The return of the Apollo samples 30 years ago brought about a great advance in our knowledge of the Moon. Recent measurements by the Clementine and Lunar Prospector spacecraft have provided a similar advance, and have once again highlighted a number of important questions that still need to be answered.
Early theories for the origin of the Moon focused on three main ideas: co-accretion of the Earth and Moon from the same dust and gas reservoir, capture of the wandering Moon by the Earth's gravitational field, and fission (the splitting) of the Moon from the Earth due to the high angular momentum of the system.
A fourth theory became generally more accepted after the Apollo missions, the giant impact theory. Early in the formation of the Earth, a large body approximately half the size of the Earth struck the Earth, depositing large amounts of the Earth's crust and mantle into orbit that eventually solidified to form the Moon. This theory explains many features of lunar geochemistry, including its oxygen isotope composition and depletion in volatiles, as well as explaining the orientation and evolution of the Moon's orbit.
To understand the origin of the Moon we need to understand its dramatic early thermal history. After the initial impact event, there is strong evidence for further heating causing a lunar magma ocean to have formed early in the history of the Moon. It is possible that the accretion of the Moon occurred quickly enough that it was still hot on formation.
Other heating may have occurred later due to radioactive heating and gravitational resettling. A detailed knowledge of the thermal history of the Moon is needed and has important consequences for establishing the size of the lunar core and the geochemistry of the lunar surface.
According to the giant impact theory a large portion of the Moon was completely molten right after its accretion, forming a lunar magma ocean. The lunar magma ocean hypothesis accurately predicts the formation of the low density crust we see in the form of the bright highland material on the Moon. The highlands represent the ancient crust that developed from the magma ocean prior to mare flooding. Therefore, the highlands provide an important record of the events that have occurred throughout lunar history. Measurements of the crust on a global scale are therefore directly applicable to theories of early lunar evolution.
Current thinking suggests that the Moon underwent differentiation during the magma ocean phase, in other words, it separated out. The lighter material formed the crust while the denser material sank; creating the regions that later sourced the volcanism that formed the mare. These are the darker areas on the surface of the Moon, which can predominantly be seen on the nearside (the side facing the Earth). Although the mare basalts probably account for only ~1% of the lunar crustal volume, their study is very important for the understanding of the lunar mantle and chemical history of the Moon.
Impact craters and impact basins offer an insight into the compositional structure of the crust, acting as bore-holes through which we can examine the layers directly beneath the surface. Put simply, the larger the crater, the deeper into the crust you can see. The South Pole-Aitken basin is the largest known impact structure in the Solar System with a diameter of 2500km and a physical depth of up to 12km. It is possible that this basin has excavated right through the crust exposing mantle material below. Although this possibility is still being debated, there is no doubt that the South Pole-Aitken basin has penetrated deeper into the lunar crust than any other impact.