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Chemical Composition

  • Grades: 6–8, 9–12

Direct 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 16-17% silicon, 6-10% aluminum, 4-6% calcium, 3-6% magnesium, 2-5% iron, and 1-2% 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 SiO2 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. Lunar Prospector, another NASA spacecraft that orbited (January 1998-July 1999) the Moon, was sent crashing into the lunar surface to see if the resulting plume might reveal the presence of water ice, but the results were negative. (The craft bore some of the ashes of planetary scientist Eugene Shoemaker, making him the first person to be "buried" on a celestial body other than the Earth.) 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 be sufficiently abundant to support a future lunar colony for some time, if extracting it did not prove prohibitively expensive.

Mineralogy. The dark crystalline materials that fill the basins of lunar maria can be described as gabbroid basalts - materials akin to lavas known on the Earth but enriched with iron and titanium. In contrast, the continental areas of high reflectivity appear to consist of feldspathic rocks similar to terrestrial granites, including a nearly pure feldspar called anorthosite. Anorthosites replaced the iron or magnesium of basaltic rocks with aluminum, making them lighter in weight as well as color. The very existence of anorthosites on the Moon implies chemical differentiation of the crust, in the course of which heavier elements such as iron were separated from lighter ingredients. Moreover, anorthosites consist mostly of coarse-grained minerals, which means that they must have cooled off slowly from the melt, and thus not on the lunar surface. The dark crystalline materials that fill the basins of lunar maria can be described as gabbroid basalts - materials akin to lavas known on the Earth but enriched with iron and titanium. In contrast, the continental areas of high reflectivity appear to consist of feldspathic rocks similar to terrestrial granites, including a nearly pure feldspar called anorthosite. Anorthosites replaced the iron or magnesium of basaltic rocks with aluminum, making them lighter in weight as well as color. The very existence of anorthosites on the Moon implies chemical differentiation of the crust, in the course of which heavier elements such as iron were separated from lighter ingredients. Moreover, anorthosites consist mostly of coarse-grained minerals, which means that they must have cooled off slowly from the melt, and thus not on the lunar surface.

The physical texture of the lunar rocks is of even more interest than the chemical composition because of what the texture reveals about the origin of the lunar surface formations. Of signal importance is the fact that 85 to 90% of the material by weight imported from the lunar continents are the breccias. Consisting of grains of various minerals, these are conglomerates of preexisting crystalline rocks, in which angular fragments of diverse origin were welded together by events subsequent to their first solidification.

The structure of such breccias indicates shock metamorphism (changes brought about by high temperatures and pressures from impact). These kinds of changes indicate, in turn, that the rocks were produced by high-velocity impacts of celestial bodies of different size on the lunar surface in the course of its long history.

Lunar-orbiting spacecraft have also revealed regions of unusually high gravitational attraction. These regions, called mascons (for mass concentration), are primarily found beneath most of the maria. They are believed to be local concentrations of deeply buried fragments of dense material either from the impacting bodies that initially created the maria or from igneous (volcanic) rocks brought from the molten interior during the lava flooding of the maria.

  • Subjects:
    Astronomy and Space
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