Lunar swirls, visible through a simple backyard telescope, create mysterious light-colored patterns on the moon's surface that have scientists baffled. New research suggests these vortices are caused by magnetized rocks that deflect the solar wind, keeping the vortices light-colored while nearby areas darken over time. Understanding these vortices could help people understand the moon's magnetic history and surface processes.

A lunar swirl is a bright, often sinuous feature that looks like an abstract painting. They are unique to the moon. In a laboratory setting, experimental petrologists at the University of Washington have recreated the magnetization reactions that may be responsible for these mysterious light-colored features on the lunar surface. Source: NASA/Goddard Space Flight Center/Arizona State University

Lunar swirls are light-colored, sinuous features on the moon's surface that are bright enough to be seen from a backyard telescope. Some people think they look like brushstrokes in an abstract painting. But these aren't mere art decorations: NASA images (above) show some lunar swirling tendrils extending hundreds of miles.

Moon swirls have long been difficult to explain, but recent modeling and spacecraft data have shed light on the twists and turns of the mystery. The data shows that rocks in the vortex are magnetized, and they deflect or change the direction of the solar wind particles that bombard the moon. Nearby rocks will be impacted. Over time, adjacent rocks darken due to chemical reactions caused by collisions, while the swirls remain light-colored.

This image from NASA's Lunar Reconnaissance Orbiter (LRO) depicts the lunar vortex Reiner Gamma, a bright spot in the otherwise dark Oceanus Procellarummare. Although visible from a backyard telescope, LRO's view from orbit shows tendrils stretching hundreds of kilometers. Source: NASA LROWAC Science Team

But how are the rocks in the lunar maelstrom magnetized? The moon today has no magnetic field. No astronauts or rovers have yet investigated the lunar vortex.

"Impacts could cause these types of magnetic field anomalies," said Michael J. Krawczynski, associate professor of Earth, environmental and planetary sciences in the College of Arts and Sciences at Washington University in St. Louis. He noted that meteorites often bring iron-rich material to some areas of the lunar surface. But in some of the vortices, we can't be sure how the impact formed something of this shape and size. It's more likely that something else locally magnetized the vortices."

Another theory is that there is molten rock underground that slowly cools in a magnetic field, creating magnetic anomalies. His findings were published in the Journal of Geophysical Research: Planets.

Krawczynski and lead author Yuanyuan Liang, who recently earned a PhD in Earth, Environmental and Planetary Sciences from the College of Arts and Sciences, measured the effects of different combinations of atmospheric chemistry and magma cooling rates on a mineral called ilmenite to determine whether they produced a magnetizing effect.

"Earth rocks are very easily magnetized because they often contain tiny particles of magnetite, a magnetic mineral," Krawczynski said. "A lot of terrestrial research looking at magnetite doesn't apply to the moon, because the ultramagnetic mineral is not found on the moon."

However, the research team found that ilmenite, which is abundant on the moon, can also react to form iron metal particles that can be magnetized under the right conditions.

"The small particles we used appear to generate stronger magnetic fields because they have a larger surface area to volume ratio compared to larger particles. Small particles are more susceptible to reduction reactions due to the greater exposed surface area," Liang said.

"Our simulation experiments show that under lunar conditions we can produce the required magnetizable material. So it is plausible that these vortices are caused by magma beneath the surface," said Krawczynski, a researcher at the university's McDonald Space Science Center.

Determining the origin of lunar vortices is considered key to understanding the processes that formed the lunar surface, the history of the moon's magnetic field, and even how the surfaces of planets and moons affect the space environment around them. This research will help interpret data obtained from future lunar missions, particularly those exploring magnetic anomalies on the lunar surface. As part of the Lunar Vertex mission, NASA intends to send a rover to the lunar swirling region known as Reiner Gamma in 2025.

"If you want to create magnetic anomalies in the way we describe, you have to have high titanium in the subsurface magma," Krawczynski said. "We see hints of this reaction producing iron metal in lunar meteorites and in lunar samples from Apollo. But all of these samples are surface lava flows, and our study shows that subsurface cooling should significantly enhance these metal-forming reactions."

For now, experimental methods are the best way to test predictions about how unseen molten rock might drive mysterious lunar swirling magnetic effects.