A new NASA study has revealed something unexpected – and rather strange – about the Moon’s deep interior.
Using data from the GRAIL spacecraft, researchers discovered that the side of the Moon facing Earth is 'flexing' more than the far side, suggesting a surprising wonky internal structure between the two hemispheres.
“We had long assumed that the Moon was spherically symmetric deep inside, so seeing clear evidence to the contrary was both surprising and challenging,” Dr Ryan Park, lead author of the study and head of the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory, told BBC Science Focus.
Published in Nature, the study uses ultra-precise gravity measurements to show that the Moon’s near side – the one visible from Earth – deforms more under Earth’s gravitational pull than the far side.
The finding hinges on a measurement known as the ‘tidal Love number’, which quantifies how much a planetary body flexes due to tidal forces. After nearly a decade of work running complex calculations with millions of parameters, the team found that the Love number is about 72 per cent higher than it should be for a symmetrically spherical Moon.
“When we first recovered the higher-than-expected lunar tidal Love number, our team was genuinely puzzled,” Park said. But the maths checked out: the Moon really was wonky inside and out.
The cause? The near side’s mantle is warmer and less rigid than the far side’s, likely due to radioactive heating early in the Moon’s history.
This appears to be due to an ancient quirk of geology. The near side of the Moon has much more radioactive material than the far side, including up to 10 times more thorium. These radioactive elements release heat as they decay.
Billions of years ago, that heat may have created pockets of partially melted rock, fuelling massive volcanic eruptions. These resurfaced the near side with dark plains called ‘mare’, while the far side remained rugged and cratered.
The research provides the most detailed gravitational map of the Moon ever produced. It could help future missions – including NASA’s Artemis programme – land and navigate more precisely, and even inform the development of a lunar navigation system.
Looking elsewhere in the Solar System, Park is aiming to use similar approaches to explore the interiors of the metal-rich asteroid Psyche and Jupiter’s watery moon Europa.
“The most surprising aspect was just how pronounced the asymmetry was,” Park added. “This forced us to confront the complexity of the Moon’s thermal and structural evolution, and it ultimately led to new insights about how the Moon – and possibly other planetary bodies – develop internal differences over billions of years.”
Read more:
- 20 of the most amazing moons in the Solar System
- The surprising ways our Moon shapes life on Earth
- Why is the Moon sometimes visible during the day?
About our expert
Ryan Park is the supervisor of the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory in Southern California. His research has been published in prestigious journals including Science, The Astrophysical Journal and Nature.