The moon’s ancient magnetic field has long puzzled scientists. A buried rock could offer new clues - CNN

An unusual rock buried on the far side of the moon is providing fresh clues for a longstanding scientific puzzle: the ancient lunar magnetic field.

Analyzing data from lunar probes, a team of researchers found a rock formation that was much denser and more strongly magnetized compared with the surrounding material. The subterranean feature could be an ancient volcanic complex that became magnetized when the magma rose from the moon’s interior and solidified before reaching the surface, according to a new study published September 23 in the journal Science Advances.

The characteristics of the large rock body, measuring about 60 kilometers (37 miles) wide and around 9 kilometers (5.6 miles) deep, suggest that the moon once had a magnetic field when the rock formed about 4.2 billion years ago. The field was likely between one-fifth and one-third the size of Earth’s own magnetic field currently active today, the study authors noted.

Life on Earth as we know it likely wouldn’t be possible without its magnetic field. It creates the magnetosphere, a vast region in space that surrounds our planet. This celestial shield keeps harmful solar wind, cosmic rays and radiation at bay, preventing the atmosphere from being stripped away.

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Molten metal constantly churning in Earth’s outer core generates electrical currents that sustain the magnetic field, in a process called the dynamo effect. The mechanical principle is similar to a bicycle dynamo, which uses the bike’s rotating wheel to spin magnets inside the generator and produce an electrical current to power lights.

The moon may also have had a core-powered magnetic field in its past, but that activity has long ceased. About 3.2 billion years ago, as the moon’s interior cooled down, the field significantly weakened, according to NASA. Without strong protection, solar radiation ripped apart the moon’s atmosphere.

The ancient magnetic field’s formation and existence, however, has long been the subject of discussion in astronomy. “The debate stretches back to the 1970s, when the Apollo missions first returned lunar rocks that showed signs of having recorded a magnetic field,” said study coauthor Anna Mittelholz, a lecturer at the Swiss Federal Institute of Technology Zurich, or ETH Zurich.

For a long time, Mittelholz added, experts broadly agreed that the moon’s past dynamo was active from roughly 4.25 billion to 3.5 billion years ago. “This is probably still what the majority of the community believes, but in the last decade, new analyses of Apollo samples have found no magnetic signal at all for parts of that same period,” she wrote in an email, “and some researchers have gone as far as questioning whether an early dynamo existed.”

The new study brings evidence for that early dynamo, but without using samples from the Apollo missions. Instead, researchers used data collected from orbit by lunar probes to analyze a region not visible from Earth called Dewar, where they detected the magnetic rock.

Pinning down the history of the moon’s past magnetic field is key to understanding the moon itself, according to study coauthor Adrien Broquet, a researcher at the German Aerospace Center in Berlin.

“Earth and the moon had drastically different geological histories. The moon was smaller and fully molten for a long time. It also had a lot of volcanism early on, and there are many questions we cannot exactly answer if we don’t know how the moon looked like back then,” Broquet said. “Analyzing the magnetic field of the moon is a good way to estimate how the moon looked like early on.”

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The findings could potentially extend beyond the moon, helping scientists study other celestial bodies, Mittelholz added. If the moon “managed to run a dynamo for any length of time, that tells us something about the minimum conditions needed to sustain a dynamo,” she said. “Getting its history right helps calibrate what we think is possible for other small rocky and icy bodies too.”

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The researchers used data from NASA probes, including Lunar Prospector, launched in 1998; Kaguya, launched in 2007; and GRAIL, launched in 2011.

One advantage of using orbital data instead of moon rocks is that the samples were pulled out of their geological context more than 50 years ago, according to Mittelholz.