New Study Sheds Light on Moon’s Magnetic Field

For decades, scientists have debated whether the Moon ever had a strong magnetic field or whether it was always weak. New analysis of lunar rocks from the Apollo missions suggests the Moon’s magnetic field was mostly weak, with only brief episodes of intense activity, potentially resolving the long-standing debate.

Published in Nature Geoscience on 27 February, the research indicates that the Moon experienced short bursts of a strong magnetic field between 3.5 and 4 billion years ago, but for most of its 4.5-billion-year history, the field remained weak.

The study found that the melting of titanium-rich rocks at the boundary between the Moon’s core and mantle generated these temporary strong magnetic fields for periods ranging from decades to a few thousand years.

The debate over the Moon’s magnetism stems from the limited number of lunar rock samples. Six Apollo missions landed between 1969 and 1972 in regions near the lunar equator. These areas, chosen for their smooth basalt plains known as maria, contain high-titanium basalts formed by ancient meteor impacts that melted original rocks.

Researchers plotted the titanium content of lunar samples against the strength of their magnetization. Rocks with less than 6% titanium displayed weak magnetic fields, while higher titanium concentrations corresponded with stronger magnetization.

This suggests that the generation of a strong magnetic field was linked to the presence of titanium-rich rocks formed from molten material deep within the Moon.

Apollo mission samples make up a large portion of lunar material on Earth. NASA records show about 650 kilograms of lunar rocks exist globally, with 382 kilograms from Apollo missions. Early conclusions that the Moon had a sustained strong magnetic field were skewed because the Apollo rocks studied were disproportionately titanium-rich and highly magnetized.

Simulation models confirmed this “sample bias,” indicating that the Moon’s full magnetic history is not represented by the rocks collected so far. Random sampling would show that strongly magnetized rocks are rare.

NASA’s Artemis program aims to land in more varied lunar locations, collecting diverse samples to better represent the Moon’s complete 4.5-billion-year history.