For decades, scientists have puzzled over the stark contrast between the Moon’s two sides: the near side, always facing Earth, with its vast dark plains, and the far side, dominated by rugged, mountainous terrain. Now, an in-depth analysis of data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission has provided compelling evidence that this asymmetry extends deep below the lunar surface.
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The new research, published in Nature, shows that the Moon’s internal structure is lopsided — with the near side significantly warmer and geologically more active than the far side. The findings offer the strongest support yet for theories that intense ancient volcanic activity on the near side shaped the Moon’s surface and internal evolution.
Probing the Moon’s Interior with Gravity
GRAIL’s twin spacecraft, nicknamed Ebb and Flow, orbited the Moon between 2011 and 2012. By measuring tiny variations in the Moon’s gravitational pull, the mission produced the most detailed gravity map of the Moon ever created.
Scientists used this data to study how the Moon flexes as it orbits Earth — a process called tidal deformation. They found that the near side bends more than the far side, a discovery that points to differences in temperature and composition within the Moon’s mantle, the layer between its crust and core.
“Our study shows that the Moon’s interior is not uniform,” said Dr. Ryan Park, lead author and supervisor at NASA’s Jet Propulsion Laboratory. “The nearside is warmer and more geologically evolved than the farside.”
Ancient Volcanism and Radioactive Heat
The near side’s distinctive lava plains, known as maria, formed billions of years ago from molten rock that surfaced and cooled. The new study confirms that intense volcanic activity helped concentrate radioactive, heat-producing elements such as thorium and titanium within the nearside mantle. These elements continue to generate heat through radioactive decay, maintaining a thermal imbalance with the farside.
Estimates suggest that the nearside mantle is 100 to 200 degrees Celsius (180 to 360 degrees Fahrenheit) warmer than the farside. This thermal asymmetry is consistent with the Moon’s observed deformation and helps explain why the two hemispheres appear so different today.
A New Model of the Lunar Interior
The findings challenge previous models that assumed the Moon’s internal layers were symmetrical. The new gravity model required the team to simulate a lopsided interior, and the result aligned closely with both surface geology and tidal data.
“The detected asymmetry in the mantle matches the pattern of the surface features,” noted Caltech planetary scientist Alex Berne. “It suggests the processes that drove ancient volcanism are still influencing the Moon’s structure today.” Seismology may provide further clarity. NASA plans to launch the Farside Seismic Suite in 2026 to monitor moonquakes and refine our understanding of the Moon’s deep interior.
Beyond the Moon
The techniques used in this study have broader applications. Researchers hope to apply similar gravitational analysis to other celestial bodies, such as Saturn’s icy moon Enceladus and Jupiter’s Ganymede — both considered promising candidates in the search for extraterrestrial life.
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Meanwhile, the updated lunar gravity map will aid future exploration. “It’s a critical tool for building lunar navigation and timing systems,” said Park. “As we return to the Moon, understanding its interior is essential for mission safety and success.” The Moon, despite its proximity, continues to surprise scientists. From influencing Earth’s tides to stabilizing its rotation, its importance is immense. As Park concluded, “Even after decades of exploration, we’re still uncovering its secrets.”