Inside the Moon
What is inside the Moon?
A thin crust, a thick rocky mantle and a small iron core. We know it from Apollo seismometers, laser ranging and gravity maps. Peel the layers apart below.
AI-generated illustration. AI-generated illustration for BotBuhdy.
Peel the Moon
Tap Peel or drag the slider to strip away one layer at a time. Tap any layer to read about it.
Layer sizes are to scale except the crust, which would be thinner than a line at this size. Depths from NASA, Weber and colleagues (2011) and Briaud and colleagues (2023).
A lopsided crust
The Moon's crust is not even. NASA's twin GRAIL spacecraft (2012) mapped the Moon's gravity in fine detail and showed that the crust is thinner on the near side facing Earth and thicker on the far side. That helps explain why the dark lava plains (maria) are almost all on the near side: magma reached the surface more easily where the crust was thin.
GRAIL also found the average crust is only about 34 to 43 km thick, thinner than earlier estimates, and that it is broken up and porous from billions of years of impacts.
Moonquakes: the Moon rings like a bell
Apollo astronauts set up seismometers at their landing sites. Apollo 12, 14, 15 and 16 stations worked together as a network until 1977 and recorded thousands of moonquakes. They found four kinds:
- Deep moonquakes, about 700 to 1,200 km down, the most common type. They repeat in step with the tides that Earth raises in the Moon.
- Shallow moonquakes, only 20 to 30 km down and rare, but the strongest, up to about magnitude 5. Some lasted over 10 minutes.
- Thermal quakes, small cracks as the frozen crust expands in the two-week day and shrinks in the two-week night.
- Meteorite impacts, from rocks hitting the surface.
On 20 November 1969, the crew of Apollo 12 deliberately crashed their lunar module's empty ascent stage back into the Moon. The seismometer recorded the Moon shaking for about 55 minutes. On Earth, a similar jolt would fade in a minute or so. The dry, cracked rock of the Moon carries vibrations for a long time with very little damping.
The Moon is slowly shrinking
As the Moon's interior cools, it shrinks a little, like a grape becoming a raisin. Because the crust is brittle, it cracks instead of wrinkling, forming step-like cliffs called lobate scarps. NASA's Lunar Reconnaissance Orbiter has found thousands of them.
Over the last several hundred million years, the Moon has shrunk by about 50 metres in radius. In 2019, scientists showed that several of the shallow moonquakes recorded by Apollo happened near these scarps, which suggests that the Moon is still shrinking and its faults may still be active today (Watters and colleagues, 2019).
How do we know any of this?
Seismometers
Moonquake waves bend and slow as they pass through different layers. Reanalysis of the Apollo records in 2011 revealed the fluid outer core and the solid inner core.
Laser ranging
Laser pulses bounced off the Apollo reflectors show how the Moon wobbles slightly as it spins, which reveals a fluid core inside.
Gravity maps
GRAIL's two spacecraft measured tiny changes in the distance between them as they flew over denser or lighter rock.
Quick answers
Does the Moon have a core?
Yes. It has a small iron core, about 330 to 360 km in radius, with a solid inner part about 240 to 260 km in radius. It makes up only about 1 to 2% of the Moon's mass.
Is the inside of the Moon hot?
Warmer than the surface. A layer just above the core may be partly molten, and the outer core is liquid, but the Moon has lost most of its heat and has no active volcanoes today.
Does the Moon have earthquakes?
It has moonquakes. Apollo seismometers recorded thousands of them between 1969 and 1977. Most are weak and very deep, but shallow ones can reach about magnitude 5.
Did the Moon really ring like a bell?
When the Apollo 12 ascent stage hit the Moon in 1969, the seismometer recorded vibrations for about 55 minutes, far longer than on Earth, because the Moon's dry, fractured rock barely dampens them.
Does the Moon have a magnetic field?
Not a global one today. Magnetised Apollo rocks show it once had a magnetic field powered by its core, which later faded away.
Is the Moon shrinking?
Yes, slowly. As its interior cools, it has shrunk by about 50 m in radius over the last several hundred million years, forming cliffs called lobate scarps.
Sources and credits
- NASA Science, Moon composition and structure.
- Weber, R. C. et al. (2011), Seismic detection of the lunar core, Science 331.
- Briaud, A. et al. (2023), The lunar solid inner core and the mantle overturn, Nature 617.
- Wieczorek, M. A. et al. (2013), The crust of the Moon as seen by GRAIL, Science 339.
- Nakamura, Y. et al. (1979), Shallow moonquakes: depth, distribution and implications, Lunar and Planetary Science Conference; NASA, Apollo Passive Seismic Experiment records.
- Latham, G. et al. (1970), Seismic data from man-made impacts on the Moon, Science 170 (Apollo 12 ascent stage impact).
- Watters, T. R. et al. (2019), Shallow seismic activity and young thrust faults on the Moon, Nature Geoscience 12; NASA, LRO news release (May 2019).
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