Earth's Moon

Our knowledge of Earth's Moon has come from years of detailed telescopic study and uncrewed and crewed space flight. Seismic detectors left by astronauts record about four hundred weak moon‐quakes a year. The seismic data indicate the Moon's interior is layered underneath a 65‐kilometer‐thick crust made up of basalt and feldspar‐rich rock. Mantle material is inferred to exist beneath the crust. Most scientists think the moonquakes are related to the gravitational pull of the earth, which causes small crustal movements along faults. Seismic calculations indicate the Moon is solid down to a depth of about 1,000 kilometers, the lower limit of the Moon's lithosphere (crust and rigid upper mantle).

The large, smooth lowlands on the lunar surface are regions covered by basaltic lava flows. Geophysical measurements suggest the mountainous regions are isostatically balanced, indicating that at one time the Moon's lithosphere was plastic enough to let the thicker crust “float.” Rock samples retrieved by crewed expeditions consist of feldspar‐rich anorthosite, an igneous rock, and basalt. Age dates vary from about 3.0 to 4.5 billion years—the proposed beginning of the solar system.