Scientists at the University of Oxford say they may have cracked the puzzle of the Moon’s magnetic field and settled a debate that has raged since the Apollo missions returned with rock samples.
NASA astronauts brought back evidence suggesting the lunar magnetic field was strong for long periods of its history, at times even stronger than Earth’s.
The findings created a puzzle, though. Scientists also considered the theory that the relatively small size of the Moon’s core – around one-seventh of its radius – means it cannot create a strong field.
New research from Oxford’s Department of Earth Sciences shows they are both right… kind of.
Led by associate professor Claire Nichols, the team analyzed the composition of a type of lunar rock known as the Mare basalts and found a new correlation between their titanium content and levels of magnetism.
Looking at the collected lunar samples, they found those with a strong magnetic field also contained large amounts of titanium, but those with less than 6 percent titanium were all associated with a weak magnetic field.
The study argues that the formation of high-titanium rocks and a strong lunar magnetic field were the result of titanium-rich material melting deep inside the Moon, which created a strong magnetic field, but only for about 5,000 years.
“Our new study suggests that the Apollo samples are biased to extremely rare events that lasted a few thousand years – but up to now, these have been interpreted as representing 0.5 billion years of lunar history. It now seems that a sampling bias prevented us from realizing how short and rare these strong magnetism events were,” she said in a statement.
“We now believe that for the vast majority of the Moon’s history, its magnetic field has been weak, which is consistent with our understanding of dynamo theory. But that for very short periods of time – possibly as short as a few decades – melting of titanium-rich rocks at the Moon’s core-mantle boundary resulted in the generation of a very strong field.”
The research team said Mare basalts made a good landing site for the Apollo missions because they are relatively flat. Since the astronauts brought back nearby rocks, they carried more titanium-rich basalts than a representative sample of the Moon’s surface would have. The result was a false impression of the length of time during which the Moon had a strong magnetic field.
Co-author Dr Simon Stephenson added: “We are now able to predict which types of samples will preserve which magnetic field strengths on the Moon. The upcoming Artemis missions offer us an opportunity to test this hypothesis and delve further into the history of the lunar magnetic field.” ®
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