Lunar swirls are beautiful optical anomalies on the surface of the moon that occur without any defined shape and exist in various sizes. These peculiar delights have puzzled scientists who tried to figure out their exact origin story. Scientists at Rutgers and University of California Berkeley have now investigated the origin of these anomalies and come back with new insight into the history of the moon.
The most famous lunar swirl is Reiner Gamma. Around 40 miles long and visible through most telescopes on a clear night, it's a popular spot for backyard astronomers.
Like many lunar swirls, Reiner Gamma has powerful, localized magnetic fields. Scientists believe the patterns of brightness and darkness intermingling seen within Reiner Gamma stem from magnetic fields deflecting particles from the solar wind. Over an extended period of time, this deflection would cause some parts of the lunar surface to weather more slowly.
"But the cause of those magnetic fields, and thus of the swirls themselves, had long been a mystery," says Sonia Tikoo, coauthor of the study recently published in the Journal of Geophysical Research - Planets and an assistant professor at Rutgers, in a . "To solve it, we had to find out what kind of geological feature could produce these magnetic fields—and why their magnetism is so powerful."
The researchers examined the geometry of lunar swirls. From there, they built out a mathematical model on the idea that each swirl was standing above a narrow magnetic object. Through that model, they found an image consistent with the appearance of lava tubes and lava dikes. Lave tubes are long, narrow structures by flowing lava in volcanoes, while lava dikes are vertical sheets of magma injected into the lunar crust. These are known geologic entities, but they're not typically seen as magnetic.
On Earth, volcanoes have a lot of destructive qualities but none of them are due to magnetism. Studies in 2009, using samples brought back from the initial moon landings of the 60s and 70s, moon rocks have a history of magnetism.
The moon's molten core may have at one point created its own magnetic field, and the rocks on the surface still share some of these qualities. Break down certain types of lunar materials with enough heat, and at the right temperature they will release metallic iron. With a strong enough magnetic field nearby, the iron would become magnetized in that direction.
This wouldn't happen on Earth, where free-floating oxygen binds with iron. Nor would it happen on the moon today, scientists say, because there is no more global magnetic field. However, last year Tikoo also discovered that the moon's historic magnetic field actually lasted lasted 1 billion to 2.5 billion years longer than previously thought.
It's possible, then, that has these lava tubes and dikes were forming they were being drawn towards the magnetic core of the moon. That would account for the startling images they've left behind.
"No one had thought about this reaction in terms of explaining these unusually strong magnetic features on the moon. This was the final piece in the puzzle of understanding the magnetism that underlies these lunar swirls," Tikoo says.
Of course, what would really help in proving this theory is doing something humanity has never managed to do—visit a lunar swirl. Tikoo plans to propose a rover that will do just that.