Chemical CompositionDirect information on the chemical
Composition of the Moon became available in 1969 with the return of
the first Apollo mission. Although the data refer only to the rocks collected on the surface, there is no reason to believe that the
composition of the interior of the Moon would be essentially different. By atomic composition, the most abundant element found on the Moon is oxygen. It composes 60% of the Moon's crust by weight, followed by 16Ð17% silicon, 6Ð10% aluminum, 4Ð6% calcium, 3Ð6% magnesium, 2Ð5% iron, and 1Ð2% titanium. All other
elements are present in amounts very much smaller than 1% by weight. The elements oxygen, silicon, and aluminum are present on the Moon in amounts comparable to their existence in the crust of the Earth. Iron and titanium contents are distinctly enhanced on the Moon, as compared to the Earth, while the alkali metals are less abundant, as are carbon and nitrogen.Of the compounds formed by these elements, silica (SiOM) constitutes between 40 and 50% of the Moon's crust by weight, compared to 48.5% in the crust of the Earth. Ferrous oxide (FeO) and calcium oxide (CaO) constitute 10 to 20% of each. All oxidized compounds appear to be present on the Moon only in their lowest states of oxidation, because they solidified at temperatures between 1,100 and 1,200¡ C (2,000 and 2,200¡ F). Any free hydrogen on the Moon would be that imported by the solar wind, and water that might be produced by its oxidation would be quickly dissociated by sunlight. A report on lunar data from the 1994 mission of NASA's Clementine spacecraft, however, suggested the presence of water ice. This was almost conclusively confirmed by Lunar Prospector, another NASA spacecraft that began orbiting the Moon in January 1998. If the ice does exist, it lies in the polar regions in permanently shadowed craters. It would be in the form of crystals mixed in with particles of other surface materials, and would constitute a small percentage of this mixture. Such ice might even so be sufficiently abundant to support a future lunar colony for some time, if extracting it did not prove prohibitively expensive.