Wait, the moon has an atmosphere, you ask? You thought it was a vacuum, right? Why, yes it does have an atmosphere - technically speaking. But for all practical purposes such as landing and habitation - no. In fact, the lunar atmosphere is so tenuous that it is really more properly known as an "exosphere", and it can only be measured with sensitive scientific instruments.
To illustrate how tenuous the lunar exosphere is, it's reported that the weight of a feather dropped on your hand would exert 1 trillion times more pressure than the lunar atmosphere. David Woods, in a recent podcast, mentioned that the total lunar atmosphere is about 10 tons. That's about the same amount as the weight of the propellant used during the lunar activities in each Apollo mission. So, essentially, each Apollo mission doubled the total weight of the lunar atmosphere (though these extra molecules very quickly dissipated due to the solar wind).
But back to helium. We knew from instruments deployed by the Apollo missions that the moon has a tiny atmosphere, composed mostly of argon and helium, with trace amounts of sodium, potassium, and hydrogen. NASA's Lunar Reconnaissance Orbiter has recently confirmed the presence of helium in the lunar atmosphere. What's new in this announcement is that the helium concentrations fluctuate, and that in turn highlights the fact that we don't know where that helium comes from. Is it outgassing from lunar minerals?
Alan Stern of the Southwest Research Institute in Boulder, Colo., said in a statement
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? If we find the solar wind is responsible, that will teach us a lot about how the same process works in other airless bodies.
More info on the LRO finding after the jump. And check out David Wood's book "How Apollo Flew to the Moon" - essential reading for those interested in the Apollo missions and one of the best books ever written on the subject. The above-mentioned Omega Tau podcast(s) with Mr. Woods is similarly outstanding and highly recommended.
Image courtesy: NASA