Scientists: Mystery of magnetic moon rocks may have an answer

Scientists: Mystery of magnetic moon rocks may have an answer

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It seems the mystery surrounding the moon’s magnetic field may finally have an answer.

A team of scientists at University of California Santa Cruz have proposed an ancient magnetic field may be responsible for magnetic lunar rocks retrieved from the surface of the moon.

A paper by scientists including Christina Dwyer from the University of California Santa Cruz, says an ancient geodynamo could have existed in the Moon’s distant past. Dwyer and colleagues say the Moon’s magnetic field was generated as the result of its liquid core once rotating differently to its overlaying solid mantle.

The team of researchers modeled the effects of differential motion in the early lunar core and mantle, when the moon orbited the Earth much more closely. The team of scientists said the model suggests that the difference in distance would have generated tidal interactions between the two bodies that could have caused the lunar mantle to rotate slightly differently from its core. The difference could have allowed for the existence of a lunar magnetic field, the team said.

The assertion comes as the “geodynamo” that generates Earth’s magnetic field is powered by heat from the inner core, which drives complex fluid motions in the molten iron of the outer core. The moon, which has long since ceased to have a global magnetic field, continues to maintain a liquid core similar to that possessed by earth. Scientists say the lunar magnetic field died down about 2.6 billion years ago.

“This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon,” Dwyer said. “The moon, like the earth, has a solid mantel made of rock and a liquid outer core.”

The published paper will likely present scientist with an alternative theory regarding rock samples returned by the Apollo astronauts, which show the moon once had a long-lasting, global magnetic field. Scientists were at a loss to explain what else might generate the required liquid motion of iron inside the moon, until now. Rocks can become magnetized from the shock of an impact, a mechanism some scientists have proposed to explain the magnetization of lunar samples. But recent paleomagnetic analyses of moon rocks, as well as orbital measurements of the magnetization of the lunar crust, suggest that there was a strong, long-lived magnetic field on the moon early in its history.

“People have been scratching their heads for 40 years, ever since Apollo,” said Dwyer. “This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon.”

A working model of a lunar dynamo, combined with more detailed paleomagnetic analysis of moon rocks, could give scientists a powerful tool for investigating the history of the moon, Dwyer said. In addition, the study presents a novel mechanism for generating a magnetic field not only on the moon, but also on other small bodies, including large asteroids.

Of note: In the same issue of Nature, another research team arrived at the same conclusion: Lunar mantle motions generated a global magnetic field on the ancient moon. However, the second study suggests that repeated impacts from enormous asteroids could have pushed the outside of the moon, causing it to slide over the inner core and create magnetism.

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