Utilizing the Alpha-Particle Spectrometer, which was designed to sniff for any faint releases of gas from underneath the Moon's surface, lunar scientists have found what they believe to be pockets of trapped gas occasionally released by the Moon's very weak remaining quakes.

These lunar quakes, which release not much more energy than a large firecracker, are mostly caused by the Moon's "creakings" as it cools and contracts a still a little further.

Since the Moon is a small world and cooled off faster after its initial formation, virtually all its volcanic activity died out over a billion years ago -- but there may still be faint traces of it left.

For this reason, the command ships of the Apollo 15 and 16 missions carried alpha-particle spectrometers as they orbited the Moon, to detect radioactive emissions from any tiny traces of radon gas that might be produced by the underground decay of uranium, and released through lunar surface vents as part of (comparatively) bigger flows of more orthodox gases such as carbon dioxide or argon.

Radon-222 is highly radioactive -- it has a half-life of only about 4 days before releasing alpha particles and decaying into polonium isotopes and then into a radioactive isotope of lead. But that lead-210 has a half-life of fully 21 years before decaying again into polonium-210 -- and while the lead doesn't release alpha particles that can be detected by this instrument, the polonium does.

So, by detecting alpha particles of the energy released by the radon, the instrument can map gas releases from the Moon's surface in the past few days -- and by detecting alpha rays of the different energy released by the polonium, it can map areas where earlier radon releases have occurred any time within the past few decades, although the radon may have traveled 200 km or so from its vent site before decaying into solid lead and polonium and encrusting the lunar soil.

During the Apollo observation, the alpha-particle spectrometer instruments were able to detect some indications of these two substances.

Radon was detected around Aristarchus, a crater that has long intrigued lunar scientists as it is one of the freshest big craters on the Moon, and the area around it includes a large volcanic valley (Schroter's Valley) that may still be venting leftover volcanic gases.

This is also the main place where astronomers have occasionally thought they sighted brief red glowing patches on the lunar surface -- although the existence of these has still not been solidly confirmed scientifically.

And concentrations of polonium were detected in several other places, mostly around the edges of lunar maria -- especially Mare Fecunditatis. But the data was very tentative -- it was taken for only a few days, too short to get a really good count -- and the Apollos orbited only over a narrow band around the Moon's equator.

Twenty five years later a similar spectrometer was carried on Lunar Prospector, which spent 18 months in a polar orbit around the Moon allowing the Moon's entire surface to be mapped in detail by a variety of instruments .

Although the Alpha-Particle Spectrometer encountered some problems that have delayed data analysis the results are finally coming in.

In an abstract at the Lunar and Planetary Science Conference this month, Dr. Stephanie Lawson and her associates report that the first preliminary results do indeed show some slight traces of polonium on the Moon -- in fact, over a wide area of the Moon's nearside, including areas in the east where the Prospector's gamma-ray and neutron instruments had failed to detect uranium and thorium on the surface itself.

Apparently underground traces of uranium are more widely distributed on the Moon than had been thought. But this LPSC paper didn't report any findings for radon.

However, Dr. Lawson now tells SpaceDaily that further analysis is starting to confirm the existence of radon concentrations as well -- confirming that there are indeed a few spots on the Moon where at least tiny traces of gas are still being vented.

Preliminary indications are that one is indeed just south of Aristarchus, confirming that this is one of the most scientifically interesting spots on the Moon's nearside for future surface investigations.

Another is located over Mare Fecunditatis, where the Apollos had already detected signs of polonium -- and the third is located near the center of the Moon's visible disk, generally around the Sinus Medii area.

So far, however, no signs of either radon or polonium have turned up on the Moon's farside -- which is not surprising, since the crust there is a good deal thicker and there were almost no eruptions of lava through it from the Moon's interior even during the Moon's earliest violent days.

It seems likely that most of the areas on the Moon where faint traces of gas are still being released are likely to be concentrated around the edges of the maria, which are actually gigantic ancient crater basins that were then flooded with lava -- the material around their edges was pushed up and disrupted by the impacts that first created them.

The Apollo instruments did indeed find indications that polonium is most concentrated there.

And this ties in with an intriguing paper at last year's Lunar and Planetary Science Conference by P.H. Schultz, in which he reported that photos of four small depressions around the edges of the Mare Imbrium show that they may actually be volcanic calderas from which gases escaped only recently, and that the 3-km wide Ina crater in particular may be only a few million years old, and in fact may be "still in the process of formation."

While the importance of these areas shouldn't be overplayed -- they are not areas of true volcanic activity, but rather the last gasps of trapped gases from inside the Moon -- they are nevertheless extremely interesting scientifically, and (to quote Schultz) "may provide a unique resource for future lunar exploration."

Further study of them may even solve the nagging mystery of those "transient glows" which may or may not appear occasionally and briefly on small patches of the Moon's surface.

Related Links
Preliminary Results From LP Alpha Particle Spectrometer: Lawson
Recent Lunar Activity: Evidence and Implications: Schultz
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