While traipsing about the Moon during the Apollo 15 and 17 missions, NASA astronauts noticed an unexplained "horizon glow" on the lunar surface just before sunrise. The sight led them to suspect that a dust cloud might be percolating miles above them — but no further evidence has been found to validate this idea. The explanation for it remained a mystery.

Now, writing in Nature, researchers from the University of Colorado have confirmed that a big, lopsided dust cloud does indeed exist on the surface of the Moon. Yay, mystery solved, right? Well, not quite.

The position and composition of this newly revealed dust cloud doesn’t quite explain the weird glow seen by NASA astronauts over 40 years ago. But the discovery does indicate that maybe other airless planetary objects have dust clouds surrounding them as well. That means rocky planets without dense atmospheres could have some interesting materials floating around them in nearby space — an important thing to consider if we ever want to visit Mars, which has an atmospheric pressure just one-hundredth of Earth’s.

Dust clouds are important things to consider if we ever want to visit Mars

When they first saw this unexplained ethereal glow, the Apollo astronauts suspected that a phenomenon called light scattering was behind it. Light scattering works like this: when sunlight hits many tiny dust particles, it scatters in various directions, giving off what looks like extra brightness. But to explain the Apollo glow, a dust cloud would have needed to have some very particular properties: It would need to be a high-density cloud, filled with particles just one-tenth of a micron in radius. And the cloud would need to be situated 10 to 60 miles above the Moon’s surface.

(NASA)

Using data collected from NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE), the uncrewed lunar probe that orbited the Moon from 2013 to 2014, study author Mihaly Horányi and his team went looking for this mysterious Apollo cloud — only to find another dust cloud altogether. The one they discovered is less than a mile up and comprised of particles about half a micron thick, indicating that this is not the cloud that issued the weird Apollo light. There hasn’t been any evidence of the Apollo cloud in the last six months of LADEE, Horányi says. "The cloud that would have explained the Apollo observations is certainly not there all the time," Horányi, a professor of physics at University of Colorado-Boulder, told The Verge.

This dust cloud, however, is likely the result of high-speed interplanetary particles colliding with the lunar surface and then kicking up materials, according to Horányi. The researchers found that the density of the cloud even grows when meteors pass by, such as the Geminid meteor shower; particles in the meteors’ tails bombard the Moon’s surface, amplifying the size of the cloud.

High-speed interplanetary particles collide with the lunar surface and kick up materials

Similar dust ejection effects have been seen previously on icy moons in our solar system, such as those surrounding Jupiter. Until now, researchers were unsure if these clouds could form over objects covered in the rocky soil found on the Moon and Mars, known as regolith.

"It could be that even though the Moon is bombarded at all times just like the icy moons of Jupiter, every single particle would bury itself deep into the surface and would not generate an outgoing population of particles," said Horányi. "But that’s not the case. Even at the Moon, this process is at work." That means it’s possible for clouds like these to be circulating above other rocky, airless planets, too. That’s important if we ever want to land humans on Mars, because we’ll have to consider everything that might stand in our way. Dust clouds surrounding planets can harm spacecraft and astronauts if they’re dense enough.

As for what caused that glow back in the 1970s? The mystery remains. It’s possible that some electrostatic forces may have kicked up the dust during those two missions. Regardless, it’s definitely not what you’re thinking.

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