New evidence from ancient lunar rocks suggests that the moon's long-lived 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 impacts.

Lawrence Livermore National Laboratory 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 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.

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.

East limb view of the Earth's moon as seen by Clementine. Image courtesy of NASA

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."

Other collaborators include researchers from Massachusetts Institute of Technology, University of California, Berkeley, University Aix-Marseille, University of California, Santa Cruz, Kyung Hee University, Brown University and University of Hawaii at Manoa.

The research appears in the Proceedings of the National Academy of Sciences this week.