The Origin and Evolution of the Solar System

26 The structure of the Solar SystemThis contrasts with the maria basalts which cooled quickly and are fine-grainedbecause the crystals had little time to grow. Like the maria material the highlandrocks contain particulate iron and are also deficient in water and volatile elementsbut in terms of chemistry and mineral compositions the two types of material arevery different. The main metallic components of the dark lava are iron, magnesiumand titanium while the lighter-coloured highland material is rich in aluminiumand calcium. More than 50% of highland rocks are plagioclase, a mixtureof albite (NaAlSi ¿ O ) and anorthite (CaAl ¾ Si ¾ O ), with varying amounts ofpyroxene ´Å µËÇ ¿ , olivine ´Å µ ¾ ËÇ and some spinel, a metallicoxide. The lower-density crust material comes from differentiation of the bulkMoon as a result of large-scale melting of surface material early in the Moon’shistory.The common minerals in the lunar basalt are clinopyroxene, a calcium-richform of pyroxene and anorthite-rich plagioclase. There can also be up to 20%olivine but in most basalt it is absent.The ages of the highland rocks, that is from the time they became closedsystems, have been deduced from radioactive dating. They are usually in therange 4.0–¾ ¢ ½¼ years except for one Apollo 17 sample which has an ageof ¢ ½¼ years, close to the accepted age of the Solar System and of theMoon itself. There seems to have been a 400 million year period when eitherrocks did not form on the surface or during which the rocks which had formedwere being destroyed in some way. The ages of lunar basalts vary from 3.16to ¿ ¢ ½¼ years, which shows that volcanism occurred on the Moon at leastduring that period. However, since older material gets covered by newer it is alsopossible that volcanism could have been earlier, even as far back as the origin ofthe Moon itself.The surface of the Moon is covered by a thick blanket of pulverized material,described as lunar soil. A component of the lunar soil is called KREEPon account of its high component of potassium (K) rare-earth elements (REE)and phosphorus (P). It also contains more rubidium, thorium and uranium thanis found in other lunar rocks. The majority of KREEP material is found in thevicinity of Mare Imbrium and could be material excavated from 25–50 km belowthe surface when the basin was formed.A characteristic of the total surface is the general deficiency of volatile elementscompared to the Earth. This is illustrated in figure 1.13 which shows theabundance of various elements relative to the Earth as a function of their condensationtemperatures. It can be seen that there is a general trend for a lesserfractional abundance of the more volatile elements in the Moon with a balancinggreater abundance of the more refractory materials. The discovery of small watericedeposits in some well-sheltered parts of the Moon in 1998 goes against thistrend. However, this ice was probably deposited by comet impacts long after theMoon’s surface had become cool.