Moon’s Magnetic Field Lasted Over 2 Billion Years, Thanks To Lunar Dynamo

Moon Rock — Apollo 15 breccia sample 15498. The rock consists of basalt fragments welded together by a dark glassy matrix that was produced by melting from a meteorite impact. Scale cube is 1 cm across. NASA

The moon, Earth’s only natural satellite, is thought to have been formed soon after the planet itself came into existence about 4.5 billion years ago after an object the size of Mars crashed into Earth and the debris from the collision coalesced together to form the moon. It was only natural that there were some common properties among Earth and the moon.

A magnetic field was one such thing, but while it continues to exist around Earth to this day — and is considered a fundamental factor for habitability — it disappeared from around the moon at some point in the past due to reasons not yet understood. Till recently, the lunar magnetic field was thought to have disappeared, or at least weakened significantly, about 3.5 billion years ago. But new research has shown that the lunar dynamo in the moon’s core — thought to be responsible for the magnetic field — was active till as recently as 1 to 2.5 billion years ago.

Researchers from the Massachusettes Institute of Technology (MIT) and Rutgers University analyzed a moon rock sample that was collected in August 1971 by the crew of Apollo 15. The relatively young rock, about 1 to 2.5 billion years old, was likely formed as a result of a meteor impact. And the analysis showed a magnetic field of about 5 microtesla at the time of its formation.

Compared to the magnetic field on Earth today, 5 microtesla is about 10 times weaker, but it is still about 1,000 times stronger than the fields that now exist in interplanetary space. About 4 billion years ago, the moon had a magnetic field measuring about 100 microteslas, which is almost twice the average 50 microtesla strength of Earth’s magnetic field today.

This finding pushes the timeline for when we thought the moon lost its magnetic field by between 1 and 2.5 billion years. After significantly dissipating about 3.5 billion years ago, for reasons not known or understood, we didn’t know whether the lunar magnetic field still lingered for some time. Until now, that is.

It is an important finding with some serious implications. For one, it can help our understanding of how the lunar dynamo in the moon’s core — the phenomenon responsible for generating the magnetic field — was powered and how it behaved over time.

Benjamin Weiss from MIT, who was a co-author of a research paper on the subject published Wednesday in the journal Science Advances, explained in a statement from the university: “The concept of a planetary magnetic field produced by moving liquid metal is an idea that is really only a few decades old. What powers this motion on Earth and other bodies, particularly on the moon, is not well-understood. We can figure this out by knowing the lifetime of the lunar dynamo.”

Then there is the matter of the relationship between a planetary body’s habitability and the magnetic field that surrounds it. Without a field, charged particles that originate at the body’s host star — in our case, the sun — will likely lead to the loss of liquid water on the body’s surface, if it had any, like what happened on Mars.

“Whenever we look at exoplanets or the moons of exoplanets that could be in the habitable zone, we can consider the magnetic field as an important player in habitability. Then the question becomes what size planets and moons should we be considering as possibly habitable worlds,” Sonia Tikoo from Rutgers, who was lead author of the study — titled “A two-billion-year history for the lunar dynamo” — said in a separate statement.

“We didn’t think that small planetary bodies could generate magnetic fields for a very long time because they have smaller cores that would cool quickly and crystallize early in their lifetimes. Because the rate of crystallization depends on the core composition, our finding may challenge what we think the lunar core is made of. It’s mostly made of iron, but something must be mixed in with it: sulfur, carbon or another element,” she explained further.

Samples from the moon that recorded magnetic activity and were also younger than 3.2 billion years are hard to come by, since lunar volcanic activity largely ceased around that time. Luckily for the researchers, some of the samples collected by the Apollo 15 crew were formed later, due to meteor impacts. The specific piece they analyzed is called Apollo 15 sample 15498, and the glassy material that welds together the minerals and bits of rock is very good at recording magnetic properties.

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