A new model for generating a global magnetic field in the ancient moon could help solve a 40-year-old mystery.

The Earth's magnetic field exists because it has a spinning solid core surrounded by hot metallic liquid, which churns around lava-lamp style and generates magnetism. But the moon is too small and cool to possess such a molten interior and therefore lacks a global magnetic field.

Yet when Apollo astronauts brought the first samples back from the moon’s surface, scientists discovered many of the rocks were magnetized.

“At first people said, ‘What are you talking about?’ since this was completely unexpected,” said planetary scientist Christina Dwyer of the University of California, Santa Cruz, who is the lead author of a paper in Nature Nov. 9 proposing a new way to create global magnetic fields.

The answer to this mystery begins with the moon’s formation more than 4 billion years ago. Researchers think that at this time an enormous Mars-sized object hit the Earth, splitting off a giant chunk. This proto-lunar lump orbited much closer to the Earth than the current moon.

Dwyer and her collaborators suggest that after this chunk cooled down, tidal forces from Earth's gravity could have acted on the moon to keep its outer mantle stirring around its core for more than a billion and a half years – the same way the moon's gravitational pull causes tides on Earth today. Instead of a spinning core, the shifting mantle would have generated the complex interior motions necessary to produce a magnetic field.

As the moon moved farther and farther away over time, the tidal force grew weaker. Around 2.7 billion years ago, it became too feeble to move the moon’s mantle anymore. This is important because no global magnetic field is observed on the moon today, so the force must have turned off at some point, Dwyer said.

In the same issue of Nature, another research team has arrived at the same conclusion: Lunar mantle motions generated a global magnetic field on the ancient moon. But this 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.

Rather than a billion and a half years, each encounter would have produced relatively short-lived magnetic fields, roughly one to 10 thousand years in duration. Currently, there isn't enough data to determine which of these accounts is more accurate.

Regardless of which model is correct, this type of mechanism for generating a global magnetic field has drawn praise from other researchers.

“I think it’s a major advance on something that’s been a real puzzle since the Apollo era,” said planetary scientist Benjamin Weiss from MIT. The mechanism might also be applied to explain possible magnetic fields on other small bodies such as asteroids, he added.

Eventually, more detailed data from magnetized Apollo rocks may be necessary to differentiate between the two teams' proposals.

Dwyer herself has suggested that both models could have some parts correct, with tidal forces pushing the mantle steadily for a time and giant impacts speeding up the motion occasionally.

“We might be able to combine the two,” she said.

Image: NASA

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