A new scientific breakthrough from NASA has shed light on one of the Moon’s long-standing mysteries: why its Earth-facing side looks so distinct from the far side we never see. Using data gathered from NASA’s twin Gravity Recovery and Interior Laboratory (GRAIL) spacecraft, scientists have discovered fundamental differences in the Moon’s internal structure that date back billions of years.
The research, recently published in Nature, reveals that the Moon’s nearside is warmer and geologically more active than the farside, largely due to intense volcanic activity in the distant past. This activity led to the accumulation of heat-producing elements like thorium and titanium on the nearside, heating its interior and shaping the surface we see today.
“The Moon’s internal makeup is far from uniform,” said Dr Ryan Park, the lead researcher from NASA’s Jet Propulsion Laboratory. “Our findings show that the nearside mantle is significantly hotter and more geologically evolved compared to the farside.
“The Moon’s nearside is dominated by large, dark plains known as maria, formed by ancient lava flows. In contrast, the far side has a more rugged and heavily cratered surface with very few lava plains.
The GRAIL mission’s high-precision gravitational data revealed that these surface differences are mirrored deep beneath the surface, particularly in the mantle, the layer between the crust and the core. One striking observation was the way the Moon flexes slightly in response to Earth’s gravitational pull — a process known as tidal deformation.
According to the researchers, the nearside bends more than the farside during the Moon’s elliptical orbit, suggesting a difference in temperature and composition between the two hemispheres. Estimates indicate that the nearside mantle is, on average, 100 to 200 degrees Celsius (180 to 360 degrees Fahrenheit) warmer than the farside.
The radioactive decay of nearside mantle-concentrated elements is probably the cause of this internal heating. These conditions may have driven the volcanic activity responsible for shaping the Moon’s distinct appearance.
The study also delivers the most accurate gravitational map of the Moon ever produced. “This updated gravity model is a key resource for the next generation of lunar missions,” Park noted. “It will be vital in developing navigation and timing systems for surface operations and orbiting spacecraft, improving mission safety and precision.
“The research holds implications beyond just lunar science. The same techniques could be used to investigate the interiors of other celestial bodies, including potentially habitable moons like Enceladus and Ganymede, which orbit Saturn and Jupiter, respectively.
Still, the Moon remains a key target for exploration. “The Moon helps stabilise Earth’s rotation and plays a major role in ocean tides, affecting ecosystems and daily life,” said Park. “Even after decades of exploration, we are still uncovering its secrets. As our nearest celestial neighbour, it continues to be a rich source of scientific discovery.”