Moon’s magnetic personality explained

An ancient lunar magnetic field that came as a puzzle to planetary scientists may have been caused by interactions between the Earth and the Moon billions of years ago.

Harrison Schmitt gathers rock samples on Apollo 17 mission
Harrison Schmitt gathers rock samples on Apollo 17 mission (NASA)

It has been known for some time that the Moon has no global magnetic field, so it was quite a surprise when Apollo astronauts retrieved lunar rocks which showed traces of magnetism. Studying the magnetic field of rocks, an area of science known as paleomagnetism, can help to discover the history of the Moon’s magnetic field.

“Billions of years ago, when the rocks were cooling, they recorded the magnetic field which was present during the cooling process,” explains Christina Dwyer of the University of California, Santa Cruz. “Today, these rocks are still magnetised, just like how more familiar magnets are still magnetised when they are at home stuck on my refrigerator, even though they were originally magnetised when they were at some factory.”

The magnetic field of the Earth is caused by fluid motion of the molten outer core. This is due to the heat of the inner core driving convective motion. However, as the Moon is much smaller than the Earth, it has cooled down to the point where no such convective motion can occur. Even if an Earth-like dynamo existed on the Moon early in its lifetime, it wouldn’t have lasted very long and the predicted magnetic field does not match that discovered in the lunar rocks. However, Dwyer and her colleagues have suggested a solution to this problem.

The Moon used to orbit much closer to the Earth than it does today. Just as the Moon causes tides on the Earth, the Earth can also influence the Moon. These tidal interactions caused the solid mantle of the Moon to rotate at a different rate than the liquid core. This differential rotation then stirred the liquid core into life, enabling it to induce a magnetic field.

The magnetic field eventually declined as the Moon drifted further from the Earth and shut down completely about 2.7 billion years ago. Paleomagnetic measurements could thus be used to find out more about the orbital evolution of the Moon early in its history.

It is possible the similar dynamos could have existed on other small bodies in the Solar System. “It’s even possible that one of those magnetised planetesimals became the asteroid 4 Vesta, which we think is magnetised because we have meteorites which we think came from Vesta which are magnetised,” Dwyer tells Skymania News. “We know that they weren’t magnetised on Earth or during the time that they were falling through the Earth’s atmosphere because the paleomagnetic analyses of these rocks show that they were magnetised before entry into the Earth’s atmosphere.”

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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