What Stirred Up the Moon’s Ancient Magnetic Field?

BY Jenny Marder  November 15, 2011 at 12:10 PM EST

In its ancient history, the moon had a magnetic field that may have lasted more than a billion years. Photo by NASA.

Update: 7 pm ET, Nov 15|

Lunar rocks collected during Apollo moon missions revealed a mysterious truth about the ancient history of the moon. These rocks were magnetized — nearly conclusive evidence that at the time they solidified, four billion years ago, the moon had a powerful magnetic field that has since disappeared.

What caused that magnetic field is a mystery that has perplexed scientists since the 1970s. Two companion studies, published last week in the journal Nature, pose two unrelated (but not necessarily contradictory) theories.

Just as the moon exerts forces that impact the Earth like ocean waves and high tide, the Earth once had a powerful influence over the moon.

According to the first theory, the moon used to be hotter and closer to the Earth than it is now and it possessed an Earth-like molten core. When it reached a certain distance from the Earth, the Earth’s gravitational tug caused this molten iron core and the surrounding solid mantle to rotate around slightly different axes. This resulted in a stirring motion, churning up interior turbulence, which eventually became vigorous enough to magnetize rocks on the moon, according to this theory.

Christina Dwyer, a Ph.D student at the University of California, Santa Cruz, and first author of the paper said the magnetic field most likely existed between 4.2 billion years ago and 2.7 billion years ago.

Whether the moon has a magnetic field is “an important property that relates to the fundamental structure of the moon,” said Dwyer, who began researching the subject as part of a senior-year, undergraduate term paper at the California Institute of Technology in 2005. “This whole thing ties into evolution of the lunar orbit. It ties into questions about life and questions about what was happening on Earth.”

The other study suggests that a fleet of asteroids smashed into the moon, forming large craters, but also jolting the moon’s axes and creating turbulence, similar to the dynamo that Dwyer describes in her paper. This period of time when these impacts occurred was known as the “late heavy bombardment,” and corresponded to the time when the magnetic field was present, said Michael Le Bars, author of one of the Nature papers and a scientist at the Research Institute for Out-of-Equilibrium Phenomena in Marseille, France.

“There is a large range of sizes of objects that impacted the Moon,” Le Bars said. “What we know for sure is that the objects that made the largest … basins were capable of delocking the Moon from its synchronous rotation.” The impact changed the rotation rate of the mantle, Le Bars said, sloshing the liquid core around like water inside a washing machine.

The Earth’s magnetic field is generated by convective motion in its outer core – a result of temperature changes, but such a dynamic can’t explain the magnetic field on the moon, which has a core too small to support convection. So scientists have been searching for an alternative. Finding a new model that generates magnetic field has been a challenge, and this model provides a new framework that can be applied to other planets and exoplanets.

The two papers complement each other, he added.

“They are based on the same idea that a dynamo can be generated by ‘mechanical stirring,’ which is a viable alternative to the convective dynamo,” he said. “They stir the core of the Moon using its precession, we stir it using impact … and both models could apply to the Moon at the same time or successively … and both models could be interesting for other planets.”

Read more on our Science page.