Lunar Geology & Composition

Information on the Composition & Geology of the moon.

Composition

Evidence of the moon's composition is based on rocks obtained by the 1st Apollo mission in 1969. The rocks were collected from the surface, but there is little reason to believe they are not indicative of the Moon's overall composition.

Oxygen, silicon, and aluminum occur in amounts comparable to Earth's crust. Iron and titanium are more common on the moon, while the alkali metals, carbon and nitrogen occur in lower amounts than that of Earth.

The only hydrogen on the moon arrives thanks to the stellar wind. While it was previously thought that water thus formed would be quickly dissociated by sunlight, recent discoveries suggest that some ice may be exist in craters on the poles, protected from sunlight by their walls.

Geology

The basins of lunar maria consist of dark crystalline materials called gabbroid basalts - materials similar to lavas but enriched with iron and titanium.

Continental areas of high reflectivity appear to consist of feldspathic rocks similar to terrestrial granites, including an almost pure feldspar called anorthosite. These rocks replace the iron or magnesium of basaltic rocks with aluminum.

The 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 singular importance is the fact that 85 to 90 percent 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). This indicates, 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.

Perturbations in the orbit of lunar-orbiting spacecraft also indicate regions of unusually high gravitational attraction. These regions, called mascons (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.

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