Surface Rocks

During the Apollo missions, astronauts collected rock samples from various locations on the Moon’s surface, including the lunar highlands (the light-colored areas on the Moon’s face as seen from Earth). Anorthosite, which makes up the lunar highlands, is an igneous rock composed mostly of plagioclase, a mineral that consists of lightweight elements. Studies of rocks on Earth indicate that plagioclase crystals float upward through magma as it cools, forming the crust; this suggests that the Moon was once covered in a "global magma ocean" shortly after its formation.

The image on the left shows a sample of anorthosite from the lunar highlands. The image on the right shows a thin section from the sample under polarized light. Under such light, the characteristics of different minerals become visible, making the minerals easier to identify. At the center of the thin section is an example of a plagioclase crystal, with the characteristic striping seen under polarized light.