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Moon’s extinct magnetic field lasted surprisingly long

The moon’s magnetic field lasted 1 billion to 2.5 billion years longer than once thought, a finding experts say could have important implications for habitability on other moons and planets throughout the universe.

“The Earth’s magnetic field is a shield that protects us from dangerous solar wind particles and ionizing radiation, so magnetic fields play a key role in the habitability of planets and, possibly, moons,” says Sonia Tikoo, assistant professor of earth and planetary sciences at Rutgers University and lead author of the study in Science Advances.

The moon now has no core-generated magnetic field—and scientists don’t know when it turned off.

“Without this shield, we’d have more radiation, we’d have lots of mutations and who knows how life would respond in an unstable environment like that,” says Tikoo, who began working on the study in 2013 while she was a graduate student at the Massachusetts Institute of Technology and who has examined more than 10 moon rocks. “It would be a harsher place to survive in.”

For the first time researchers successfully heated a lunar rock brought to Earth during an Apollo space mission to retrieve an accurate intensity for the lunar magnetic field. The energetic cores of planets and moons generate magnetic fields and rocks can record magnetic fields to which they were exposed.

For the study, researchers re-analyzed a moon rock collected by the Apollo 15 crew on August 1, 1971, on the southern rim of Dune Crater within eastern Mare Imbrium. The small, young rock—partially coated with melted glass—likely formed during a meteor impact on the lunar surface.

To analyze the lunar rock, researchers used a magnetometer—a device that measures the strength and direction of magnetic fields in rocks. The rock was heated to 1,436 degrees Fahrenheit in a controlled atmosphere chamber at MIT to reveal its original magnetization.

The researchers think the moon’s magnetic field declined by about 90 percent from its high point 3.56 billion years ago or earlier. That’s when the moon’s magnetic field was about the same strength as Earth’s is today—an average of about 50 microtesla, a measure of magnetism.

The lunar rock Tikoo tested, which is about 1 billion to 2.5 billion years old, recorded 5 microtesla. The moon now has no core-generated magnetic field—and scientists don’t know when it turned off. Lingering questions include trying to figure out when the field ceased and what the field was like between 3.56 billion and 2.5 billion years ago.

“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,” Tikoo says.

How the moon got its off-kilter orbit

“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.”

When a planet’s magnetic field dies, ionizing particles from its sun can lead to the loss of its water over hundreds of millions of years, “That’s a big deal in terms of habitability,” Tikoo says. For example, Mars once had lots of water but lost nearly all of it after its magnetic field died about 4 billion years ago.

The inside of the moon is surprisingly wet

“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.”

Other coauthors are from the University of California, Berkeley and Massachusetts Institute of Technology.

Source: Rutgers University

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