Alpha-particle spectrometer

Although the Moon appeared to be a very dead world to anyone who looked at it. scientists wondered if some traces of volcanism were still spluttering in some corner of the globe. Tantalisingly, some telescopic observers had reported seeing ’emissions’ in the form of brief 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, was the locale for some of the reported emanations seen by telescopic observers. These tentative indications of possible ongoing lunar activity should be seen in the light of studies of a crater, Lichtenberg, on the western side of Oceanus Procel – larum. 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 has 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 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 the KREEP elements that would have found it difficult to become part of the rock’s crystal lattice. Geologists now believe that the violence of the Imbrium impact event nearly four billion years ago was enough to punch through the crust and excavate KREEPy rocks to the surface. A lot of this slightly radioactive rock was covered by the lava flows that drowned the western portion of the Imbrium Basin over three billion years ago. Then half a billion years ago the impact that formed Aristarchus drilled through the layers of basalt to re-excavate KREEPy material.