Mystery moon swirls caused by blasts of comet gas?

© NASA/GSFC/Arizona State University
This NASA Lunar Reconnaissance Orbiter observation shows the vast swirls of Reiner Gamma in the Oceanus Procellarum region of the moon, to the west of the crater Reiner. Scientists have long pondered the origin of these swirl patters and others like them and new computer simulations point to cometary impacts being a possible cause.

Strange bright swirls have long been known to exist on the moon's surface and their origin is steeped in mystery. Often stretching thousands of miles across the lunar landscape, scientists have tried to make connections with the elegant curved shapes with the moon's interior magnetism or interactions between moon dirt and the solar wind, but these explanations have fallen short.

Now, inspired by the Apollo moon landings and armed with a powerful computer model, researchers at Brown University think they have an alternative answer for these swirly patterns.

Over the past 100 million years, many small comets impacted the moon's pockmarked surface. Along with the icy nuclei that carved craters into the moon rock, the gaseous comet atmospheres — known as a comet's coma — would have also blasted into the moon's uppermost layer of regolith, possibly leaving the swirly imprint.

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

Like a strong gust of wind blowing across a dry, dusty beach, Schultz and co-investigator Megan Bruck-Syal (who is now a researcher at the Lawrence Livermore National Laboratory) believe that as a comet hits the moon's surface, the coma also blasts into the surface, blowing away the dark, lightweight upper layers of regolith (the moon's fine dusty "soil"), exposing the brighter layers below.

This hypothesis takes its lead from the Apollo lunar modules that landed on the moon in the 1969′s and 70′s.

"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," said Schultz in a Brown press release. "That was part of what got me started thinking comet impacts could cause the swirls." These findings have been published in the journal .

Through the use of computer simulations, Schultz and Bruck-Syal have revealed that the impact of comets on the moon could indeed create the vast swirls — the eddies and vorticies created by the gaseous coma impacting the lunar surface appear to leave their own special kind of imprint in the uppermost layers of regolith, often far from the parent comet's impact crater.

These simulations may also explain the magnetic anomalies discovered near the swirls. During a comet impact on the moon, small iron-rich particles would melt and cool and align in the direction of the magnetic field carried by the comet.

"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."

These are some of the most sophisticated computer simulations of the origin of the mysterious swirls and they certainly provide a compelling explanation, but further observational evidence is needed.

"Everything we see in simulations of comet impacts is consistent with the swirls as we see them on the moon. We think this process provides a consistent explanation, but may need new moon missions to finally resolve the debate."