Mysterious bright swirls on the moon's surface may have been created by comet collisions that occurred over the past 100 million years.

Researchers used computer models to simulate the effect comet impacts may have on the moon's soil, and determined the events could cause the features, known as "lunar swirls," Brown University reported.

"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.

The swirls do not look like typical impact craters, so the source of them has been a subject of debate for years. They were first discovered in the 1970s, and at the time scientists believed they were related to the moon's crustal magnetic field. In this scenario, rocks below the surface containing remnant magnetism would deflect solar wind (which at the time was thought to darken the lunar surface), creating brighter areas. Schultz believed the swirls were caused by something completely different after observing lunar modules land on 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," he said. "That was part of what got me started thinking comet impacts could cause the swirls."

Comets have their own gaseous atmosphere, called "coma," and this gas is believed to displace loose soil on the lunar surface upon impact. This could potentially create what appear as bright swirls on the moon. The structure of grains in the upper layer of the moon (referred to as the "fairy castle structure") have the ability to scatter the Sun's rays, creating darker areas. When this dust is scoured away, the surface below would appear brighter when struck by the Sun's rays.

Using state-of-the-art computer simulations, the researchers showed the impact of a comet coma and its icy core would have the ability to blow away the small-grained top layer of sand and create patterns consistent with the size and shape of the swirls.

"This is the first time anyone has looked at this using modern computational techniques," Schultz said. "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." 

The findings were reported in papers published in the journals Icarus and Nature.