Oxford scientists have discovered that the Moon’s ancient magnetic field was mostly weak but experienced rare, powerful bursts. These “heartbeats” were recorded in titanium-rich rocks, which biased previous Apollo data for decades

University of Oxford researchers have resolved a decades-long scientific debate regarding the Moon’s prehistoric magnetic field. For years, scientists were split: some argued the Moon once had a powerful magnetic field like Earth’s, while others maintained it was always weak.

A new study published in Nature Geoscience reveals that both sides were correct. By reanalysing Apollo mission rock samples, the team discovered that while the Moon’s magnetism was usually faint, it experienced rare, explosive “heartbeats” of intense magnetic strength.

The mystery of the Lunar Dynamo

In the early history of the Moon—between 3.5 and 4 billion years ago—the lunar core was active. However, because the Moon’s core is only about one-seventh of its total radius, many physicists doubted it could sustain a steady, powerful magnetic field for millions of years. This led to a contradiction: rock samples from the Apollo missions recorded high magnetic strengths, but computer models of the lunar interior suggested those strengths were impossible to maintain.

The Oxford team found that these intense magnetic phases were not permanent. Instead of lasting for half a billion years as previously thought, these “super-strong” bursts likely lasted no more than 5,000 years, and possibly only a few decades. For the vast majority of lunar history, the field remained weak, aligning with traditional dynamo theory.

Titanium hotspots and sampling bias

The key to the mystery lay in the chemistry of the rocks themselves. The researchers examined Mare basalts—volcanic rocks from the Moon’s surface—and found a direct correlation between titanium content and magnetic strength. Every rock that recorded a powerful magnetic field was rich in titanium (specifically above 6 wt.%).

The study proposes that when titanium-rich material melted deep at the Moon’s core-mantle boundary, it temporarily triggered an unusually powerful magnetic dynamo. The disagreement in the scientific community was caused by a “sampling bias.” Because the Apollo missions prioritised landing on the flat, smooth surfaces of the titanium-rich Mare regions, astronauts unknowingly collected a disproportionate number of rocks from these rare magnetic hotspots. This gave the false impression that the entire Moon was highly magnetic for a long time.

Looking ahead to Artemis

This discovery changes how scientists interpret the geological history of the Moon and other small planetary bodies. By understanding which rock types preserve specific magnetic signatures, researchers can now predict the magnetic history of unexplored lunar regions.

The findings provide a specific hypothesis for the upcoming Artemis missions to test. By collecting samples from a more diverse range of locations outside the titanium-rich Mare regions, future astronauts can help confirm the intermittent nature of the Moon’s ancient magnetic heartbeats.