Inside the Moon: Crust, Mantle, Core and the Evidence Beneath the Surface
How Apollo seismology, laser ranging, samples and gravity mapping constrain the Moon's internal structure and thermal evolution.
Key takeaways
- The Moon is differentiated into crust, mantle and a small metallic core.
- Seismic waves and gravity constrain structure indirectly; they are not direct images of the interior.
- Returned basalts sample melts derived from the mantle.
- GRAIL greatly improved knowledge of crustal thickness and mass distribution.
A differentiated rocky body
The Moon is not compositionally uniform. Early melting and differentiation separated materials by density and chemical behaviour. Buoyant plagioclase-rich material helped form ancient crust, while denser materials remained below and later mantle melting supplied mare basalts.
What Apollo seismometers revealed
Apollo surface stations recorded moonquakes, impacts and other vibrations. Seismic waves change speed and direction as material properties change, allowing researchers to infer boundaries and deep structure. Sparse station coverage limits resolution, which is why modern lunar geophysics continues to revisit the Apollo record and plan new networks.
Gravity as an interior constraint
A spacecraft's motion responds to the distribution of mass below it. GRAIL measured tiny changes in separation between twin spacecraft and converted those changes into an exceptionally detailed gravity field. Combining gravity with topography helps estimate crustal thickness and identify basin-related mass concentrations.
The mantle and volcanic samples
Mare basalts are products of partial melting inside the Moon. Their chemistry records mantle source composition and the conditions under which melts formed. Because returned samples come from limited locations, scientists combine laboratory geochemistry with global spectroscopy to judge how representative those sources are.
The small lunar core
Geophysical analyses support a small metallic core relative to the Moon's radius. Core size and state affect models of lunar cooling and magnetic history. Questions remain about the detailed deep interior, making future seismology and heat-flow measurements especially valuable.
Sources and further reading
- Primary / institutional source - https://science.nasa.gov/moon/inside-the-moon/: Background and factual verification for this guide
- Primary / institutional source - https://science.nasa.gov/mission/grail/: Background and factual verification for this guide