After more than 50 years, scientists have finally uncovered the moon’s interior structure, showing that our closest celestial companion has a fluid outer core and a solid inner core, similar to Earth’s. A team of researchers from the Côte d’Azur University and the Institute of Celestial Mechanics and Ephemeris Calculations (IMCCE) in France detailed these findings May 3 in a study published in Nature(opens in new tab).

Astronomers have puzzled over the moon’s structure since well before any probes landed there. A hot debate raged in the first half of the 20th century as to whether the moon was a “primitive” rocky world, like Mars’s moons Phobos and Deimos, or whether it had a rich inner geology.

The first hints that the moon had an Earth-like interior came from NASA’s Apollo missions. Data gathered by the lunar landers’ instruments suggested that the celestial body was differentiated — or layered with denser material at the center and less dense material nearer the surface — as opposed to uniform rock all the way through. Apollo astronauts even left seismometers on the moon, which later revealed that it experiences moonquakes, according to NASA(opens in new tab).

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