NASA Reveals Moon’s Hidden Heat Imbalance

NASA scientists have discovered a substantial temperature difference between the Moon’s nearside and farside hemispheres, potentially solving one of lunar science’s most enduring mysteries.

The finding, published in Nature on May 14, reveals a 100-200 degree Kelvin temperature variation hidden deep in the lunar mantle that has persisted for billions of years. This thermal asymmetry helps explain why the Moon’s Earth-facing side features vast dark volcanic plains while its farside has a thicker, lighter-colored crust—one of the most striking dichotomies in our solar system.

The pioneering research comes from a team led by Ryan Park, supervisor of the Solar System Dynamics Group at NASA’s Jet Propulsion Laboratory, who used gravitational data from NASA’s GRAIL (Gravity Recovery and Interior Laboratory) spacecraft to peer deep inside Earth’s only natural satellite without landing on its surface. The same analytical approach was also applied to the asteroid Vesta, revealing surprising insights about its internal structure.

Tidal Tomography: Seeing Inside Celestial Bodies

The researchers developed a new method called “extraterrestrial tidal tomography” that examines how planetary bodies flex and deform in response to gravitational forces. By precisely measuring tiny variations in the Moon’s gravity field as it orbits Earth, scientists could map its interior structure with unprecedented detail.

“Gravity is a unique and fundamental property of a planetary body that can be used to explore its deep interior,” said Park. “Our technique doesn’t need data from the surface; we just need to track the motion of the spacecraft very precisely to get a global view of what’s inside.”

The key finding came from an unexpected measurement called a “degree-3 gravitational tidal Love number” (k3), which was approximately 72% higher than scientists would expect for a uniform Moon. This higher number indicates significant differences in the lunar interior between the nearside and farside.

The Surprising Asymmetry

After analyzing GRAIL data, researchers determined the Moon’s asymmetry represents a 2-3% difference in the shear modulus—a measurement of a material’s elasticity—between the two hemispheres, most likely caused by a persistent temperature difference of 100-200 degrees Kelvin.

“We found that the Moon’s near side is flexing more than the far side, meaning there’s something fundamentally different about the internal structure of the Moon’s near side compared to its far side,” said Park. “When we first analyzed the data, we were so surprised by the result we didn’t believe it. So we ran the calculations many times to verify the findings. In all, this is a decade of work.”

While scientists have long observed the visible differences between the Moon’s hemispheres—with the nearside dominated by dark volcanic plains (maria) and the farside featuring a more rugged, cratered landscape—the underlying cause remained elusive until now.

Heat Driving Lunar Evolution

What could cause such a persistent temperature difference? The research team considered several possibilities, including variations in chemical composition, before concluding that the most likely explanation involves the distribution of radioactive elements that generate heat.

The study provides strong evidence that heat-generating radioactive elements accumulated deep inside the near side’s mantle billions of years ago, creating conditions where magma could rise more easily toward the surface to form the distinctive dark maria visible today.

This thermal imbalance has several important implications:

• The warmer nearside mantle likely facilitated volcanic activity 2-3 billion years ago
• Some partial melting may still exist deep within the lunar interior
• The distribution of deep moonquakes correlates with regions where partial melt may be present
• The thermal anomaly has survived for billions of years despite the Moon’s gradual cooling

From Ancient Volcanoes to Modern Moonquakes

The researchers found that the location of partial melting in the lunar mantle coincides with the distribution of deep moonquakes. This suggests that small amounts of molten material—less than 5% by mass—may promote seismic activity by creating stress concentrations in regions that deform due to Earth’s gravitational pull.

Can this thermal asymmetry explain the more recent volcanic activity detected on the Moon? Possibly. The gradual cooling of the lunar interior over billions of years is consistent with the discovery of young volcanic material (approximately 120 million years old) from China’s Chang’e-5 sample return mission.

Applying Gravity Analysis Across the Solar System

Park’s team didn’t stop with the Moon. In a companion study published in Nature Astronomy on April 23, they examined the asteroid Vesta using similar techniques. By analyzing data from NASA’s Dawn spacecraft, which orbited Vesta from July 2011 to September 2012, they found something unexpected: instead of having distinct interior layers as many planetary scientists predicted, Vesta appears to have a remarkably uniform internal structure.

“Our technique is sensitive to any changes in the gravitational field of a body in space, whether that gravitational field changes over time, like the tidal flexing of the Moon, or through space, like a wobbling asteroid,” explained Park.

The researchers have already extended this analytical approach to Jupiter’s volcanic moon Io and the dwarf planet Ceres, revealing that Io likely lacks a global magma ocean and that Ceres has a partially differentiated interior. “There are many opportunities in the future to apply our technique for studying the interiors of intriguing planetary bodies throughout the solar system,” Park added.

NASA’s Data Archives: The Gift That Keeps Giving

These discoveries highlight the enduring value of NASA’s mission data archives. The GRAIL spacecraft completed their mission in December 2012, yet scientists continue to extract groundbreaking insights from the data they collected. This pattern of ongoing discovery from archived mission data is common across NASA’s scientific portfolio.

“NASA’s science data is one of our most valuable legacies,” said Kevin Murphy, NASA’s chief science data officer at NASA Headquarters in Washington. “It carries the stories of our missions, the insights of our discoveries, and the potential for future breakthroughs.”

Modern analytical techniques, including advanced image processing and artificial intelligence, are helping researchers find new significance in observations made decades ago. For example, the Voyager 2 data from 1986 recently yielded insights about Uranus’s magnetosphere that went unnoticed during initial analysis.

Looking Forward: New Missions to Test the Findings

How will these discoveries influence future lunar exploration? The thermal asymmetry could indicate variations in moonquake depth or frequency between the Moon’s hemispheres. While Apollo seismic data showed few farside moonquakes, it remains unclear whether this reflects actual differences in seismic activity or limitations in the data collection.

Fortunately, several upcoming missions will help test these findings directly, including the Farside Seismic Suite planned for 2026, the Lunar Environment Monitoring Station planned for Artemis III, and the proposed Lunar Geophysical Network mission.

As NASA works to establish a sustainable human presence on the Moon through the Artemis program, understanding the lunar interior structure becomes increasingly important for both scientific discovery and practical considerations like resource utilization and habitat construction.

This research demonstrates that even after decades of lunar exploration, our closest celestial neighbor still holds fascinating secrets waiting to be uncovered. By combining innovative analytical techniques with archived mission data, scientists continue to deepen our understanding of not just the Moon, but potentially of planetary formation and evolution throughout the solar system.