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The Moon’s Inner Core Is Solid Iron After All

Jun 07, 2023Jun 07, 2023

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Just like Earth's.

While NASA is busy sending astronauts back to the Moon's surface, other astronomers are trying to figure out what lies beneath that surface.

A new study from Université Côte d’Azur, along with contributions from other French astronomy institutions, confirms a long-held theory that the Moon's inner core is made of iron, just like Earth's.

Our study of the inner composition of the Moon began back toduringthe Apollo missions of the late 1960s and early 70s, when NASA astronauts recorded seismic data to unlock some lingering lunar mysteries.

This study, which was published in the journal Nature last month, revisits this data in combination with various other points of information—including lunar laser ranging experiments and measurements from other space missions (such as NASA's GRAIL mission)—to create an accurate interior profile of the Moon. Astronomers ran this profile through a modeling application to test multiple scenarios and see which ones matched up with real-world data.

These models all but confirmed that the Moon's core was most likely solid iron, a theory first put forward by NASA in 2011. Back then, NASA estimated the Moon's solid inner core was roughly 240 km in radius, and this data confirms a likely radius of 258 km. NASA also concluded that the Moon's core would weigh in at about 8,000 kilograms per cubic meter. The scientists at Université Côte d’Azur estimated a very close 7,822 kilograms per cubic meter.

But this study does more than just fact check NASA's previous work. It also puts forward the idea that, before the Moon's interior became solid, it experienced what's known as lunar mantle overturn—a process by which denser material falls toward the core's center while lighter, warmer material rises through the mantle. This could explain how traces of iron found on the lunar surface today likely arrived via volcanic activity in the satellite's distant past.

Knowing about this solid iron core is important for understanding the Moon's formation, as well as the formation of the early Solar System as a whole. Not long after the Moon came into being some 4.5 billion years ago, Earth's one and only satellite had a magnetic field just like its planetary host. However, a little more than a billion years later, that magnetic field began to decline. Because magnetic fields are produced in part by convection, which can take place in a celestial body's core, it's important to understand exactly what's in that core to truly grasp the Moon's geologic past.

Hopefully, once NASA finally puts boots back on the Moon, we’ll be able to make some on-site observations and finally confirm the inner machinations of our closest celestial neighbor.

Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.

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