In the final chapter of “The Mystery of the Lunar Swirls,” planetary scientists present a new theory to explain these strange markings on the Moon’s surface. The theory invokes underground magmas and strange magnetic anomalies.
Lunar swirls are sinuous formations that appear much more distinct than the surrounding landscape. They stretch for hundreds of miles, and no one is sure why they exist. No astronaut has ever visited one of these strange regions, but that hasn't stopped us from exploring them. Scientists avoid speculating based on images and magnetic field measurements. “Impacts could cause these kinds of magnetic anomalies,” said Michael J. Krawczynski, an associate professor of earth, environmental and planetary sciences in Arts & Sciences at Washington University in St. Louis. Krawczynski notes that meteorites deliver iron-rich material to areas of the moon’s surface. However, these swirls exist in regions that aren’t necessarily disturbed by meteorites. So what else could explain the swirls?
“Another theory is that there are underground lavas that slowly cool in a magnetic field and create the magnetic anomaly,” said Krawczynski, who, along with postdoctoral student Yuanyuan Liang, designed experiments to test this explanation. They measured the effects of different atmospheric chemistries and magmatic cooling rates on a mineral called ilmenite and found that under certain conditions, cooling underground lavas could be causing the ghostly lunar swirls.
Using Earth-based geological principles to understand lunar swirls
Even though more than a dozen people have walked on the Moon, no one has visited a lunar swirl or collected samples of its dust. That has forced planetary scientists to use Earthly analogs of lunar rocks to understand lunar magnetism. “Earthly rocks are very easily magnetized because they usually have little bits of magnetite, which is a magnetic mineral,” Krawczynski said. “A lot of the Earthly studies that have focused on things with magnetite are not applicable to the Moon, where there is no hypermagnetic mineral.”
So the research team turned to ilmenite as a test material. It is a titanium oxide mineral with a weak magnetic signal. Ilmenite exists all over the Moon and readily reacts to form magnetizable iron metal particles. “The smaller grains we worked with seemed to create stronger magnetic fields because the surface area to volume ratio is larger for smaller grains compared to larger grains,” Liang said. “With a larger exposed surface area, it is easier for smaller grains to undergo the reduction reaction.”
Interestingly, planetary scientists have observed a similar reaction that creates metallic iron in lunar meteorites in samples from the Apollo missions. The difference, however, is that those samples came from surface lava flows. Krawczynski and Liang's study focused on types of magma that cooled underground.
“Our analog experiments showed that under lunar conditions we could create the magnetizable material we needed. It is therefore plausible that these eddies are caused by underground magma,” Krawczynski said. “If magnetic anomalies are to be created using the methods we studied, then the underground magma must have a high titanium content.”
Why study eddies on the Moon?
Those mysterious dust patterns aren't there by chance. They contain clues about the processes that shaped the lunar surface. Moreover, if magnetism is involved in their formation, that says something about magnetism on the Moon as a whole.
Until astronauts can get to the Moon to study these eddies for themselves, the ilmenite experiment offers a good way to test the idea of underground magma from afar, according to Krawczynski. Of course, it would be nice to get actual samples of underground rocks on the Moon, but that will have to wait. “If we could drill down, we could see if this reaction is happening,” he said. “That would be great, but it’s not possible yet. Right now, we’re stuck with the surface.”
Studies like Krawczynski and Liang’s will be very useful when NASA sends future lunar missions to the surface. There’s an entire rover project, part of a mission called Lunar Vertex, planned to study Reiner Gamma. That’s one of the Moon’s best-known swirls. Vertex is due to launch this year and is a predecessor to the larger return to the Moon that NASA is planning for later this decade. That mission could confirm whether or not the swirls are related to the magnetic field. If not, then there’s something else going on at Reiner Gamma and other swirl sites.
For more information
Lunar “whirlpools” may be magnetized by invisible magmas
Possibility of magmatism in the lunar crust producing strong magnetism in the crust
Lunar Vertex Mission
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