Moon Lost Its Magnetic Field After Its Internal Dynamo Shut Down One Billion Years Ago

The moon appears to have lost its magnetic field after its internal dynamo ceased around one billion years ago, scientists have found. By analyzing lunar rocks, researchers showed how the magnetic field had all but disappeared by this point, most likely after the core fully crystallized.

The team, led by Saied Mighani, from MIT's Department of Earth, Atmospheric, and Planetary Sciences, and Huapei Wang, from the China University of Geosciences, have been studying the moon's magnetic field for several years.

Earth has a magnetic field that protects the planet from the stream of charged particles coming from the sun. It is thought to be generated by electric currents that are produced by the movement of liquid metal at the planet's core.

The moon is believed to have formed about 4.5 billion years ago after a massive collision between Earth and a Mars-sized object. The debris from the collision was eventually drawn together by gravitational forces and became our planet's satellite.

Initially, the moon was far closer to Earth than it is today. Over time it drifted away and it is still moving away from us now, at a rate of about one inch per year. Initially, it had a strong magnetic field, but this was eventually lost.

"A strong field on the moon may have shielded the surface from the solar wind, the supersonic plasma emitted from the sun," study author Benjamin Weiss told Newsweek. "This may have prevented the surface from being space weathered and the soil becoming rich in solar gases like it is today. Furthermore, the Moon was likely about twice as close to the Earth during the time the Moon was inferred to have a strong magnetic field compared today."

What generated the magnetic field and when the dynamo ceased was not known, however.

In previous research, the team found that four billion years ago, the moon's magnetic field was around 100 microteslas—about double the strength of Earth's today. This was determined by looking at lunar rock samples, mostly those that had been ejected during volcanic eruptions, MIT Technology Review reports. As the rocks cooled and solidified, tiny grains aligned in the direction of the magnetic field, capturing a record of it.

However, volcanic activity had mostly stopped by three billion years ago. "The past three billion years of lunar history has been a mystery because there's almost no rock record of it," Weiss told the magazine.

In 2017, Weiss and colleagues found evidence to suggest that by 2.5 billion years ago, the magnetic field had dropped to about 10 microteslas.

In a study published in Science Advances, they have now identified rocks from the lunar surface that were melted by a large impact about one billion years ago. As they cooled, they recorded the strength of the moon's magnetic field—showing that by this time, it had dropped to a maximum 0.1 microteslas. This indicates that by one billion years ago, the internal dynamo that produces the magnetic field had shut down.

full moon
The full moon in December, 2019. Researchers say the moon's internal dynamo ceased between 1.5 and one billion years ago. Matt Cardy/Getty Images

"The magnetic field is this nebulous thing that pervades space, like an invisible force field," Weiss told MIT Technology Review. "We've shown that the dynamo that produced the moon's magnetic field died somewhere between 1.5 and one billion years ago."

He told Newsweek it was surprising that the lunar dynamo lasted for at least two billion years. "This is because the moon is a small planetary body, such that it cooled off much more quickly than planets do," he said. "Mars has diameter about twice that of the moon, and yet it is widely thought that Mars dynamo ended by before four billion years ago. The moon's dynamo must have had an unusually long-lived power source."

Findings suggest the magnetic field may have been powered by two different processes. Initially, the strong field was the result of its proximity to Earth. Being so close, gravitational forces may have stirred up the liquid in the moon's core, producing the dynamo.

As the moon moved further away from Earth, this effect diminished and another process, called core crystallization, may have taken over. As the inner iron core crystallized, the liquid core was stirred up. When the core fully crystallized, the dynamo ceased.

The team say more research on younger lunar rock will be needed to understand if the dynamo stopped permanently or if it entered a "stop-start regime" before eventually ceasing altogether.

They now hope to go beyond measuring the strength of the moon's magnetic field. "We are now trying to see if we can measure the direction of the ancient field using Apollo samples whose original orientations we can construct," Weiss said.

This article has been updated to include additional comments from Benjamin Weiss.