The moon is scarred by thousands upon thousands of craters. But, one feature of the moon has been up for debate. What are the swirls of bright soil on the moon’s surface like the one in the image below?

Reiner Gamma Moon formation

Some of these swirls stretch for thousands of miles across the moon’s surface.

One weird feature of the swirl is where they are mostly located. The swirls tend to only be found on the far side of the moon. But, one swirl called Reiner Gamma (pictured above, extra GIF below) can be seen with a telescope on the southwestern corner of the Moon’s near side.

reiner gamma swirl

One theory suggests they are caused by anomalies in the lunar crust’s magnetic field. Researchers from Brown University have a new theory. They were created by several comet impacts over the last 100 million years.

Using state-of-the-art computer models, the researchers simulated comet impacts on the moon’s surface. It suggests these impacts are responsible for the mysterious swirls.

“We think this makes a pretty strong case that the swirls represent remnants of cometary collisions,” said Peter Schultz, a planetary geoscientist at Brown University.

Schultz’s comet theory originated from NASA’s visits to the Moon during the Apollo program.

“You could see that the whole area around the lunar modules was smooth and bright because of the gas from the engines scoured the surface,” Schultz said. “That was part of what got me started thinking comet impacts could cause the swirls.”

Schultz hypothesized that a comet’s coma (gaseous part of the comet) could cause the swirls as the comet slammed into the moon.

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The computer simulations showed that the impact of a comet’s core and coma could produce the swirly features on the surface of the Moon. The simulations also showed the swirls stretch for thousands of kilometers from the impact point.

Schultz says comet impacts would also explain the magnetic field anomalies. “Comets carry with them a magnetic field created by streaming charged particles that interact with the solar wind,” Schultz said. “As the gas collides with the lunar surface, the cometary magnetic field becomes amplified and recorded in the small particles when they cool.”

Schultz believes the simulations provide a solid explanation for the swirls, but a future Moon mission could answer the question once and for all.

Schultz and co-author Megan Bruck Syal write in the study’s abstract:

Regional scouring by an impacting comet explains both the structure and albedo variations: large dynamic pressures entrain the smallest grains within a near-surface flow of dusty plasma, disrupting the backscattering, “fairy-castle” structure of lunar soils in equilibrium with the airless environment. The resulting surface is brightened by compaction of the previously open, porous macrostructure. Darker lanes observed within swirl regions are interpreted as possible melt and/or vapor deposits. Finally, the intense magnetic fields generated during high-speed cometary impacts provide an explanation for correlations between swirl locations and magnetic anomalies.

Image credits: NASA

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