The Moon has lacked a magnetic field for most of its history – new research solves mystery sparked by rocks reported on Apollo

The surrounding Earth is a strong magnetic field created by liquid iron swirling around the core of the planet. Earth’s magnetic field is perhaps almost as old as the Earth itself – and in stark contrast to the Moon, which totally lacks a magnetic field today.

But has the Moon’s core generated a magnetic field in the past?

In the 1980s, geophysicists studying rocks brought back by astronauts from Apollo concluded that the Moon once had a magnetic field as strong as that of the Earth. But a strong magnetic field requires a power source, and the Moon’s core is relatively small. For decades, scientists have struggled to solve this conundrum: How could such a small nucleus create a strong magnetic field?

I am a professor of geophysics and have been studying the Earth’s magnetic field for over 30 years. I recently assembled a team to use new scientific techniques to reexamine the evidence for lunar magnetization. We discovered that the Moon actually does not have a long-lived magnetic field. Not only does this discovery change the modern understanding of the moon’s geological history, it also has major implications for the presence of resources on the moon that could be critical to future human exploration.

Why a magnetic moon?

Some rocks have the extraordinary ability to retain records of past magnetic fields when they contain minerals with iron atoms that align with a magnetic field as the rock cools and solidifies. The best magnetic minerals for preserving evidence of a field are tiny – a thousand times smaller than the width of a human hair – because it takes a lot of energy to rearrange their atoms.

Geophysicists who study ancient magnetism recreate this process, heating rock samples in the presence of known magnetic fields and comparing the new alignment of iron atoms with the orientation of iron atoms before the rock is heated. This allows researchers to learn more about past magnetic fields.

The first researchers studying the first rocks brought from the Moon by American astronauts wanted to use this method to study the magnetism of the Moon. But they ran into problems. Moon rocks contain a certain type of iron – called native iron – which is easily damaged by heat. Additionally, the native iron grains in moon rocks are sometimes relatively large, making them less likely to reliably register past magnetic fields.

Starting in the 1970s, geophysicists used alternative non-heating methods to study the magnetism of the Moon. They found that some lunar samples recorded strong magnetic fields, suggesting that the Moon had a magnetic field for more than 2 billion years.

But this result only deepened the enigma. The question of how the Moon’s core could produce a strong magnetic field remained unanswered.

An alternative theory

In the experiments, some Apollo samples showed signs of strong magnetic fields, but others did not. Some researchers have attributed the missing magnetization to the presence of large grains of native iron which were poor magnetic recorders. But many samples also contained small grains of iron that should have recorded a field.

There have long been doubts about the heatless techniques used by researchers on the Apollo samples. Some scientists have called them “last resort” methods and conclude that the uncertainties in the data collected in this way were so great that any interpretation should be considered speculation.

Alternatively, another group of scientists have suggested for decades that when meteorites strike the moon, they create a dusty plasma – a gas of ions and electrons – that could generate a strong magnetic field and magnetize moon rocks near the impact area.

In 2008, geophysicist Kristin Lawrence decided to revisit the question of lunar magnetization using an improved heating technique. Unlike the researchers who initially studied the samples, she could not detect any definitive evidence of a past magnetic field. The approach used by Lawrence and his team was better than testing without heating, but his results were still inconclusive. She felt like she was on to something, however, and that’s when she turned to me and my lab for help.

In 2011, Lawrence brought us a collection of lunar samples to test. We had developed techniques to identify individual millimeter-sized silicate crystals that contain only very small grains of iron and have ideal recording properties. We then used an ultra-sensitive superconducting magnetometer and a special carbon dioxide laser to quickly heat these samples so as to avoid altering their iron minerals. We found that almost all of the rocks had deeply weak magnetic signals.

At the time of this first test we were still improving the method, so we could not say for sure if the samples had formed on a Moon without a magnetic field. But we improved our testing methods and last year we decided to review the Apollo samples.

We definitely found that some of the samples did contain magnetic minerals capable of preserving the high fidelity signals of old magnetic fields. But the rocks hadn’t registered any of these signals. This suggests that the Moon has lacked a magnetic field for most of its history.

So, what explains the previous discoveries of a magnetic moon? The answer was in one of the samples: a small piece of dark glass containing tiny particles of iron-nickel.

The glass was made by a meteorite impact and showed clear evidence of a strong magnetic field. But it formed only about 2 million years ago. Almost all geophysicists agree that the Moon did not have a magnetic field at that time, because after 4.5 billion years of cooling, there was not enough heat left to fuel the churning of iron in the core of the Moon to generate a field. The magnetic signature of the glass corresponded to simulations of magnetic fields that could be generated by meteor impacts. This showed that meteorite impacts alone can create strong magnetic fields that magnetize nearby rocks. This could explain the previously reported high values ​​for some Apollo rocks.

Together, I think these findings solve the mystery of a seemingly magnetic Moon.

Magnetic shielding and lunar resources

This new view of lunar magnetism has huge implications for the potential presence of valuable resources as well as information about the ancient Sun and Earth that could be buried in lunar soils.

Magnetic fields act as shields that prevent solar particles from reaching a planet or moon. Without a magnetic field, the solar wind can strike the moon’s surface directly and implant elements like helium-3 and hydrogen into the ground.

Helium-3 has many applications, but most importantly, it could be a fuel source for nuclear fusion and future planetary exploration. Hydrogen’s value comes from the fact that it can combine with oxygen to form water, another crucial resource in space.

Since the Moon did not have a long-lived magnetic field, these elements could have accumulated in soils for billions of years longer than previously thought.

There is also scientific value. Elements encrusted by the solar wind could shed light on the evolution of the Sun. And when the Moon crosses the Earth’s magnetic field, elements of the Earth’s atmosphere can be deposited on the lunar surface, and these can contain clues about the oldest Earth.

The absence of a long-lasting magnetic field on the Moon could be a loss for some, but I think it could unlock a scientific boon and a valuable pool of potential resources.

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