Mystery of 'lunar swirls' finds answers in the moon's volcanic, magnetic past

The findings resolve a long-standing puzzle as to the phenomena linking magnetism and lunar swirls.

One of the most enigmatic features of our solar system are the curly, long stretches of flat clouds above the moon's surface called lunar swirls.

After nearly six decades of investigation, lunar scientists from University of California Berkeley and Rutgers University may have demystified the phenomenon.

Lunar swirls are strange markings on the Moon made by pale clouds of moondust, as NASA describes it.

These swirls resemble cream in a cup of coffee, but on a much greater scale – stretching for many kilometres over the moon's surface.

Part of what puzzled lunar scientists about these swirls was how perfectly flat they are, and the magnetic field around where they are located – far stronger than the moon's own field in the area.

The study suggests that these swirls are a product of ancient volcanic activity and the moon's magnetic field at the time.

The most recognised and widely studied lunar swirl is the Reiner Gamma – a 64-kilometre-long snake-like cloud (pictured), wildly popular with backyard astronomers.

 Mystery of lunar swirls finds answers in the moons volcanic, magnetic past

The Reiner Gamma lunar swirl on the moon near the iconic Reiner crater. Image courtesy: NASA

The researchers believe that the patterns seen in these clouds – which alternate between dark and bright shades – could be from solar wind particles being deflected by the magnetic field in the area. This would cause some areas on the moon to weather more slowly than others, the study notes.

NASA, too, has reported in the past that lunar swirls were almost always found in regions with a powerful magnetic field.

"The cause of those magnetic fields, and thus of the swirls themselves, had long been a mystery," Sonia Tikoo, coauthor of the study recently published in the Journal of Geophysical Researchsaid to Rutgers University Press.

"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 developed a mathematical model for these hotspots by putting together what is already know about the geometry of the swirls with data they collected about the magnetic field that surrounds them.

The findings show that swirls like the Reiner Gamma develop above patches of the moon that have a narrow, long and magnetic object buried just under the lunar surface.

The researchers think these objects are likely lava tubes – formed as lava flows in an eruption, or lava dikes – vertical sheets of magma that settle into the crust of the moon, based on their observations.

Lunar soil collected from Apollo 17 displayed in the National Museum of Natural History in . Image courtesy: Wikimedia Commons

Lunar soil collected from Apollo 17 displayed in the National Museum of Natural History in Washington. Image courtesy: Wikimedia Commons

The magnetism of these volcanic rocks finds answers in a simple geochemical reaction – some minerals release magnetic iron when exposed to temperatures as high as 600ºC in an oxygen-deprived environment, the study explains.

If a magnetic field is present during this reaction, the metallic iron aligns itself along the same direction, making the magnetism stronger in just that location. But iron getting magnetised doesn't just need an oxygen-free environment, it needs a global magnetic field.

The Apollo mission brought back evidence to show that our moon did have a strong magnetic field once, which diminished over time and left the moon as weakly magnetic as we've known it for over 3 billion years now.

"No one had thought about this reaction in terms of explaining these unusually strong magnetic features on the moon," Tikoo said. "This was the final piece in the puzzle of understanding the magnetism that underlies these lunar swirls."

Towards that end, Tikoo is serving as a committee member on a moon mission to collect samples of the swirls and study them first-hand.

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