The chemical composition of the Moon

The overall chemical composition of the Moon, the Earth's only satellite, is now known in broad terms, but ideas on its internal structure and detailed chemistry are still conjectural. Hypotheses for the origin of the Moon are numerous, but considerations of lunar chemistry as well as of celestial dynamics strongly limit the possibilities.

The low mean density of the Moon (3340 kg/m3 ) indicates that, relative to the Earth, the Moon is depleted in iron (the most plentiful heavy element in the solar system), and this is reflected in the presence of only a small metallic iron core, independently deduced from evidence of a lunar magnetic field.

4.9 : A selection of tektites to show the range of aerodynamically moulded forms

Samples of rock returned from the Moon during the Apollo missions maybe separated into two classes on the basis of their mineral content and chemistry. In the first group is the rock type called anorthosite. forming a lunar crust that is 40 km thick; the second group comprises basalts which occupy the lunar maria (plains).

The lunar anorthosite crust separated from a magma ocean perhaps 400 km thick about 4400 million years ago. by upward flotation of plagioclase feldspar (CaAl₂Si₂O₈ ). Because of the preponderance of plagioclase. the lunar crust has a notably high content of calcium and aluminium. Lunar anorthosites have chemical characteristics unlike those of comparable terrestrial counterparts; in particular the calcium content of plagioclase is unusually high, and the abundances of certain volatile elements (Br. Cd. Ce. Sb, Te, Zn etc) are elevated relative to mare basalts. Furthermore, the entire lunar crust has been subjected to intense meteorite bombardment, contaminating surface rocks with material of a chondrite type of chemistry.

There also exists a group of crustal rocks termed norites, which are notable for either high or low contents of certain 'incompatible' elements, including potassium (K), the rare earth elements (REE) and phosphorus (P). to which the acronym KREEP is applied. Incompatible elements are so termed because during partial melting they are selectively concentrated from the solid into the melt fraction. KREEP norites appear to have formed by local partial melting of the anorthositic crust.

Lunar basalts resemble terrestrial basalts in overall chemistry, each having been derived by partial melting of the mantles of their respective parent bodies. However, two major differences exist in lunar basalts : a wide variation in abundance of titanium and certain other elements in some mare basalts, coupled with extreme depletion of the volatile trace elements, water and carbon dioxide in all basalts sampled. Depletion of the most volatile major elements, silicon and sodium, is also evident in lunar basalts relative to their terrestrial counterparts. It is believed that low-titanium basalts were derived by simple partial melting of the lunar mantle, whereas the high-titanium basalts were contaminated by mixing with the residual heavy mineral sink' left over beneath the crust, including the titanium mineral ilmenite (FeTi0₃).

Lunar basalts possess abundances of non-volatile siderophile elements (Ni, Co, W, Os, Ir, P) which closely resemble those in the Earth's mantle. Given that the low terrestrial mantle abundances are the result of scavenging of these elements into the metallic iron core, due to their affinity with iron, it follows that the comparable lunar abundances also require that the lunar mantle equilibrated chemically with a metallic iron phase. Yet the Moon possesses only a small core.

These and other considerations have led to the idea that the Moon split off from the Earth. It is postulated that during the Earth's melting the metallic iron segregated into the core, sweeping the siderophile elements out of the mantle. Transfer of mass towards the Earth's centre led to an increased rate of spin, such that part of the Earth's mantle was 'thrown off' by centrifugal force into a disc orbiting the Earth. The Moon formed by coalescence of the disc, which accounts for certain geochemical similarities to the terrestrial mantle and for loss of volatiles in the pre-lunar disc during fission. In this context it is significant that the volatile siderophile elements (Cu. Ga, Ce. As. Ag. Sb. Au) are depleted in the Moon relative to the Earth precisely because of this devolatilization episode.