Like pours of cream spilled into coffee, lunar swirls appear to curl in patchy globs across the surface of the moon. The most famous one is the tadpole-shaped Reiner Gamma. At 40 miles long, it’s caught the eye of astronomers since the Renaissance. Exactly why the moon is replete with these natural features, however, has been a point of academic debate for the last 50 years. Now, scientists from Rutgers University and the University of California, Berkeley say they have an answer.

In a paper published in the Journal of Geophysical Research the team claims it all comes down to the geological history of the moon. Recently, scientists have begun to accept that lunar swirls like Reiner Gamma emerged in places where local magnetic fields once shielded parts of the moon’s surface. The moon doesn’t have a strong magnetic field anymore, but researchers estimate that it did have one between 2.7 and 4.2 billion years ago. Remnants of that ancient magnetic field are now embedded in the lunar crust beneath the swirls.

Studying how the moon’s magnetic fields arose billions of years ago helped reveal why lunar swirls exist, study co-author and planetary scientist Sonia Tikoo, Ph.D., explained on Thursday. “We had to find out what kind of geological feature could produce these magnetic fields,” she said.

lunar swirl
Swirls east of the moon's Firsov crater.

The team developed a mathematical model, incorporating what was already known about the geometry of the swirls and the strength of the magnetic field hotspots beneath them. This model indicated that the underlying magnetized rocks would have to be shallow, narrow, and strongly magnetized — factors that point to the existence of archaic lava tubes.

“We suggest that these rocks were likely injected into the crush in the form of dikes or subsurface channels of flowing lava and they cooled slowly, leading to the enhancement of their metal content and enabling the rocks to capture a stable record of the Moon’s ancient magnetic field,” the scientists write.

The lava tubes theory fits nicely with what we already know about magnetism and moon rocks. Previous experiments demonstrated that moon rocks become highly magnetic when heated above 600 degrees Celsius. On Earth, where there’s free-floating oxygen, not much would happen at those temperatures, but on the moon, high temperatures — like those potentially found in lava tubes — cause minerals to break down and release metallic iron. If the iron is near a strong magnetic field, it will then become magnetized itself.

The researchers believe this process creates a swirl. The only real way to find this out though is to visit a lunar swirl and study it directly — something that, if NASA’s moon colony plan stays on track, could happen in the relatively near future.