What is the Moon made of? And how did it form?

The Moon is made of rock: a crust, a thick mantle and a small iron core, much like a scaled-down Earth with most of the iron left out. Its rocks are mostly oxygen, silicon, magnesium, iron, calcium and aluminum, bound into minerals such as feldspar and pyroxene. We know this because astronauts and robots brought pieces back. Most likely, the Moon formed from the debris of a planet-sized collision with the young Earth about 4.5 billion years ago.

In short

• The bright parts of the Moon are ancient crust rich in calcium and aluminum; the dark patches are later lava flows rich in iron and titanium.
• The Moon has far less iron than Earth, which is one of the main clues that it formed from the outer layers of a giant collision.
• The same face always points at Earth because the Moon spins exactly once for every orbit, and its phases are just how much of its sunlit half we see.

What is the Moon made of, layer by layer?

Seismometers left by Apollo astronauts recorded moonquakes for years, and the way those vibrations traveled shows the Moon has layers. According to NASA's guide to the Moon's composition, they are:

• Crust. The outer shell, made mainly of oxygen, silicon, magnesium, iron, calcium and aluminum. It is thicker on the far side than on the near side, and nobody yet knows why.
• Mantle. The thickest layer, mostly solid rock rich in the minerals olivine and pyroxene, partly molten near the bottom.
• Core. A solid, mostly iron inner core inside a molten outer core, with some nickel and sulfur. It reaches only about 20% of the way from the center to the surface.

There is no air to speak of, no liquid water on the surface and no plate tectonics. The surface is covered in regolith, a layer of dust and broken rock made by billions of years of meteorite impacts. It clings to everything, as the Apollo crews found when it worked into their suits and equipment.

There is water, though not as lakes or rain. In 2009 NASA's LCROSS mission crashed a spent rocket stage into a permanently shadowed crater near the south pole and detected water in the plume it threw up. How much ice such craters hold is still being measured. It matters because ice could one day give crews drinking water, air and rocket fuel.

Why does the Moon have light and dark patches?

The two shades are two kinds of rock of different ages. The bright highlands are the original crust, a pale rock called anorthosite made largely of feldspar, rich in calcium and aluminum. The dark patches, called maria (Latin for seas, because early astronomers took them for water), are basalt: lava that welled up and flooded giant impact basins between about 4.2 and 1.2 billion years ago, according to NASA. Basalt carries more iron and titanium, which makes it darker. The "man in the Moon" is a pattern of these lava plains.

The Moon's rocks are now some of the best-studied material in science. Between 1969 and 1972, six Apollo missions brought back 382 kg of rock, pebbles, sand and dust, in 2,200 separate samples, according to NASA's lunar sample curator. Three Soviet robotic Luna missions returned about 300 grams more. Decades later, China's Chang'e 5 brought back samples in 2020, and Chang'e 6 returned the first samples from the far side in 2024. How the samples settle doubts about the landings themselves is the subject of whether we really landed on the Moon.

How did the Moon form?

The leading explanation is the giant impact hypothesis. About 4.5 billion years ago, a body sometimes called Theia, perhaps the size of Mars, struck the young Earth a glancing blow. The collision melted and threw out a huge cloud of rock, mostly from the outer layers of both bodies, and that debris came together in orbit as the Moon.

The evidence is easiest to see side by side. Figures are from NASA's Moon fact sheet and its page on the Moon's formation.

Clue	Earth	Moon	What it suggests
Average density	5.51 g/cm³	3.34 g/cm³	The Moon is made mostly of lighter, mantle-like rock
Core as a share of mass	about 30%	about 1.6–1.8%	The impact flung out rock while Earth kept most of the iron
Oxygen isotopes, a chemical fingerprint	—	almost identical to Earth's	The two share a common origin
Easily evaporated elements	plentiful	scarce	They boiled away in the heat of the impact
Early surface	—	a deep ocean of molten rock	Only a very violent birth melts a whole world

Here is the density clue worked through. Iron is about 7.9 g/cm³, ordinary rock about 3. Earth's 5.51 means a large iron core. The Moon's 3.34 is barely more than rock alone, which leaves room for only a tiny core. A Moon that formed beside Earth from the same raw material should have a similar share of iron. One made from Earth's rocky outer layers should not, and that is what we find.

Earlier ideas, that Earth captured a passing Moon or that the two formed side by side, fell out of favor after Apollo.

Still argued

That a giant impact made the Moon is widely accepted; how it happened is not. In the classic version, most of the Moon comes from Theia, which should have left it chemically different from Earth, yet the two match almost exactly. Some models answer with a faster, more violent collision that mixed both bodies thoroughly, others with a Theia that formed near Earth from similar material, or with a series of smaller impacts rather than one. NASA lists the Moon's iron, its orbit and the differences between its near and far sides among the questions still open.

Why do we always see the same side of the Moon?

Because the Moon turns once on its axis in exactly the time it takes to orbit Earth, about 27.3 days. This is called tidal locking. Earth's gravity stretched the young Moon slightly, and the pull on that bulge slowed its spin until spin and orbit matched, as described in NASA's explanation of tidal locking. The far side is not dark: it gets as much sunlight as the near side. It is only hidden from us.

Why does the Moon change shape?

It doesn't. Half of the Moon is always lit by the Sun, and as it orbits Earth we see different amounts of that lit half. A common mistake is to think the phases are Earth's shadow; that only happens during a lunar eclipse. A full cycle, from new moon to new moon, takes about 29.5 days, longer than the 27.3-day orbit because Earth has moved around the Sun in the meantime. Because the phase sets where the Moon is relative to the Sun, it also sets when the Moon is up, which is a quick way to tell the phase without a calendar.

Phase	Days after new moon	Rises around	Sets around	Best seen
New moon	0	sunrise	sunset	not visible, it is near the Sun
First quarter, right half lit	about 7	noon	midnight	in the evening
Full moon	about 15	sunset	sunrise	all night
Last quarter, left half lit	about 22	midnight	noon	after midnight and in the morning

The halves are reversed when seen from the Southern Hemisphere. The Moon is lit by the Sun, and it is one of hundreds of moons in the solar system, unusually large for the planet it circles. It is still the only other world in the universe that people have walked on.

Check yourself

• What two kinds of rock make the Moon's light and dark areas, and which is older?
• How does the Moon's low density support the giant impact hypothesis?
• Why isn't a half moon caused by Earth's shadow, and when would you see a first-quarter moon in the sky?

Sources

• Moon composition — NASA
• Moon formation — NASA
• Moon fact sheet — NASA Space Science Data Coordinated Archive
• Lunar rocks and soils from Apollo missions — NASA Astromaterials Acquisition and Curation Office
• Tidal locking — NASA
• Moon phases — NASA