It looks like it's back to the drawing board for scientists trying to solve a decades-old lunar mystery.
Some Apollo astronauts orbiting the moon in the 1960s and 70s saw a glow along the horizon just before sunrise, leading scientists to hypothesize that the glow was created by dust lofted above the moon's surface by impacts with space debris, including small pieces of comets.
But new data gathered by NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, which was designed to help solve this lunar mystery, confirms the dust cloud, but still leaves the glow unexplained, according to a new study published in the journal Nature on Wednesday.
So, one mystery down, one big one left to go.
Scientists have known about dust clouds surrounding other moons of the solar system orbiting Jupiter or Saturn, but this marks the first time a spacecraft has detected the cloud of dust around the Earth's moon.
The new study may also help scientists further understand the way that dust behaves in the solar system at large.
Other rocky cosmic bodies in the universe may also play host to clouds of dust like the one discovered surrounding the moon, and studying that dust may help researchers learn more about the stuff that makes up our cosmic neighborhood.
"We expect similar clouds to engulf all airless bodies: Mercury, asteroids, Phobos and Deimos - the moons of Mars," Mihaly Horanyi, physicist and co-author of the Nature study, told Mashable via email. "So, similar measurements in orbit around them could also be used to learn about their surface properties, as well as their interplanetary dust environment."
The dust cloud LADEE detected extends out about 155 miles, or 250 kilometers, from the surface of the moon, the study found.
Scientists think that the moon's permanent dust cloud was created by the "steady rain" of particles that impact the lunar surface, scattering lunar dust above the surface of the moon, according to Rick Elphic, a LADEE project scientist who was unaffiliated with the study.
"The Earth/moon system orbits the sun with an average speed of 67,000 miles per hour, and like bugs on a car windshield, the interplanetary micrometeoroid materials smack into the 'upstream' side of the Earth and moon," Elphic told Mashable in an email.
"On Earth these cause meteors, which burn up in the atmosphere, but with the almost negligible atmosphere on the moon, these particles smash right into the lunar surface with tremendous speed."
This means that the dust cloud is asymmetrical, thicker on one side of the moon than the other, suggesting that the dust cloud may have been created by particles from comets hitting the moon because of the way the cloud is angled, Elphic said.
What could the horizon glow be?
Even though scientists found a permanent cloud of dust around the moon, they could not explain the lunar glow the Apollo astronauts reported.
"We have found no evidence of the high density small particle population that could have explained the Apollo reports," Horanyi said.
Astronauts weren't the only ones to pick up on some kind of lunar horizon glow, though. NASA's Surveyor spacecraft also saw a glow from its spot on the surface of the moon in the 1960s.
It's possible that researchers will never get to the bottom of the mystery, Elphic said.
If an astronaut were to orbit the moon today, he or she probably wouldn't see any kind of glow before sunrise, Horanyi said, but the dust environment does increase around the moon at different times of year thanks to meteor showers bombarding the moon and Earth.
LADEE was operational for about 6 months and experienced one major meteor shower — the Geminid shower — during that time.
It is possible that the Apollo astronauts saw some kind of transient dust event which caused the glow on the horizon, rather than one caused by a known, regularly recurring meteor shower, Horanyi added.
There are other experiments that scientists could perform in order to get a better sense of what caused the strange glow.
For example, if China's Chang'e 3 moon lander still has an operating camera, it could be used to look at the horizon after sunset and compare those measurements to what the Surveyor spacecraft saw.
"There are two parts of this horizon glow question," Elphic said. "One is what the astronauts said that they saw looking out the windows just before orbital sunrise, and then there's another part of this that may be related to what's going on at the surface of the moon, and that's something that LADEE wouldn't have been able to detect."
Elphic thinks that it's still possible that charged dust grains are lofted just a few meters above the surface of the moon, possibly causing the glow that Surveyor saw, but LADEE wouldn't have detected that light as it orbited relatively far above the moon's surface.
LADEE launched to space on Sept. 6, 2013. The moon mission ended when it ran out of fuel and made a planned impact into the lunar surface on April 17, 2014, possibly stirring up more of the dust it studied during its moon observing life.
