Radical! Neon Found on the Moon
An illustration depicts NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft in orbit above the moon.
Credit: NASA Ames/Dana Berry

The moon doesn't have any gaudy casinos or buzzing diner signs, but it does have neon.

NASA's LADEE spacecraft has made the first-ever detection of neon in the wispy lunar atmosphere, which is properly known as an "exosphere" because it's so thin — about 100 trillion times less dense than that of Earth at sea level.

"The presence of neon in the exosphere of the moon has been a subject of speculation since the Apollo missions, but no credible detections were made," study lead author Mehdi Benna, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland, Baltimore County, said in a statement. "We were very pleased to not only finally confirm its presence, but to show that it is relatively abundant." [The Moon: 10 Surprising Lunar Facts]

But the gas is not abundant enough on the moon to generate the famous neon glow, NASA officials said.

LADEE — which is short for Lunar Atmosphere and Dust Environment Explorer — studied the moon's exosphere from orbit for seven months, from September 2013 through the end of its mission in April 2014.

The spacecraft's Neutral Mass Spectrometer (NMS) instrument determined that the moon's atmosphere is composed mainly of helium, argon and neon. Most of this material comes from the solar wind, a diverse stream of particles flowing from the sun at about 1 million mph (1.6 million km/h). (Other elements in the solar wind tend to stick to the lunar surface, because they're more volatile than helium, argon and neon, NASA officials said.)

But the NMS data showed that some of the exospheric gases come from moon rocks, via the process of radioactive decay. About 20 percent of the helium probably came from the decay of uranium and thorium, and some of the argon from the decay of potassium-40 into argon-40, researchers said.

"We were also surprised to find that argon-40 creates a local bulge above an unusual part of the moon's surface, the region containing [the dark volcanic plains] Mare Imbrium and Oceanus Procellarum," Benna said. "One could not help [but] notice that this region happens to be the place where potassium-40 is most abundant on the surface. So there may be a connection between the atmospheric argon, the surface potassium and deep interior sources."

LADEE's measurements also revealed that argon abundance changed by about 25 percent over the course of the spacecraft's mission, possibly as a result of outgassing caused by the Earth's strong gravitational tug, researchers said.

The new results, which were published May 28 in the journal Geophysical Research Letters, should give scientists a better understanding of exospheres in general, and that of the moon in particular, researchers said.

"It's critical to learn about the lunar exosphere before sustained human exploration substantially alters it," Benna said.

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