An unprecedented discovery has found neon in the extremely thin atmosphere of the Moon. Although it had previously been theorized to exist, its detection is an important confirmation of this hypothesis, and it could reveal more about how these thin atmospheres are able to form.
Don’t expect to see a lunar Las Vegas any time soon, though. The Moon’s tenuous atmosphere, commonly referred to as an exosphere, is 100 trillion times less dense than Earth’s atmosphere at sea level. This means the amount of neon in the atmosphere is very, very low.
The discovery, published in Geophysical Research Letters, was made by NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, which orbited the Moon from October 6, 2013, to April 7, 2014, when it was purposefully crashed into the surface when it ran out of fuel. Much of the data it gathered is still being analyzed though. In this case, LADEE's Neutral Mass Spectrometer (NMS) instrument found that the Moon’s exosphere was made up of almost entirely argon, helium and neon.
"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," said lead author Mehdi Benna of NASA’s Goddard Space Flight Center and the University of Maryland, in a statement. "We were very pleased to not only finally confirm its presence, but to show that it is relatively abundant."
The Moon’s exosphere is replenished by the solar wind, which is mostly made of hydrogen and helium but contains many other elements too. Most of the elements stick to the surface except for argon, helium and neon, which are able to evaporate and form the thin exosphere. Helium was also found to be emitted from the decay of radioactive thorium and uranium in lunar rocks, at a rate of about seven liters per second across the surface.
Another interesting discovery was that the different elements peak at different times: 1 a.m. for helium (in Moon time), 4 a.m. for neon and sunrise for argon. Argon was found to be particularly interesting, as over the course of the mission its abundance in the exosphere increased and decreased by about 25%, possibly due to tidal stresses on the surface altering its emission, known as outgassing.
The discovery could have implications for future human operations on the Moon. So thin is the atmosphere that rockets launching on the surface could significantly alter its composition. "It's critical to learn about the lunar exosphere before sustained human exploration substantially alters it," added Benna.
Exospheres are also the most common type of atmosphere in the Solar System. They have very few atomic collisions compared to fully fledged atmospheres, which are driven by such collisions. Thus, studying the Moon’s exosphere could provide further insight into how they work.