Researchers have ended a long-standing debate about the Moon’s core. They confirmed it isn’t hollow or molten but has a solid, iron-rich inner core. By using lunar seismic data, laser measurements, and computer models, they created a detailed picture of the Moon’s interior. This shows an active mantle and an iron-heavy core, much like Earth’s.
Longstanding Questions About the Lunar Core
Since the Apollo missions placed seismometers on the Moon, scientists have tried to determine if its core is solid or fluid. Initial models allowed for both a molten center and a small solid core surrounded by liquid metal due to limited seismic data.
A 2011 NASA study suggested the Moon might have a small solid core about 240 kilometers in radius, with a density of around 8,000 kilograms per cubic meter. However, the evidence was not definitive. Without more accurate seismic data or additional probe missions, uncertainty persisted. Is the Moon’s core similar to Earth’s, featuring a solid inner core and a fluid outer layer, or is it fundamentally different?
Combining Multiple Lines of Evidence
Astronomer Arthur Briaud from France’s Centre National de la Recherche Scientifique and his team used an innovative, multi-disciplinary method to answer the question:
• Lunar Laser Ranging—Reflections of laser pulses off retroreflectors left on the Moon by Apollo astronauts provide ultra-precise measurements of the Moon’s distance and orientation over time. These data reveal tiny variations in the Moon’s rotation and tilt that depend sensitively on its internal structure.
• Gravitational Deformation—Spacecraft measure the Moon’s gravity to understand its shape and bulges caused by Earth’s tides, revealing how its mass is distributed.
• Seismic and Geophysical Data—Apollo’s seismic data, though low-resolution, reveal an outer fluid layer. The study’s models had to align these findings with other data.
The team combined three observation types and systematically tested interior profiles to identify core configurations that best matched nature’s clues.
Active Mantle Overturn: A Dynamic Moon
The study reveals a fascinating discovery: evidence of mantle overturn deep within the Moon. This process involves denser materials sinking to the core, while lighter minerals rise to the upper mantle. Unlike Earth’s ongoing mantle convection that drives plate tectonics, the Moon’s overturn was likely a singular or long-dormant event, significantly influencing its thermal history.
Dense materials in the Moon’s mantle sank to the core, pushing lighter materials up. This movement released heat, possibly causing early volcanic activity and explaining the makeup of lunar surface basalts.
The authors claim this scenario offers key insights into the Moon’s early bombardment, cooling, magnetic field changes, and volcanic history in its first billion years.
Defining the Solid Inner Core
Briaud’s team used detailed modeling to determine the Moon’s core size and makeup:
• Outer Fluid Core Radius: Approximately 362 kilometers (225 miles) from the lunar center.
• Solid Inner Core Radius: Roughly 258 kilometers (160 miles), about 15 percent of the Moon’s total radius.
• Inner Core Density:The Moon’s core density is about 7,822 kilograms per cubic meter, similar to pure iron. This aligns with a 2011 study, confirming its accuracy. The consensus is that the Moon has a two-layer metallic core, like Earth, with a liquid outer layer and a solid, iron-rich inner core.
Implications for the Lunar Magnetic Field
Exploring the Moon’s core aims to uncover the secrets of its ancient magnetic field. Lunar rocks show remanent magnetization, suggesting the Moon once had a magnetic dynamo, driven by liquid metal convection in its core.
The discovery of a solid inner core encased by a fluid outer layer explains how a dynamo is sustained. As the core cools, the solid inner core expands, releasing gravitational and latent heat. This process drives convection in the liquid outer layer, creating magnetic fields.. Geochemical data indicates the lunar dynamo started weakening about 3.2 billion years ago. The team’s findings on the core’s size and composition are vital for dynamo simulations, aiding in determining the timing and reasons for the lunar magnetic shield’s cessation.
Unraveling the Moon’s Formation and Evolution
Unraveling the Moon’s interior reveals its origins. The dominant theory, the giant-impact hypothesis, suggests a Mars-sized object collided with early Earth, flinging debris that formed the Moon. A dense metallic core, akin to Earth’s, implies that metal-rich material from both the impactor and proto-Earth blended effectively during formation.
The young Moon was once hot, enabling large-scale differentiation and dynamic mixing before solidifying. This suggests a once-active lunar interior, resembling a miniature Earth, unlike the cold, inactive Moon we see today.
A New Frontier for Planetary Seismology
The study highlights the strength of palaeoproteomics and advanced bioinformatics in tracing evolutionary DNA. These techniques, adapted for planetary science, enhance data extraction. By comparing them to protein sequencing methods used to infer ancient human lineages, researchers improved the extraction and interpretation of seismic and geodetic signals hidden in noisy datasets..
“Machine learning and AI can now explore planetary interiors, Briaud stated. These technologies help solve complex issues like electrode-electrolyte interactions in batteries or solvent-surface chemistry in catalysts. They tackle systems where interactions are too complex for basic calculations.”
Looking Ahead: Missions and Next Steps
Apollo seismic data are invaluable, but future lunar missions can enhance interior models with modern seismometers and laser reflectors. The Chinese Chang’e program and NASA’s Artemis missions aim to expand the network of surface instruments, boosting resolution and regional coverage..
Briaud’s team urges the use of gravity, laser ranging, and remote-sensing data from lunar orbiters like NASA’s Lunar Reconnaissance Orbiter and ESA’s BepiColombo to constantly refine models of the Moon’s deep structure.
A Solid Foundation for Solar System History
Scientists have confirmed the Moon’s solid iron core, providing crucial insights into the early Solar System. The Moon acts as a “fossil record,” revealing planetary differentiation and periods of intense bombardment. Understanding its core helps model the evolution, cooling, and magnetic shield formation of terrestrial planets, essential for habitability.
The study, featured in Science and Cell, is a breakthrough in lunar science. It answers a key question about the Moon and highlights the complex links between how planets form, their internal processes, and surface changes. According to Briaud and team, “The Moon’s solid inner core changes our view of lunar history and adds depth to the story of the Solar System’s creation and development.
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