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Earth's Moon

Giant Impact

Giant Impact

A Mars-sized object collided with Earth, vaporizing, melting, and throwing debris from the impactor and Earth's outer layer into orbit around Earth, creating an encircling debris ring.

The Moon Forms

The Moon Forms

Material in the debris ring accreted to form our Moon, possibly within a few hundred years. The young Moon was much closer to Earth, and orbited the planet once every few days.

Lunar Magma Ocean

Lunar Magma Ocean

The heat from accreting particles caused the Moon to at least partially melt, creating a lunar magma ocean.

Magma ocean graphic courtesy of NASA Goddard Space Flight Center.

Ancient Lunar Atmosphere

Ancient Lunar Atmosphere

Lava erupting onto the lunar surface ~3.5 billion years ago released gases above the surface faster than those gases could escape to space. This created a temporary atmosphere that dissipated as the frequency of volcanic eruptions decreased.

Ancient lunar atmosphere graphic courtesy of NASA Marshall Space Flight Center.

Oldest Moon Rocks

Oldest Moon Rocks

The Apollo missions returned samples of ancient lunar crustal rocks. These rocks are about 4.5 billion years old, indicating that parts of the Moon’s crust solidified soon after the Moon formed.

Photographs courtesy of NASA Johnson Space Center.

Lunar Crust

Lunar Crust

The lunar magma ocean cooled and crystallized, forming a crust about 40 kilometers thick. Asteroids continued to bombard the Moon, leaving impact craters.

Lunar Volcanism

Lunar Volcanism

Portions of the Moon’s interior remained hot enough to produce magma for more than a billion years after it formed. Molten rock flowed onto the lunar surface through cracks in the crust, spreading out and filling the low regions in the impact basins. Thelava cooled quickly, forming the fine-grained, dark rocks — basalts — sampled during the Apollo missions. The dark areas seen on the Moon are basaltic lava plains.

Apollo 17 image AS17-2444 courtesy of NASA.

Moon Becomes Geologically Inactive

Moon Becomes Geologically Inactive

Lunar volcanism decreased significantly by 3 billion years ago and ceased completely by about 1 billion years ago as the interior of this small body cooled.

Near-side image of the Moon, courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Longer Days More Distant Moon

Longer Days More Distant Moon

The length of our Earth day has increased through time. Approximately 900 million years ago, each day was about 18 hours long. By 370 million years ago, the day was 22 hours long. Today, of course, Earth experiences a 24-hour day. The drag of the tides, caused by the gravitational pull of our Moon, slows Earth’s rotation.

Lunar Reconnaissance Orbiter image showing the ringed Mare Orientale basin, courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Copernicus Crater

Copernicus Crater

Copernicus Crater formed on our Moon less than a billion years ago when an impactor, several kilometers across, struck the surface. The impact produced a circular crater nearly 100 kilometers across and blew material out in prominent rays. The Apollo 12 astronauts collected samples from one of the rays. These samples provide evidence of the timing of the impact.

Lunar Reconnaissance Orbiter image of Copernicus crater courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Closer Moon

Closer Moon

The Moon currently orbits the Earth at a distance of ~384,400 kilometers. It is estimated that 3.9 billion years ago, the Moon orbited the Earth at a distance of ~133,800kilometers. This would have caused the Moon to appear about 3 times larger in the sky.

Images of the Moon courtesy of NASA/Goddard Space Flight Center/Arizona State University.

Tycho Crater

Tycho Crater

Tycho Crater, about 85 kilometers across, is clearly visible on our Moon’s surface. The freshness of the crater and the rays of material radiating from it suggest that this is a young crater; there has been little time to erode it.

Image of the Moon courtesy of NASA/Goddard Space Flight Center/Arizona State University.