Although the Moon appeared to be a very dead world to anyone who looked at it, many scientists wondered if some traces of volcanism were still spluttering in some corner of the globe. Tantalisingly, some telescopic observers had reported seeing occasional 'emissions' on the Moon in the form of glows and hazes, which kept alive hopes of finding extant activity. The alpha-particle spectrometer was designed to look for indications of such activity.
Lunar rock samples from earlier missions were found to contain traces of uranium and thorium, two elements which, through their radioactivity, decay to form gaseous radon-222 and radon-220 among other elements. The alpha-particle spectrometer could detect these substances from lunar orbit by their emission of alpha-particle radiation - essentially the nuclei of helium atoms - as they further decayed and, by inference, locate areas of possible volcanism or other features that might cause the concentration of uranium and thorium to vary. Any emissions from the Moon of gases such as carbon dioxide and water vapour would also be detectable as they would be expected to include a small amount of decaying radon gas.
The major result to come from this instrument was that there is a small degree of outgassing of radon at various locations on the Moon, especially in the vicinity of the prominent crater Aristarchus - a result confirmed a generation later by the Lunar Prospector probe. Interestingly, Aristarchus, which is also one of the brightest places on the Moon, has been the locale for some of the reported emanations seen by telescopic observers. These tentative indications of possible current lunar activity should be seen in the light of studies of a crater, Lichtenberg, on the western side of Oceanus Procellarum. This crater exhibits a ray system that is believed to be just less than a billion years old, which is quite young by lunar standards. Yet, on a world where most of the basalt is much older, a distinctive dark lava flow can be seen to have obliterated much of its southern ray system. From this evidence, and as far as is known, the final gasps of lunar volcanism occurred about 800 million years ago. To put this into a terrestrial context, this is 300 million years before any kind of complex multicellular life appeared on Earth.
The detection of radon gas, particularly at Aristarchus, is best explained by the effect of the huge impact that formed the Imbrium Basin, within which Mare Imbrium now lies. The current magma ocean theory of the Moon's early evolution not only explains the richness of aluminium in the upland regions of the Moon, but also predicts that, as the magma ocean cooled, the last vestiges of lava to solidify would have been rich in elements that would have found it difficult to become part of the rock's crystal lattice, particularly potassium, phosphorus and various rare earth elements, including uranium and thorium. Rocks that are rich in these elements were found at the Apollo 12 landing site and later at the Apollo 14 site, and are known as
KREEPy rocks (K is the chemical symbol for potassium, P for phosphorus and REE means rare earth elements, and the 'y' makes it an adjective). Geologists now believe that the violence of the Imbrium impact event nearly 4 billion years ago was enough to deeply excavate the Moon's crust and bring KREEPy rocks up to the surface. A lot of this slightly radioactive rock was covered by the lava flows that drowned the western rim of the Imbrium Basin over 3 billion years ago. The impact that formed Aristarchus occurred only half a billion years ago, drilling through the layers of basalt and excavating KREEPy material from Imbrium's rim.
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