Crashing comets caused Moon's mystery swirls

Scientists have found that lunar swirls - wispy bright regions scattered on the Moon's surface - were created by several comet collisions over the last 100 million years.
Researchers used state-of-the-art computer models to simulate the dynamics of comet impacts on the lunar soil. The simulations suggest that such impacts can account for many of the features in 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.
Lunar swirls have been the source of debate for years. Most are found on the unseen far side of the Moon, but one famous swirl called Reiner Gamma can be seen by telescope on the southwestern corner of the Moon's near side.

In the 1970s, scientists discovered that many of the swirls were associated with anomalies of the Moon's crustal magnetic field.
This led to one hypothesis for how the swirls may have formed. Rocks below the surface in those spots might contain remnant magnetism from early in the Moon's history, when its magnetic field was much stronger than it is now.

It had been proposed that those strong, locally trapped magnetic fields deflect the onslaught of the solar wind, which was thought to slowly darken the Moon's surface. The swirls would remain brighter than the surrounding soil because of those magnetic shields.

However, Schultz had a different idea for how the swirls may form. He proposed that comet impacts could cause the swirls.
Comets carry their own gaseous atmosphere called a coma. Schultz thought that when small comets slam into the Moon's surface the coma may scour away loose soil from the surface. That scouring may produce the bright swirls.
Schultz first published a paper outlining the idea in the journal Nature in 1980. That paper focused on how the scouring of the delicate upper layer of lunar soils could produce brightness consistent with the swirls.
As computer simulations of impact dynamics have gotten better, Schultz and colleagues decided to take a second look at whether comet impacts could produce that kind of scouring.

The new computer simulations show that the impact of a comet coma plus its icy core would indeed have the effect of blowing away the smallest grains that sit atop the lunar soil.
The simulations showed that the scoured area would stretch for perhaps thousands of kilometres from the impact point, consistent with the swirling streaks that extend across the Moon's surface.
Eddies and vortices created by the gaseous impact would explain the swirls' twisty, sinuous appearance.
The comet impact hypothesis could also explain the presence of magnetic anomalies near the swirls. The simulations showed that a comet impact would melt some of the tiny particles near the surface.

When small, iron-rich particles are melted and then cooled, they record the presence of any magnetic field that may be present at the time.