Looking back at data from the geology experiments left on the lunar surface by the Apollo missions half a century ago, scientists have finally determined that the Moon has a solid core surrounded by a fluid outer core similar to the Earth's.
When the six Apollo missions landed on the Moon from 1969 to 1972, the astronauts left behind instrument packages on the surface that included geology instruments. Among them were seismographs, geophones, and even mortars and explosive charges. In addition, NASA deliberately crashed S-IVB boosters and lunar module ascent stages on the surface to produce some artificial moonquakes.
The experiment packages collected data until 1977, when support operations were shut down, although one passive laser experiment can still operate for centuries to come. Scientists are still studying the information decades later and are making major discoveries.
Scientists from the French National Centre for Scientific Research (CNRS), along with the Université Côte d'Azur, the Côte d'Azur Observatory, Sorbonne Université, and the Paris Observatory-PSL, looked at the Apollo seismic record combined with studies of irregularities of the Moon's rotation and then developed models to determine what the interior structure would produce such readings.
It was already shown in the 1990s that the Moon had a fluid outer core heated by tidal forces, but the nature of the inner core was still up for grabs. What they found was that the lunar inner core is solid and about 500 km (310 miles) in diameter or about 15% of the Moon's total size and is made of a metal with roughly the density of iron.
According to the team, this is important because the existence of the core was hard to find because of its small size. It also helps explain how the Moon's magnetic field disappeared even though it was once 100 times as strong as the Earth's present field.
The research was published in Nature.
