Across the dark planes of the Moon, known as the maria, there are bright swirling marks, some of which can be easily spotted by amateur astronomers. These swirls are highly magnetized and by studying their shape researchers have been able to uncover some crucial details regarding these lunar rocks.
The patterns are caused by electrically charged particles from space being deflected by the magnetic rocks. This deflection allows areas of the Moon to weather more slowly. The Moon has no magnetic field but it used to billions of years ago. Researchers used this fact and the pattern of the swirls to propose a solution. Their findings are published in the Journal of Geophysical Research-Planets.
"The cause of those magnetic fields, and thus of the swirls themselves, had long been a mystery," co-author Professor Sonia Tikoo, from Rutgers University, said in a statement. "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 team suggested that these swirls must be in close proximity to a magnetic object that is narrow and close to the surface of the Moon. Two structures fit the bill. Lava tubes, long and narrow structures seen in many places on Earth, where lava used to flow as the surface solidified, or lava dikes, vertical sheets of magma that got stuck in the lunar crust.
Those solutions are extremely interesting and they explain the shape of the swirls, but they do not explain the magnetism. Then the team realized that the lunar lava would experience a reaction that wouldn’t happen on Earth. Certain minerals would break down at high-temperature, releasing iron. Without oxygen to react with, the iron is free to become magnetized if exposed to a strong enough magnetic field. The magnetized iron then remained stuck in place as the lava cooled down billions of years ago. These interesting swirls are just a visible consequence of that.
"No one had thought about this reaction in terms of explaining these unusually strong magnetic features on the Moon," Professor Tikoo explained. "This was the final piece in the puzzle of understanding the magnetism that underlies these lunar swirls."
The explanation is very intriguing but the researchers want more proof to back their solution. Professor Tikoo serves on a committee proposing a new lunar rover who could get there to study them directly.