The Moon’s interior is surprisingly irregular

Although the Moon is Earth’s closest neighbor, some of its fundamental properties are not well understood. A better grasp of the composition and thermal structure of the lunar interior, for example, would help researchers trace the evolution of the Moon and the origins of its volcanic deposits and other surface features seen today. To help probe the Moon’s interior, two spacecraft with NASA’s GRAIL mission, Ebb and Flow, collected observations in lunar orbit in 2012. Small displacements in the orbits of the instruments were used to generate a map of the lunar gravitational field.

From early analyses, it appeared that the Moon’s deep interior was roughly spherically symmetric. Many researchers, therefore, assumed that the observed compositional and temperature asymmetries were too small to help explain how the Moon formed and evolved. That assumption, however, now seems to be overturned. Using GRAIL data, Ryan Park of the NASA Jet Propulsion Laboratory and colleagues found an unexpectedly large time-varying gravity signature that they report is consistent with uneven temperatures in the Moon’s deep interior.

The Moon’s gravitational field is typically estimated using the mathematics of spherical harmonics. In the equations, the dimensionless Love number k characterizes a body’s response to tidal forces—in this case, from Earth—over time. The GRAIL results published a decade ago found a value of k consistent with a spherically symmetric Moon. They were based on data from the primary mission, which ran from March through May 2012. Park and colleagues developed their gravity map using the primary data plus measurements acquired during the extended mission, from August through December 2012.

In using the entire suite of GRAIL data, the researchers found a puzzling, physically unrealistic k value: It was 72% higher than what was expected for a spherically symmetric Moon. The team spent years painstakingly testing its high-resolution gravity map against alternative possibilities and have concluded that the Moon’s interior must be asymmetric.

The asymmetry can’t be explained by variations in mantle composition. If it were, the Moon’s center of mass would be offset from its geometric center by much more than what’s observed. The more likely explanation is that the Moon’s nearside interior is 100–200 K warmer than the farside. Lunar models show that compared with the Moon’s farside, its nearside has more radiogenic elements, including thorium and titanium. They could have supplied differing quantities of heat to the interiors. Over billions of years, the uneven heating could have led to the hemispheric differences in basalt that are observed on the Moon’s surface today. (R. S. Park et al., Nature 641, 1188, 2025 .)

This article was originally published online on 17 June 2025.