When the Moon was Active
The Moon appears a dead world. Once it was geologically active. New evidence from ancient lunar rocks suggests that the moon's own magnetic dynamo -- a molten, convecting core of liquid metal that generated a strong magnetic field -- lasted 160 million years longer than originally estimated and was continuously active until well after the final large surface impacts. Lawrence Livermore scientist William Cassata and a group of international collaborators analyzed two rocks gathered during the Apollo 11 mission and found that they were magnetized in a stable and surprisingly intense magnetic field. The study of these slowly cooled, unshocked rocks demonstrates that the moon had a core dynamo as late as a mere 3.55 billion years ago.
"The important implication of this discovery is that the moon possessed a magnetic field much later than would be expected for a body of its size," Cassata said.
The Moon has an external magnetic field of about 1—100 nanoteslas, less than one-hundredth that of the Earth. It does not currently have a global dipolar magnetic field, as would be generated by a liquid metal core geodynamo, and only has crustal magnetization, probably acquired early in lunar history when a geodynamo was still operating.
Exactly what powered the dynamo remains a mystery. One possibility is that the lunar dynamo was self-sustaining, like Earth’s: As the planet has cooled, its liquid core has moved in response, sustaining the dynamo and the magnetic field it produces. In the absence of a long-lived heat supply, most planetary bodies will cool within hundreds of millions of years of formation.
One hypothesis posits that the moon’s dynamo may have been powered by Earth’s gravitational pull. Billions of years ago, the moon was much closer to Earth than it is today; terrestrial gravity may have had a stirring effect within the moon’s core, keeping the liquid metal moving even after the lunar body had cooled.
Cassata said the study shows that the moon likely possessed a long-lived, core dynamo, much like the one that currently exists on Earth, but generated by a different mechanism. The Earth's core dynamo is generated by thermally driven convective motions in the liquid outer core. But, because of its size, the moon was too cool to sustain core convection as late as 3.55 billion years ago.
In the past, however, when the moon was closer to Earth, it's greater angle of precession would allow for mechanical stirring of the liquid metal core by the overlying rocky mantle. These motions can induce a global magnetic field.
A gradual decrease in the moon's precession angle as it moved further away from Earth and increase in its core viscosity as it cooled may have caused the dynamo to decline between 1.8 billion and 2.7 billion years ago.
Global magnetic dynamos can also be generated by large impacts, which have the potential to unlock the core and mantle from synchronous rotation. This differential rotation allows for fluid motions in the liquid metal core and a temporary dynamo, but can only occur if the impact creates a crater larger than 300 kilometers in diameter.
"Our data rule out the impact-generated dynamo hypothesis and support a model wherein the moon's magnetic field is generated by continuous stirring of the liquid metal core by the overlying rocky mantle due to Earth's gravitational pull," Cassata said.
"The lifetime of the ancient lunar core dynamo has implications for mechanisms of field generation on other planetary bodies," Cassata said. "Our results require a continuous power source like precession of the lunar mantle or thermal convection."
For further information see Moon Dynamo.
Core image via Wikipedia.