Reanalysis in 2023 found a solid ball of iron about 500 kilometres across at the Moon's centre, wrapped in a molten outer core — the same layered arrangement Earth has.

The evidence is indirect: decades of laser ranging and orbital tracking, plus the old Apollo seismic records, fed into models of how the Moon deforms. The best fit needs a solid inner core with roughly the density of iron. It settles an argument that had run since the seismometers were switched off in 1977.