In the late 1960s, NASA sent a series of unmanned Surveyor spacecrafts to the Moon and encountered a mystery that's still unsolved today: Viewed from certain angles before sunrise, part of the Moon's horizon seems to glow. Why's this weird? Because with the moon having barely any atmosphere, there's seemingly nothing that could be doing the glowing.

Later, during the Apollo missions, Moon-walking and orbiting astronauts also reported the same inexplicable phenomenon. They took notes and snapped photos. And before the three members of Apollo 17—the last men to walk the Moon—left for Earth, they even sketched the glow, trying to make sense of the mystery.

Today, a team of astrophysicists led by Mihály Horányi at the University of Colorado, Boulder, has come as close as we've ever been to explaining the mysterious phenomenon. Thanks to the the recently launched lunar orbiter LADEE (Lunar Atmosphere and Dust Environment Explorer) the team has discovered that an a incredibly thin but ever-present cloud of dust that hangs over part of the Moon. It's the first lunar dust cloud ever discovered. Horányi's discovery lends credence to our best theory explaining the lunar glow: that it's caused by sunlight energizing small flakes of floating Moon-dust. Their study appears in Nature.

According to Horányi, the newfound cloud is composed of lunar soil that's continually kicked up by the surprisingly heavy rain of debris from comets. This debris is the same thing that can cause shooting stars and meteor showers like the Perseids here on Earth. "Every day the Earth receives around 10 tons of cosmic debris, which burns up in our atmosphere. That's not the case for the Moon, which receives about 5 tons of material each day," Horányi says. The cloud even ebbs and flows with cometary meteor showers (like the Geminids and Northern Taurids) which Horányi says is even more evidence that comets are the cause.

So why hadn't scientists ever identified this cloud before? "The cloud is so thin that any sort of optical instrument would have an incredibly hard time finding it," he says. It took LADEE, which was specifically designed to look for such clouds, passing through the cloud to detect the miniscule amounts of dust particles.

Still, though, this cloud doesn't entirely solve the lunar glow mystery. Horányi says that his cloud is too thin and too close to the surface to explain what the Apollo astronauts saw. However, Horányi says, the existence of this cloud makes is more likely scientists could find a higher, denser cloud responsible for the weird effect. "We certainly can't discount the possibility," he says, "but this isn't the last page on the mystery."

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