Why one side of the moon still glows from within

The moon has two strikingly different sides. The side facing Earth is mostly smooth, with dark, flat plains formed by ancient lava flows. In contrast, the far side is rugged and filled with craters.

But why do they look so different? What could have caused such a stark contrast between the two halves? Scientists have been searching for answers, and recent findings suggest that the key may lie deep within the moon’s interior.

NASA’s GRAIL mission has provided new insights. Two spacecraft, Ebb and Flow, orbited the moon to measure its gravity. They discovered a temperature difference deep inside the moon. This could explain why one side looks smooth and the other rough.

Hidden differences inside the moon

Ryan Park and his team used GRAIL data to peer beneath the surface. They weren’t looking at rocks or craters. They were looking at gravity. The moon’s gravitational field fluctuates slightly as it moves, and Park’s team measured those tiny shifts.

The research was focused on a parameter called the gravitational tidal Love number, k3. It’s a number that reveals how the lunar mantle deforms under gravitational forces – and it revealed something very surprising.

“We report our estimated value of k3 = 0.0163 ± 0.0007, which is about 72% higher than that expected for a spherically symmetric moon,” noted the researchers.

The discovery is significant. It shows that the moon’s mantle isn’t the same throughout. One side – the nearside – is warmer, softer, and more flexible.

Meanwhile, the farside is cooler, stiffer, and less likely to deform. This difference could explain why the two sides of moon look so different.

Why one side of the moon is hotter

Why is the nearside hotter? Park’s team points to radioactive elements – thorium and titanium – locked in ancient lava flows.

Billions of years ago, volcanic eruptions spread molten rock across the nearside. As the lava cooled, it trapped these heat-producing elements. They are still there, and they are still releasing heat.

Park and his colleagues estimate that the nearside mantle is up to 170°C (338°F) hotter than the farside. That heat keeps the rock softer and more flexible, allowing it to deform more easily under gravitational stress. And that’s why the k3 value is so high.

Forces stretch the moon’s interior

Tidal forces don’t just move Earth’s oceans. They also affect the moon. As the moon orbits Earth, gravitational forces stretch and squeeze its interior on both sides.

GRAIL’s measurements showed that the moon’s mantle doesn’t react the same way on each side.

The warmer, softer nearside stretches and bends more under these forces. Meanwhile, the cooler, stiffer farside resists deformation, staying more rigid. This uneven response reveals differences in the moon’s internal structure.

This discrepancy creates gravitational anomalies. And it could also explain why deep moonquakes cluster in certain regions.

Melting rock and moonquakes

When rock gets extremely hot, it begins to melt. Park’s team believes that there are areas deep within the moon’s mantle where rock has partially melted, particularly under the nearside. These pockets of molten rock could act like stress points.

As the moon orbits Earth, tidal forces stretch and compress its interior. The molten zones might respond more intensely to this pressure, causing moonquakes.

These melt zones aren’t just anywhere – they are located at depths ranging from 800 to 1,250 km (497 to 777 miles). That’s the same depth where many deep moonquakes occur.

This overlap suggests a connection between these molten areas and the origins of moonquakes, pointing to a possible link between the moon’s internal heat and its seismic activity.

A volcanic past and a chilled future

The moon wasn’t always so divided. Billions of years ago, lava gushed across the surface, burying craters and creating vast plains. Today, those plains are the dark spots we see on the nearside – the mare regions.

Park’s team thinks that as the lava cooled, it trapped radioactive elements, creating a thermal imbalance that persists today.

The farside, lacking such lava flows, cooled more quickly. And that’s why it’s more rugged, cratered, and colder.

Mapping the moon’s interior

Park’s approach didn’t involve digging into the moon or landing any probes. Instead, he and his team focused on measuring the moon’s gravity.

As the moon moves around Earth, gravitational forces pull and squeeze its interior. By tracking these subtle changes in gravity, Park’s team effectively created a detailed map of the moon’s internal structure – like taking an “X-ray” of its insides.

This technique, called tidal tomography, uses gravitational data to study what’s hidden deep within. And it isn’t just for the moon.

Implications for other worlds

The moon is not the only celestial body with two distinct sides. Mars has flat, smooth plains in the north and rough, elevated highlands in the south.

Enceladus, one of Saturn’s moons, shoots icy plumes into space, hinting at internal activity. Ganymede, Jupiter’s largest moon, has a strong magnetic field, suggesting complex processes beneath its surface.

If tidal tomography can reveal hidden structures inside the moon, it could do the same for these worlds.

By analyzing gravitational changes, scientists could uncover what’s happening deep within these distant bodies – all without landing a single probe.

Hidden activity beneath the surface

NASA’s upcoming lunar missions include the Farside Seismic Suite and the Lunar Geophysical Network. These missions will monitor moonquakes and map the lunar mantle in even greater detail.

The missions could confirm the existence of melt zones, pinpoint their depths, and maybe even detect ongoing volcanic activity.

Meanwhile, the GRAIL findings raise new questions. How much heat still lingers in the nearside mantle? Could residual magma still be flowing beneath the surface? And what does that mean for future moonquakes?

The moon’s geological story

NASA’s GRAIL mission did more than just measure gravity – it uncovered pieces of the moon’s history.

The thermal differences between the two sides suggest that ancient volcanic eruptions and radioactive decay left the nearside warmer and more flexible than the farside.

The nearside’s lingering warmth and softer mantle indicate that the moon’s interior hasn’t fully cooled. This ongoing heat suggests that the moon’s geological story isn’t over – there might still be activity beneath its surface.

The study is published in the journal Nature.

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