However, it has a very thin atmosphere which is detectable with very sensitive instruments. This week's episode #370 of the podcast The Skeptic's Guide to the Universe reported a story about a new observation from one of the satellites that is currently orbiting the moon. Lunar Reconnaissance Orbiter's LAMP spectrometer detects helium in Moon's atmosphere is an article describing the findings.
The surprising thing about this was not the finding of helium, but that the panel was not able to understand where this helium would come from. The podcast is generally excellent and I learn a lot from it. If you are not a regular listener then I would recommend it to you very strongly. (I know a few people have abandoned it recently because of the unholy row started by Rebecca Watson, but I still listen to it. I must admit that this week's episode was more to my liking because I found none of the panel members annoying!)
However, like many great science podcasts, SGU lacks adequate input from anyone who understands the physical sciences adequately, and time after time they miss something really obvious.
Someone else, Brad the Barbarian, beat me to the answer on the SGU forum but that's not unusual since I usually don't get the podcast download until the Saturday or Sunday after the Wednesday release. He explained that helium is the result of alpha decay of radioactive isotopes. I'll add a little more detail.
This process happens on the earth too, and it keeps the concentration of helium in the air fairly constant at a few parts per million. Yes, as they said on the show, helium escapes relatively easily from the atmosphere because it is so light that it can reach escape velocity, but it is constantly being topped up from below, albeit rather slowly.
|Helium - light enough to escape|
One of the best illustrations of this is the observation that helium is a major inconvenience to the industries that drill for natural gas. In some fields the natural gas contains more than 20% helium. Helium is inert and does not burn, and if the helium is not removed it has a very detrimental effect on the calorific value, and therefore the economic value of the gas that they wish to sell.
Fortunately this is where we get most of our industrial helium from. The unwanted contaminant has value in itself. At the moment there is a world shortage which is causing some inconvenience.
The helium collects in natural gas reservoirs for the same reason that the natural gas collects. Above the oil field there is a rock layer that is impervious to gases, so they collect as a kind of bubble. By drilling through the top of the bubble the gas is allowed out.
The helium comes from a specific type of radioactive decay of elements in the rocks below - namely alpha decay. Some elements decay into others releasing an alpha particle. An alpha particle only needs to collect two electrons from the environment to become a helium atom, and electrons are very easy to find if you have a positive attitude - or should I say a positive charge.
Incidentally, for the SGU panel there was a clue. The instrument that measured this property of the moon's atmosphere is known by the acronym LAMP. The A in that acronym is 'Alpha'.
A Skeptic's Guide to Helium
Helium-3. The precious little sister of helium-4.