Researchers using NASA’s Lunar Reconnaissance Orbiter have made the first spectroscopic observations of the noble gas helium in the thin atmosphere of the Moon.
These remote-sensing observations complement in-situ measurements taken in 1972 by Apollo 17 astronauts.
The scientists, using the Lyman Alpha Mapping Project (LAMP) aboard the LRO, examined the far ultraviolet emissions visible in the tenuous atmosphere above the lunar surface, detecting helium over a campaign spanning more than 50 orbits.
Because helium also resides in the interplanetary background, several techniques were applied to remove signal contributions from the background helium and determine the amount of helium native to the Moon.
“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?” said Dr Alan Stern of Southwest Research Institute, LAMP principal investigator and lead author of the paper published in the Geophysical Research Letters.
In a related study led by Dr Paul Feldman of Johns Hopkins University, published in the journal Icarus, observations showed day-to-day variations in helium abundances, possibly varying with the solar wind, and also significantly decreasing when the Moon passed behind Earth out of sight from the solar wind.
“If we find the solar wind is responsible, that will teach us a lot about how the same process works in other airless bodies,” Dr Stern explained.
If spacecraft observations show no such correlation, radioactive decay or other internal lunar processes could be producing helium that diffuses from the interior or that releases 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,” Dr Stern said.
Bibliographic information: Stern et al. 2012. Lunar Atmospheric Helium Detections by the LAMP UV Spectrograph on the Lunar Reconnaissance Orbiter. Geophysical Research Letters, vol. 39, doi: 10.1029/2012GL051797