Mysterious Lunar Swirls

To study the tail, "we built two small satellites and asked NASA to put them in orbit around the Moon." The Moon is a great place to sample the Earth's magnetotail, he explains, because the Moon passes through the tail once a month as it orbits Earth.

Courtesy of NASA An artist's concept: an Apollo sub-satellite leaves the Service Module.

NASA said yes, and two "sub-satellites" were deployed by the crews of Apollo 15 in 1971 and Apollo 16 in 1972. "The astronauts pushed a button and the satellites were shoved into space by a spring," says Lin. Free of the Service Module (the Apollo mothership), they orbited the Moon, gathering data collected by onboard electron detectors and magnetometers.

"We learned a lot about Earth's magnetic tail," says Lin. But they learned even more about the Moon. As the sub-satellites flew just 60 miles above the lunar terrain, they passed in and out of strange magnetic domains. Magnetic force fields were sprouting out of the lunar surface, reaching up and affecting the satellites' sensors. "We realized that the crust of the Moon must be magnetized," he recalls. It wasn't a global magnetic field like Earth's, but rather a crazy-quilt of magnetic patches.

The strongest fields were located above Lunar Swirls. "The swirls have magnetic fields measuring a few hundred nano-Tesla (nT) at ground level," says Lin. (Earth's magnetic field, for comparison, is 30,000 nT.) "If you walked around a swirl with a magnetic compass, the needle would swing back and forth in a confusing way. You'd quickly get lost because the magnetic fields are so jumbled."

Lin believes these strange fields are an important clue to the origin of swirls, and he offers this possibility: "Almost four billion years ago, the Moon had a liquid iron core and a global magnetic field. Suppose an asteroid hit the Moon. The blast would make a cloud of electrically conducting gas ('plasma') that would sweep around the Moon, pushing the global magnetic field in front of it. Eventually, the cloud would converge at a point directly opposite the impact, concentrating the magnetic field at that point." Eons later, the Moon's core cooled and its global magnetic field faded away. Only the strongest, tangled patches remained--the swirls.