Moon’s Magnetic Field: Weak for Most of History, With Brief Strong Bursts
For decades, scientists have debated whether the moon ever possessed a strong magnetic field, or if it was consistently weak. New analysis of Apollo-era moon rocks suggests the moon’s magnetic field was likely weak for most of its history, punctuated by brief, intense bursts of activity – potentially resolving the long-standing mystery.
The research, published in the journal Nature Geoscience, demonstrates that the moon’s magnetic field intensified for short periods in its early history, roughly to years ago. However, for the vast majority of the moon’s -year lifespan, the magnetic field remained weak.
“For very short periods of time – no more than 5,000 years, but possibly as short as a few decades – melting of titanium-rich rocks at the moon’s core-mantle boundary resulted in the generation of a very strong field,” explained Claire Nichols, an associate professor of the geology of planetary processes at the University of Oxford, in a statement.
A Long Debate
The debate surrounding the moon’s magnetic field stems from the limited sample of lunar rocks available for study. Between and , six Apollo missions landed on the moon, all within a relatively small area around the lunar equator. These missions consistently touched down in zones characterized by basaltic rocks.
The landing sites were chosen for their flat, basaltic areas – known as maria – which are ancient lava plains formed after large meteorite impacts melted the original rock. These Apollo landing zones are particularly rich in titanium basalts.
The new research correlated the titanium content of lunar samples with the strength of their magnetic fields. Scientists found that samples containing less than 6% titanium exhibited weak magnetic fields, while those with higher titanium concentrations displayed stronger magnetism.
This suggests a connection between the formation of high-titanium rocks and the generation of a strong lunar magnetic field. Researchers believe both were triggered by the melting of titanium-rich material deep within the moon, temporarily creating a powerful magnetic field.
A Limited Sample
The limited nature of the Apollo sample collection has historically influenced interpretations of the moon’s magnetic history. Approximately 1,433 pounds (650 kilograms) of moon rocks are currently on Earth, with around 842 pounds (382 kg) originating from the Apollo missions, according to NASA.
The fact that many of the analyzed Apollo rocks are titanium-rich led to the perception of a long-lasting, strong magnetic field. However, this contradicted the understanding that the moon’s relatively small core – about one-seventh of its radius – would be insufficient to sustain a powerful magnetic dynamo for extended periods.
The researchers validated the sampling bias through modeling, demonstrating that a random selection of moon samples would likely yield only a small number of strongly magnetized rocks. The hope is that future missions, such as the NASA-led Artemis program, will land in more diverse locations, collecting samples that represent a broader range of the moon’s 4.5 billion-year history.
“If we were aliens exploring the Earth, and had landed here just six times, we would probably have a similar sampling bias – especially if we were selecting a flat surface to land on,” said study co-author Jon Wade, an associate professor of planetary materials at Oxford, in the statement. “It was only by chance that the Apollo missions focussed so much on the mare region of the moon – if they landed somewhere else, we would likely have concluded that the Moon only ever had a weak magnetic field and missed this important part of early lunar history entirely.”