[zeugma]

"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.

I'd bet on the former. 

How many millibars of atmosphere does the moon actually have? It's generally referred to as 'airless', but any body as large as the moon will have some trace atmosphere due to gravity.

[MikeD]

The lunar exosphere is 1/trillionth of an atmosphere. Apollo 17 measured helium on the lunar surface, but LAMP can measure helium density as a function of altitude, since it makes the measurement from lunar orbit.

MD, LAMP Instrument Scientist