The Magnetic Moon
Marcus Woo
Tina Dwyer is fascinated with the moon. The former Caltech undergrad has been interested in astronomy and science ever since she was a kid, she says. But it wasn't until she did a Summer Undergraduate Research Fellowship (SURF) project at Caltech that her passion for the moon and planetary science ignited.
"My SURF project kicked my interest in the moon to high gear," she says. She worked with professor of geobiology Joe Kirschvink and then-postdoc Ben Weiss on mapping the magnetic fields of tiny moon rocks -- glass beads found in the lunar soil. "I spent two summers on that project, and it was awesome."
Then, in the spring of 2005, she took a planetary-interiors course taught by professor of planetary science Dave Stevenson. For the class, students had to do a small research project, and one of the suggested topics was about solving a decades-long lunar mystery: how did the ancient moon power its now-defunct magnetic field?
"I grabbed onto that idea," she says.
Earth's magnetic field is powered by energy from its core, which causes the molten outer core to churn. Because the liquid outer core is primarily made out of electrically conductive iron, the fluid motions generate electric currents, which then produce a global magnetic field. The moon, however, is too small, so it doesn't have enough energy in its core to sustain a magnetic field. Scientists were puzzled, then, when the Apollo astronauts brought back magnetic moon rocks, which could only arise in the presence of an ambient magnetic field. Since then, researchers have been trying to come up with a satisfying explanation.
"For 40 years, people have been sitting there, scratching their heads, going, how do we do this?" Dwyer says.
For her research project, she proposed that instead of being powered by heat -- like in the Earth -- the moon's magnetic field could have been driven by the physical stirring of its liquid core. After graduating from Caltech in 2006, she went to graduate school at the University of Washington, where she studied experimental geochemistry. Now she's pursuing her PhD at UC Santa Cruz, where she's returned to planetary science -- and the research project she started at Caltech. With Stevenson and Francis Nimmo of UCSC, Dwyer refined her earlier work, and the team has published their findings in the journal Nature.
"Our story ties in with ideas of how the moon formed and evolved in its orbit," Stevenson says.
Earth's gravity pulls on the moon in a way that causes the moon's liquid core and mantle to spin around axes that are at a slight angle with respect to each other. As a result, instead of spinning as a single object, the core and mantle rotate separately. The differences in their motions are small today, but the moon -- which is currently moving away from Earth at a rate of a few centimeters per year -- was much closer to Earth when the lunar magnetic field existed a few billion years ago. Because of its closer distance, the gravitational interactions were more powerful, leading to a bigger difference in rotation between the core and mantle. Dwyer and her colleagues calculated that, in the past, the difference was pronounced enough to generate a magnetic field.
Over time, as the moon drifted farther away, the difference in motion lessened, and the magnetic field eventually died. "The fact that we have a way to turn off the magnetic field is a very exciting aspect of this model," Dwyer says, although she stresses that more research -- including the development of computer models to study the mechanism in detail -- is necessary to show that the theory is viable.
