Why The Moon Once Had Its Own Magnetic Field

Rocks from the Moon are magenetized if they are more than 3.5 billion years old, but until now we've struggled to work out why. Helen Field/NASA/Shuttestock

The Moon doesn't have a magnetic field, but magnetized rocks returned by the Apollo missions show it once did. Now, more than 40 years after humanity last visited the lunar surface, we have an explanation for how this came to be.

Rocks that formed in the first billion years of the Moon's existence are magnetized, demonstrating the presence of a magnetic field at the time. At one point, the field may even have exceeded the Earth's in strength. But the field weakened some 3.5 billion years ago, and vanished entirely about half a billion years later. Geologists have struggled to explain its cause.

In Earth and Planetary Science Letters, the field is attributed to crystallization occurring within the lunar core. Dr Kevin Righter of NASA's Johnson Space Center and colleagues proposed that the Moon's nickel/iron core was low in impurities such as sulfur and carbon, creating a high melting point. The Moon's small size compared to planets means it lost the heat of formation relatively quickly. If Righter is right about the high melting point, temperatures would have dropped to this level quite early in the Moon's history.

Once the melting point was reached, the Moon's core started crystallizing, and Righter argues that the energy released in this process drove the magnetic field. A team at Johnson tested the idea by heating powders formed from mixtures of the elements thought to have predominated within the Moon's interior. As long as the sulfur/carbon content was low, crystallization occurred at temperatures consistent with the theory. Although the exact composition of the core cannot be determined in this way, a trial with 0.5 percent sulfur and 0.375 percent carbon by weight had, at realistic pressures, a melting point of 1,550ºC (2,822ºF). This is high enough to fit with what we know of the field's timing.

Co-author Dr Lisa Danielson described the team in a statement as “excited, because this work shows that a specific geochemically-derived composition can explain many geophysical aspects of the lunar core.”

This is not the first explanation offered. In 2014, a model was proposed for a lunar dynamo similar to the one thought to be responsible for the Earth's field. However, skepticism remained that an object as small as the Moon could sustain the convection forces needed to maintain such a powerful dynamo, particularly for so long.

Understanding the Moon's magnetic history is of more than simply theoretical interest. It is generally believed that long-lasting magnetic fields are essential for life, at least beyond the microbial stage. Understanding the circumstances in which planets and moons form fields can help us predict the prospects for life on other worlds.

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