inner one (240 km).