New evidence suggests the strange swirls on the surface of the moon are the remnants of cometary impacts. Until recently, the theory was one of a number of explanations lacking strong support.
One of the most famous of these strange shapes—called Reiner Gamma—can be seen via a small telescope pointed toward the moon's southwestern corner, best seen when the moon is waning. "It was my favorite object to look at when I was an amateur astronomer," says Brown University's Professor Peter Schultz. Two similar objects were subsequently revealed when a spacecraft returned images of the far side of the moon. "They simply look as if someone had finger-painted the surface," Schultz says.
The discovery of the far side's swirls was followed by observations that the swirls are associated with anomalies in the moon's magnetic field. This paved the way for competing theories. One holds that certain rocks maintain a memory of a time when the lunar magnetic field was much stronger. These pockets of strong magnetism interfere with the solar wind. According to this hypothesis, particles from the wind darken the lunar surface, so protected areas look lighter in comparison.
However, Schultz noticed that the areas around the Apollo landing sites looked similar to those brighter regions. "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."
As comets approach the sun, they generate a temporary atmosphere called a coma. The cometary tail is formed from the coma being swept behind the comet by the solar wind. In the journal Icarus, Schultz and Dr. Megan Syal of the Lawrence Livermore National Laboratory modeled the possibility that when comets hit the moon, their comas cause white markings on the surface.
“Our results show that cometary impacts entrain the finest fraction of lunar soil grains (<10 μm) over regional scales (∼100–1000 km), produce large masses of vaporized material, and likely generate transient magnetic fields that could exceed the Earth’s surface field strength by a factor of 104,” the pair report. Consequently, they argue, cometary impacts could account for the bright features seen in the swirls, particularly the apparent removal of fine soil grains.
“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,” they write. “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.”
These bright features would fade with time, so the authors suggest the impacts must have occurred within the last 100 million years.
Somewhat counter-intuitively, Schultz and Syal proposed earlier this year that comets are responsible for the fact that Mercury's surface is darker than the moon.
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