Observations from NASA's Lunar Reconnaissance Orbiter (LRO) have revealed the presence of helium in the thin atmosphere surrounding the moon.
Scientists made the discovery using the Lyman Alpha Mapping Project (LAMP) spectrometer, usually used for mapping the lunar surface, combined with measurements taken as far back as 1972 by Apollo 17's by the Lunar Atmosphere Composition Experiment (LACE).
However, the team was able to use the spectrometer to examine the far ultraviolet emissions visible in the atmosphere, detecting helium over a period spanning more than 50 orbits.
"The question now becomes, does the helium originate from inside the moon, for example, due to radioactive decay in rocks, or from an exterior source, such as the solar wind?" says Dr Alan Stern, LAMP principal investigator.
"If we find the solar wind is responsible, that will teach us a lot about how the same process works in other airless bodies."
If it turns out not to be the solar wind, another possibility is that radioactive decay or other internal lunar processes could be producing helium that diffuses from the interior or is released during lunar quakes.
"With LAMP's global views as it moves across the moon in future observations, we'll be in a great position to better determine the dominant source of the helium," says Stern.
The LACE observations indicated that helium became more abundant as the night increased, something the team plans to follow up - along with LACE's apparent detection of argon in the atmosphere.