Key Ideas

Appearance of the Moon: The Earth-facing side of the Moon displays light-colored, heavily cratered highlands and dark-colored, smooth-surfaced maria. The Moon’s far side has almost no maria.

• Virtually all lunar craters were caused by space debris striking the surface. There is no evidence of plate tectonic activity on the Moon.

Internal Structure of the Moon: Much of our knowledge about the Moon has come from human exploration in the 1960s and early 1970s and from more recent observations by unmanned spacecraft.

• Analysis of seismic waves and other data indicates that the Moon has a crust thicker than that of Earth (and thickest on the far side of the Moon), a thick mantle, and a small iron core.
• The Moon has a solid inner core surrounded by a liquid outer core.
• The Moon has no global magnetic field today, although it had a weak magnetic field billions of years ago.

Geologic History of the Moon: The anorthositic crust exposed in the highlands was formed between 4.3 and 4.0 billion years ago. The Late Heavy Bombardment formed the maria basins between 4.1 and 3.8 billion years ago, and the mare basalts solidified between 3.8 and 3.1 billion years ago.

• The Moon’s surface has undergone very little change over the past 3 billion years.
• Meteoroid impacts and the solar wind have been the only significant “weathering” agents on the Moon. These weathering processes formed the Moon’s regolith, or surface layer of powdered and fractured rock.
• All of the lunar rock samples are igneous rocks formed largely of the same minerals found in terrestrial rocks on Earth. However, material in lunar rocks appears to have been exposed to higher temperatures than terrestrial rocks.

Origin of the Moon: The collisional ejection theory of the Moon’s origin holds that the proto-Earth was struck by a Mars-sized protoplanet and that debris from this collision coalesced to form the Moon. This theory successfully explains most properties of the Moon.

• The Moon was molten in its early stages, and the anorthositic crust solidified from low-density magma that floated to the lunar surface. The mare basins were created later by the impact of planetesimals and later filled with lava from the lunar interior.
• Tidal interactions between Earth and the Moon are slowing Earth’s rotation and pushing the Moon away from Earth. This also causes the length of Earth’s day to slowly increase.