A 50-year-old mystery about etchings of a magnetic field on lunar rocks gets a couple new theories.
Rock samples show the moon had a magnetic field for more than 400 million years.
The field is caused by fluid flow inside the moon, but keeping it going long enough to explain the magnetic impacts found in the rocks has been a puzzle.
Two new theories point to mechanical forces, either by Earth or from meteorite impacts, as the smoking gun.
Scientists have a new explanation for how the moon got -- and lost -- its magnetic groove; The Earth did it, at a time when the moon was closer than it is today.
Rock samples returned by the Apollo astronauts show the moon once had a long-lasting, global magnetic field, though it has none today.
On Earth, it is heat from the inner core swirling fluids in the molten iron outer core that triggers the magnetic field-producing phenomenon known as a dynamo.
Computer models show the moon is too small for heat to keep this scenario going long enough to account for the magnetic footprints found in the lunar rock samples.
Now, new research suggests it might have been gravitational tugging from Earth that triggered the moon's dynamo at a time when the moon was closer to its mother planet.
Scientists believe the moon formed some 4.5 billion years ago at a distance of about 12,000 miles from Earth. Currently, the moon is located about 239,000 miles away.
Studies show that when the moon was between about 103,000 and 190,000 miles away, tidal interactions between the Earth and the moon would have caused the moon's solid mantle to rotate differently than its liquid core, triggering the dynamo.
"At distances greater than 48 Earth radii, (about 190,000 miles) we predict no magnetic field by our mechanism," lead researcher Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz, told Discovery News.
The field would have died out about 2.7 billion years ago, she added.
Another team suggests that the moon didn't need Earth to jolt its dynamo into action -- it could have been done by the heavy bombardment of meteors, asteroids and other bodies crashing into the lunar surface.
"Each of the six large Nectarian impacts (impacts that occurred between about 3.92 billion years ago and 3.85 billion years ago) was large enough to desynchronize the moon and generate a magnetic field for about 10,000 years," planetary scientist Michael Le Bars, with France's National Center for Scientific Research and the University of Provence, wrote in an email to Discovery News.
But those might not have been the only ones. "One can indeed imagine that some large events have been hidden by the subsequent activity," Le Bars said.
Each event could have driven a dynamo for 10,000 years, magnetizing the crust. Smaller impacts that happened in a relatively short span of time also could have had similar effects, he added.
Both theories support the evidence for a lunar magnetic field that persisted for more than 400 million years sometime around 4 billion years ago. The research has implications for understanding how magnetic fields can arise in bodies like asteroids, which like the moon, are too small to sustain dynamos from internal heat.
The research appears in this week's Nature.