Bright "swirl" patterns on the surface of the moon, long a cosmic mystery, may be the signature of ancient comets slamming onto the moon's surface, scientists say.
For years, scientists have debated the origin of the tangled, swirling bands of brighter soil twisting through the darker dust on the moon's surface.
Now, Brown University researchers say they've come up with computer models of comets crashing into the lunar soil that produce results that match the bright, wispy traces seen across the lunar landscape.
At first glance, they note, the swirls seem not to be associated with any known large lunar impact craters.
"They simply look as if someone had finger-painted the surface," says Brown planetary scientist Peter Schultz. "There has been an intense debate about what causes these features."
Schultz says watching lunar landing modules touch down on the moon's surface during NASA's Apollo program suggested a possible solution.
"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 says. "That was part of what got me started thinking comet impacts could cause the swirls."
Comets are enveloped in a thin gaseous atmosphere called the coma that they carry along with them as they hurtle through space.
When small comets — perhaps too small to leave apparent impact craters — crash onto the moon's surface, Schultz suggests, that coma may scrub loose soil away from the surface to produce the swirls, similar to how the gas from the lunar modules' engines scoured it away.
When the top layer of lunar soil, which appears darker because of the manner in which it scatters sunlight falling on it, is stripped off, the exposed smoother surface would appear brighter than surrounding areas, Schultz says.
Schultz first came up with the hypothesis decades ago, but with the availability of better computer simulations of the dynamics of cosmic impacts, he decided it was a good time to take another look at comets and their potential to produce the kind of surface scouring he had been proposing.
A paper he published with his former Brown graduate student Megan Bruck-Syal, now at the Lawrence Livermore National Laboratory, described how the simulations confirmed that the impact of a comet's coma and icy core could easily strip away the grains of dust sitting at the very top of the lunar soil.
That scoured area could easily extend for thousands of miles from the site of the impact, which would be consistent with the swirling streaks long observed by astronomers, Schultz and Bruck-Syal say.
"We think this makes a pretty strong case that the swirls represent remnants of cometary collisions," says Schultz.
