Meteorites colliding with the moon sometimes set off tiny lights dancing across its surface. Now scientists think they know what powers these lunar lightbulbs, in the absence of any atmosphere that would otherwise set incoming meteors ablaze: The flashes result from superhot material kicked up by the tiny objects striking the moon’s surface.
“You have just a small piece of cometary material or asteroid, about 10 centimeters, that can do a very bright flash visible from the Earth,” says study coauthor Sylvain Bouley, a planetary scientist at the Paris Observatory.
The study, which will appear in March in Icarus, settles an old debate about where the twinkling lunar lights come from. Observed for more than half a millennium, lunar impacts occur hundreds of times each year. Meteor showers, like the Leonids in November, can dump as many as 20 objects on the moon in one night.
At first, scientists didn’t think the flashes necessarily came from the moon; they might have been reflections from tumbling satellites or some other kind of phenomenon. Then, debate revolved around whether impacts or something within the moon such as volcanism produced the transient flashes. Most recently, researchers couldn’t decide between hot, charged particles or liquid droplets kicked up by impacts as the culprit.
To answer the question, Bouley and his colleagues looked at lunar flashes recorded between 1999 and 2007. They calculated the brightness of each flash, plus the probable sizes and speeds for 54 collisions. Most impactors were around 10 centimeters in size and traveled at speeds of up to 72 kilometers per second, Bouley says.
Knowing the ingredients and brightness allowed the scientists to estimate the temperature and energy produced during each collision. They found that impacts were hot enough to release a mix of gas and liquid from the destroyed impactor. Some of that liquid, called melt droplets, produces light as it cools, creating the flash.
“Something is melting, and because it’s so hot, it radiates in the visible wavelength until it cools down,” says planetary scientist Carolyn Ernst of Johns Hopkins University’s Applied Physics Laboratory in Laurel, Md.
Astronomer Bill Cooke, who leads NASA’s Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Ala., has created impact flashes in the laboratory by shooting aluminum spheres into simulated lunar dirt. The new study “pretty much confirms what we were suspecting,” he says. ”But these guys are the first to put that suspicion into hard numbers.”
Lunar impacts have been studied closely only over the past decade, says Ernst, but they are important for several reasons. The impacts produce measurable seismic waves, allowing scientists to fine-tune their understanding of the moon’s seismic activity. Knowing how often objects hit the moon improves estimates of the ages of features on the lunar surface.
And characterizing impact hazards is useful for anyone thinking about developing a future moon base, says study coauthor and planetary scientist David Baratoux of the University of Toulouse in France. “It will be important to know how much of this impact will form on the moon,” Baratoux says. “How big are the projectiles, and how fast they go and so on.”