An alternative theory
In the experiments, some Apollo samples showed evidence of strong magnetic fields but other samples did not. Some researchers attributed the missing magnetization to the presence of large native iron grains that were poor magnetic recorders. But many of the samples also contained small iron grains that should have recorded a field.
There have been long-standing doubts about the nonheating techniques researchers used on the Apollo samples. Some scientists have called them methods of “last resort” and conclude that the uncertainties in data collected in this way were so large that any interpretation must be viewed as speculation.
Alternatively, another group of scientists has 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 lunar rocks near the impact zone.
In 2008, geophysicist Kristin Lawrence decided to revisit the question of lunar magnetization using an improved reheating technique. In contrast to the researchers who originally studied the samples, she was unable to detect any definitive evidence for a past magnetic field. The approach Lawrence and her team used was better than the nonheating tests, but her results were still not conclusive. She felt she was on to something, though, and that is when she turned to me and my lab for help.

By using a new technique, researchers were able to isolate and test tiny samples – like the piece seen here mounted inside a quartz cube – for magnetic evidence. Adam Fenster/U. Rochester, CC BY-ND
In 2011, Lawrence brought us a collection of lunar samples to test. We had been developing techniques to identify individual millimeter-size silicate crystals that contain only very small iron grains and have ideal recording properties. We then used an ultrasensitive superconducting magnetometer and a special carbon dioxide laser to rapidly heat those samples in a way that avoids altering their iron minerals. We found that nearly all the rocks had profoundly weak magnetic signals.
At the time of this first test we were still improving the method, so we couldn’t say with certainty whether the samples had formed on a Moon without a magnetic field. But we have been improving our testing methods, and last year we decided to revisit the Apollo samples.
We definitively found that some of the samples did indeed contain magnetic minerals capable of preserving high-fidelity signals of ancient magnetic fields. But the rocks had recorded no such signals. This suggests that the Moon lacked a magnetic field for nearly all of its history.
So, what explains the previous findings of a magnetic Moon? The answer was in one of the samples: a small, dark piece of glass containing tiny iron-nickel particles.

This small piece of lunar glass was formed and magnetized by a meteorite impact and could explain the strong magnetic readings from the past. Rory Cottrell/U. Rochester, CC BY-ND
The glass was made by a meteorite impact and showed clear evidence of a strong magnetic field. But it was formed only about 2 million years ago. Nearly all geophysicists agree 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 power the churning of iron in the Moon’s core to generate a field. The magnetic signature of the glass matched simulations of magnetic fields that can be generated by meteor impacts. This showed that meteorite impacts alone can create strong magnetic fields that magnetize rocks nearby. This could explain the high values previously reported from some Apollo rocks.
Taken together, I believe these findings resolve the mystery of a seemingly magnetic Moon.

Earth’s magnetic shield blocks solar wind, whereas the lack of a magnetic field on the Moon allows the solar wind to directly hit its surface and deposit elements. Michael Osadciw/U. Rochester, CC BY-ND