Color image of astronaut with equipment on the moon.

The Passive Seismic Experiment was the first seismometer placed on the Moon’s surface. It allowed scientists to learn about the internal structure of the Moon. More ›

The Moon is a differentiated world. This means that it is composed of different layers with different compositions. The heaviest materials have sunken down into the Moon’s center, and the lightest materials have risen to the outermost layer. Seismic, rotational, and gravity measurement studies have allowed us to gain insights into the different layers within the Moon.

Illustration of seismic instrument
The Apollo 11 Passive Seismic Experiment contained four seismometers powered by two panels of solar cells, which converted solar energy into electricity. It used three long-period seismometers and one short- period vertical seismometer for measuring meteorite impacts and moonquakes, recording about 100 to 200 hits by meteorites during its lifetime. Data regarding the strength, duration, and approximate direction of the seismic event were relayed to tracking stations on Earth.

At the center is the Moon’s dense, metallic core. The core is largely composed of iron and some nickel. The inner core is a solid mass about 480 km in diameter. Surrounding the solid inner core is a fluid outer core, that brings the total diameter of the core to about 660 km. The Moon’s core is small (about 20% of the Moons diameter) as opposed to other terrestrial worlds (like the Earth) with cores measuring closer to 50% of their diameters.

Above the core are the mantle and crust. Differences in compositions between these layers tell a story of the Moon being largely, or even completely, composed of a great ocean of magma in its very early history. As the magma ocean began to cool, crystals began to form within the magma. Crystals of denser mantle minerals, such as olivine and pyroxene sank down to the bottom of the ocean. Lighter minerals, notably anorthositic plagioclase feldspar, crystalized and floated to the surface to form the Moon’s crust. The mantle, with a thickness of roughly 1350 km is far more extensive than the crust, which has an average thickness of about 50 km. Interestingly, the crust of the Moon seems to be thinner on the side of the Moon facing the Earth, and thicker on the side facing away. Researchers are still working to determine why this might be.

Seismometers left on the surface of the Moon by the Apollo astronauts have revealed that the Moon does experience moonquakes. Deep moonquakes, occurring broadly around 700 km beneath the lunar surface are tidal events, caused by the pull of Earth’s gravity tugging and stretching the internal structures of the Moon. Moonquakes originating on or near the surface can be caused by meteoroid impacts with the Moon. Another type of extremely shallow moonquake can come from thermal expansion and contraction of rock on or near the surface as it goes from the extremely frigid lunar night to the very hot lunar daytime. A fourth type of moonquake originates at the moderately shallow depths of 20-30 km, can register up to a startling 5.5 on the Richter scale, and can last for over 10 minutes! The causes of this fourth type of moonquake are still being investigated.

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