Does the Moon Have Innards?
published during a waning crescent moon.




The moon’s perforated exterior has inspired a few myths, namely of a man who may or may not be made of cheese. It has a calm presence in our sky, its surface stoic and motionless. But its innards are actually suspected to have a broiling core similar to Earth’s: solid in the middle with piping-hot rock shell.

The predominant theory of how the moon came to be our companion is that of a violent wreck. Roughly 4.5 billion years ago, Earth is thought to have smashed into Theia, a celestial body the size of Mars. The debris then congealed to form a magma ocean that slowly began to crust over.

The party didn’t stop there: As the molten ball crystallized, rocks lower in density would float upward and form the crust. Meanwhile, iron — the heaviest of the mingling elements — descended to the bottom. Volcanoes likely spewed lava all over the place, leaving dark flows on the surface that we can see if we look up tonight.


Credit: NASA/JPL-Université Paris Diderot – Institut de Physique du Globe de Paris

We’ve never actually been to Earth’s core, but we do know a lot about it. Planetary scientists can surmise a lot about our planet’s insides using motions of Earth’s topography, its magnetic field — even experiments done in space, such as those aboard the International Space Station. One team, for example, extrapolated the inner core’s temperature when they watched how iron crystals acted under intense pressures.

But to get to the middle, we’d first have to break through a thin but tough crust. Then, we’d find Earth’s mantle, which is semi-liquid rock that makes up about 84% of the Earth by volume. The mantle is mostly solid, Fraser Cain of Universe Today explains, but because the layer is so close to a melting point, it acts like an extremely thick fluid. Volcanoes, meanwhile, are portals for magma from the mantle.

Finally, you’d reach that mystical iron-rich core, home to essentially the rest of Earth’s volume, planted nearly 6,300 kilometers deep (or about 4,000 miles). To compare, the deepest humans have ever drilled was 12 km (or just over 7.5 miles). The Kola Superdeep Borehole is in Russia, a standing tribute to Soviet-era science that’s now a tourist attraction.


Credit: NASA/MSFC/Renee Weber

The Earth and moon, then, have a core, mantle, and crust, but they do differ in a few ways. Earth’s crust is fairly uniform, but one of the moon’s greatest mysteries is just how its crust ended up nearly twice as thick on its far side. The moon also has a layer that Earth doesn’t have called the partially molten layer, and it stretches over a 300-mile radius.

It’s unlikely we’ll ever be able to peer under our planet and moon’s brittle blankets. But as scientists continue to refine the data, the more we’ll learn about their peculiar dance in space — and what the rest of the elegant, moon cheese-filled party that they’re hosting may look like.