In August, AmericaSpace reported about the detection of helium in the Moon’s atmosphere. A new paper by the same group now reports on how the amount of helium varies as the lunar day progresses.
The team continued to use the LAMP instrument on the Lunar Reconnaissance Orbiter to search for the helium, as before. The Lyman Alpha Mapping Project was originally intended to use the light emitted by hydrogen gas throughout the universe to map parts of the Moon never illuminated by the Sun.
The original search for helium took place during June of 2011. The team made new observations in December and January after LRO’s orbit was adjusted from a circular 50 km orbit to an elliptical one with a periapsis of 30 km and an apoapsis of 200 km. After this adjustment, LAMP was able to look directly down onto the night-side surface of the Moon. Employing a protocol developed for the Apollo 17 Ultraviolet Spectrometer, it was easy to recover the 58.4 nm wavelength light signal from the helium.
The LAMP team continued to make measurements from December 2011 into January 2012, or about one month. This allowed the team to measure the variation in the amount of helium as the Moon traveled in its orbit.
They found that the amount of helium detected was proportional to the solar wind intensity. This caused the team to theorize that the helium is caught by the Moon from the solar wind. They suggest that helium ions in the solar wind hit the regolith of the lunar surface and lose much of their energy. They also gain electrons. The helium atoms are released from the regolith at a rate governed by the temperature of the area. This helium remains trapped around the Moon for a short time, as the Moon is not massive enough to hold onto the light gas.
There was one massive departure from this model, beginning January 7 and lasting five days. During this time, the Moon was passing through the Earth’s magnetotail, shielding it from the solar wind.
The team noted that there was a large coronal mass ejection January 23-24, massively increasing the solar wind flux. This should have resulted in the detection of more helium as the Moon trapped the increased amount of incoming gas. But the CME also released a large number of high-energy electrons that interfered with the LAMP instrument, and the increased signal was not detected.
The team planned to make additional observations over the summer.
The paper, by lead author Dr. Paul Feldman, is being published in the November-December issue of the planetary science journal Icarus.