Scientists think the Moon formed when an object about the size of Mars crashed into the young Earth, and this giant-impact idea is the leading explanation for how the Moon formed. Older ideas, that Earth captured the Moon or that it formed beside Earth from the same debris, fell out of favor after Apollo brought back Moon rocks. Those rocks match Earth rocks very closely in their chemistry, a clue about how the Moon formed, although scientists at the University of New Mexico found that Earth and Moon rocks have distinct oxygen isotope compositions, not identical as once thought, which matters for theories of how the Moon formed. The rocks also hold only small amounts of the volatile elements, the ones that vaporize when heated, and compared with Earth, the Moon is depleted in them.

The Moon has a rocky surface covered with craters, which were made when space rocks crashed into it, and scientists estimate how old a surface on the Moon is by counting its craters. Its lunar soil, called regolith, is pulverized rock made by countless meteorite impacts over geologic time. The dark maria are giant impact basins that were later flooded with lava, and the flows were laid down over billions of years, with some of the youngest flows about 1.2 billion years old. Dark lava plains called maria cover much of the near side of the Moon but are almost missing from the far side, and the Moon had erupting volcanoes long ago, but its volcanoes are quiet today.

The young Moon was melted to great depth in a global magma ocean, and as it cooled, the lighter, less-dense rock floated upward toward the top. The Moon's crust is thicker on the far side than on the near side, by up to about 12 miles, and scientists do not yet fully understand why it is so uneven. Lunar mascons, hidden concentrations of extra mass under the Moon's surface, formed when huge impacts dug out basins on the ancient Moon, according to an analysis of GRAIL gravity data.

The Moon has a very small core, and its core has two parts: a solid inner core inside a fluid, molten outer core. Seismometers left by Apollo astronauts recorded 28 shallow moonquakes between 1969 and 1977, and deep moonquakes, which start far below the surface, are driven by Earth's gravity tugging on the Moon's interior, in the same way the Moon's gravity raises tides in Earth's oceans.

The Moon is slowly shrinking as its hot interior cools, and scientists link this shrinking to cracks in the crust called thrust faults and to some moonquakes. Scientists studied the Apollo seismic records and found that some shallow moonquakes were near young faults, which suggests the Moon is still tectonically active.

The Moon is about 2,160 miles (3,475 kilometers) across and less than a third as wide as Earth, and Earth has about 80 times as much mass as the Moon. The Moon takes about 27.3 days to circle Earth once, but about 29.5 days to go from new Moon to new Moon, and it travels around Earth in a path called an orbit that is not a circle: the Moon's distance from Earth changes as it moves on an oval orbit, from about 363,300 kilometers (225,700 miles) at its closest on average to about 405,500 kilometers (252,000 miles) at its farthest.

The pull between the two bodies works in both directions. The Moon's gravity pulls on Earth's oceans and is the main cause of the tides, and it tugs on the solid Earth too, not just on the oceans. Ocean water bulges out on the side of Earth nearest the Moon and on the side farthest from it, and these bulges are the high tides. The Sun is about 390 times farther away than the Moon, and its pull on the tides is about half as strong as the Moon's. Tides act like friction and slow Earth's spin, so Earth's day gets longer by a couple of milliseconds every century, and Earth's pull on the young Moon slowed the Moon's spin until it matched its orbit, which is why the same side of the Moon now always faces Earth.

Apollo 11 astronauts left a mirror array on the Moon, and scientists on Earth bounce laser pulses off it to measure the distance to the Moon. On August 1, 1969, scientists on Earth detected laser light that bounced back from the mirror array, and it improved the measured distance to the Moon. On average the Moon is about 239,000 miles (384,000 kilometers) from Earth, and the mirrors are still used today to measure that distance and to track its orbit. The Moon is slowly drifting away from Earth, by about 1.5 inches (3.8 centimeters) each year.

Words to know

• isotope: atom variety
• seismometers: quake detectors
• mascons: hidden mass patches
• volatile: easily vaporized