One of the Apollo 14 excursions was in order to deploy seismometer equipment on the Moon's surface.
THE INTERIOR OF THE MOON
 
Thanks to Apollo we have a fairly good idea of the interior structure of the Moon. The astronauts bought back 382 kg of lunar material, which is still being studied today. They set up experiments on the surface. Close up pictures and their descriptions all contributed enormously to our understanding of the Moon and its evolution.
 
More recently, the Clementine mission has allowed researchers to investigate the composition of the Moon over large areas in the way other satellites are used on Earth to detect different types of minerals. Seismometres (earthquake recorders) were set up on the surface of the Moon to record "moonquakes" or tremors. They were to measure the speed of shock waves through the rocks in order to build a picture of the internal structure. The seismometres were left on the Moon and the tremors monitored from Earth
 

A close-up view of the Passive Seismic Experiment, set up on the Moon by the Apollo 14 astronauts.
 
When the seismometres were set up it was not known if they would record anything. As it turned out there are something like 3000 moonquakes a year. They're very mild compared to quakes on Earth. They would only register as much as 2.0 on the Richter Scale, a value of two being the weakest that can be felt by someone standing on the surface.
 
Some natural moonquakes are triggered by meteorites. In 1972 a 1000 kg meteorite hit the Moon - the shock waves were recorded by the seismometres. Most moonquakes result from internal movements and occur at a depth of 600 to 900 kilometres. These are generated by tides, the effect of Earth's gravity on the shape of the Moon. Shallower ones have been attributed to the expansion and contraction of the lunar surface as it is heated and cooled.
 
The Moon has a fairly low density, lower than the Earth's, showing that it is relatively poor in heavy metals like iron and nickel. Although some iron has been detected, the Moon is richer in lighter elements. The discovery that the Moon is mostly made from lighter rocks fits with the theory that it was formed from a jet of molten mantle material escaping from the Earth in a collision.
 
So what do we know about the Moon's structure? Estimates of the thickness of the crust give it an average depth of about 70 kilometres. The crust on the near side though is thought to be a thinner, while the farside crust may exceed 100 kilometres in places. Averaged out, the volume of the crust is just over 10% of the Moon's globe.
 

Cross-section through the Moon.
 
As different processes were operating at the same time at periods in the Moon's history, the crust is thought to be quite layered. A cross section though the crust might reveal alternating layers of lava, various types of crater deposits, old surface layers, and thin layers of eruption material like volcanic glass. Cratering complicates the picture because it excavates older materials, placing it on top of younger rocks.
 
The seismic experiments showed that the Moon has a mantle beneath the crust. But though we have a good idea of the composition and structure of the upper layers, the core of the Moon is still a mystery. No magnetic field has been detected that would suggest that the Moon has a surviving magnetic core. The only magnetism recorded, is in the samples returned to Earth showing that their particles were aligned by a magnetic field when they were molten, early in the Moon's development.
 
Experiments were set up by the Apollo astronauts on the surface of the Moon, to see if there was any heat left inside the Moon. They showed that at a depth of 2 metres there was a rise in temperature of 1o. This small difference suggests that the deep interior of the Moon is still hot enough to be molten.