The Lunar Reconnaissance Orbiter has identified trace amounts of helium gas in the Moon’s tenuous atmosphere. Using its LAMP (Lyman Alpha Mapping Project) instrument, it identified the gently glow of elemental helium surrounding the Moon.
The LAMP instrument was originally designed to map perpetually dark parts of the Moon’s surface, regions untouched by sunlight. Instead, it would use the light emitted by excited hydrogen gas, called the Lyman alpha line, with a wavelength of 121.6 nm. Since hydrogen gas is everywhere, this faint light pervades the universe.
LAMP was usually pointed straight down at the lunar surface to allow mapping operations. But in November and December LAMP was pointed 83 degrees away from vertical to look out over the Moon’s limb. This was meant to look for far ultraviolet emissions from the lunar atmosphere. But a campaign to search for helium began on the 22 of June, 2011 and continued to June 30, lasting 54 orbits.
A previous experiment had already detected a small amount of helium gas around the Moon. This experiment, called the Lunar Atmosphere Composition Experiment (LACE), was left by the Apollo 17 astronauts. It lasted only 9 lunar days before failing, but did identify helium around the Moon.
During the 54 orbit search, LAMP did see the a faint light from excited helium, and identified the He I line with a wavelength of 58.4 nm.
Since interplanetary space is also populated with helium, it was necessary to make sure that LAMP wasn’t just seeing helium from the interplanetary medium. So LAMP was pointed straight up, directly vertical from the lunar surface. This allowed the instrument to determine how much of the helium signal was coming from the interplanetary medium. This observation was performed in November and December of 2011. Scientists noted that there might still be some helium from the Moon’s atmosphere above LRO’s orbit, but they considered most of the helium LAMP saw as belonging to interplanetary space.
This observation allowed the scientists to put a distinct value on the density of helium in the Moon’s atmosphere: 7.0×103 atoms/cm3
After LAMP was pointed back down to the lunar surface to continue mapping operations, it detected a faint helium signal during the lunar twilight, confirming that there was definitely helium belonging to the Moon.
Now the search is on to determine how the helium got to be around the Moon. Could it have been captured from the solar wind? Or perhaps it comes from the radioactive decay of unstable elements inside the Moon itself.
The LAMP team plans to look for correlations between solar activity and the intensity of the helium signal from the Moon. If no such signal is found, it might imply that the Moon’s helium is native to the Moon.
They also suggest that future investigations look for regions on the Moon with a stronger helium signal than other regions. That might imply that some areas on the Moon have more unstable elements emitting helium as part of their decay, confirming the hypothesis that at least some of the Moon’s helium originates inside the Moon.
The LAMP team next plans to look for signals from argon around the Moon. The LACE instrument left by Apollo 17 detected quantifiable amounts of argon, so the team hopes that LAMP will be able to detect it as well. However, they note in their paper that if the LACE measurements were correct, the argon signal will be 10-50 times fainter than the helium signal LAMP detected. However, they are confident that LAMP’s capabilities will allow them to confirm the presence of argon.