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For decades, the Moon’s interior has been a subject of speculation. Recent research, published in May 2023, presents compelling evidence that the lunar core is solid rather than molten. The study also finds the core’s density is comparable to that of iron, suggesting striking parallels with the Earth’s own core.
Gathering data on a planetary core is notoriously difficult—much more so when the body is a distant satellite. Nonetheless, such investigations are vital; on Earth the inner core powers the geomagnetic field that shields us from harmful cosmic radiation.
Because the prevailing hypothesis holds that the Moon formed from debris generated by a Mars‑sized impact with early Earth, it is reasonable to expect similar compositional traits in both bodies. The emerging evidence supports this notion.
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Unlike Earth, the Moon has virtually no atmosphere or magnetic field, which makes it vulnerable to meteorite bombardment and the familiar cratered surface. Yet this lack of an external shield does not imply an inactive interior.
Current models indicate a dense, metallic core composed mainly of iron with minor nickel content. Although solid, the core is relatively small: it occupies roughly 20 % of the Moon’s diameter, compared with about 50 % for Earth’s core. The mantle and crust envelop this core and are largely stagnant.
While the core itself is likely solid, the Moon’s mantle may still host pockets of molten material. Studies of lunar chemistry point to a once‑vigorous magmatic interior, where heavy elements sank toward the center while lighter materials rose toward the surface, cooling over time.
A 2011 study published in Science examined seismograms and suggested that up to 60 % of the Moon’s core could be liquid. However, that analysis also indicated a solid inner core surrounded by a partially liquid outer layer.
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Our understanding of the Moon’s interior rests on three main data sources: ground‑based telescopic observations, remote sensing from robotic lunar missions, and rock samples returned by astronauts. The most revealing of these came from the detection of anorthite clasts on the surface—structures that form only in a molten environment—confirming that the Moon once experienced a global magma ocean.
Much of what we infer about the lunar interior is therefore extrapolated from limited evidence. Scientists acknowledge significant gaps in our knowledge, and every new measurement offers the potential for revision.
Despite these uncertainties, the consensus points toward a solid core with a magmatic mantle, echoing the internal structure of Earth, albeit on a smaller scale. The Moon’s core appears to be a close, albeit weaker, twin to our own.
