Apollo 15 Mission
Science Experiments - X-ray Fluorescence Spectrometer
The X-ray Fluorescence Spectrometer Experiment measured the composition of the lunar surface.
The Apollo moon landings returned samples for study on Earth from only six locations on the Moon. In order to better understand the Moon's overall chemical composition, the X-ray Fluorescence Spectrometer and the Gamma-ray Spectrometer studied the composition of the Moon's surface from lunar orbit. These experiments were flown on both Apollo 15 and Apollo 16.
When X-rays from the Sun strike the lunar surface, they can cause some elements to emit additional X-rays, a process known as fluorescence. The elements that emit the X-rays can be identified based on the energy of the X-rays that are emitted. The abundances of these elements can be determined from the intensity of the emitted X-rays (the greater the intensity of the X-rays, the greater the abundance of the element). The X-ray Fluorescence Spectrometer measured the abundances of the elements magnesium, aluminum, and silicon. In all, about 9% of the Moon's surface was studied by this experiment. Features as small as 30 kilometers across can sometimes be detected in this data. In addition to studying the Moon, the X-ray Fluorescence Spectrometer was also used on the return voyage to Earth to study astronomical objects that emit X-rays. In particular, the objects Cygnus X-1 and Scorpius X-1 were both intensively studied.
Generally, over mare regions such as Mare Crisium, Mare Serenitatis, and Mare Tranquillitatis, low abundances of aluminum and high abundances of magnesium were measured. Over the lunar highlands, the opposite pattern (high aluminum abundances and low magnesium abundances) were measured. More detail about these measurements and their relationship to lunar mineral and rock composition are described in the following documents.
Aluminum-Silicon Abundance Ratio
The Apollo X-Ray Fluorescence Spectrometer did not directly measure the abundances of individual elements. Instead, results were determined as the ratio of the abundance of various elements to the abundance of the element silicon. The above figure shows a map of the ratio of aluminum abundance to silicon abundance. Red is a high ratio and blue is a low ratio. Because Apollo samples show a relatively constant abundance of silicon in different parts of the Moon, this map is approximately equivalent to a map showing variation of aluminum abundance on the Moon's surface. Data is only available for parts of the Moon that were flown over by Apollo 15 and Apollo 16. Moreover, because this experiment depended on X-rays from the Sun, only portions of the Moon that were flown over in daylight could be measured.
The Al/Si ratio is lowest over mare regions such as Mare Crisium, Mare Serenitatis, and Mare Tranquillitatis. The Al/Si ratio is about a factor of two higher over non-mare regions, such as the semicircular arc of highlands south of Mare Crisium. Aluminum is particularly abundant in the mineral plagioclase, which is a common constituent of rocks in highland (non-mare) regions on the Moon. The combination of low aluminum abundance in the mare, together with the high iron abundance in these regions measured by the Gamma-ray Spectrometer, is evidence that the mare are generally composed of basalt, even in regions that were not directly sampled by Apollo. (From Plate 10.4 of Lunar Sourcebook, Cambridge University Press, 1991.)
Magnesium-Silicon Abundance Ratio
The Mg/Si ratio has less of a systematic pattern than the Al/Si ratio, but the highest values tend to occur in mare regions, whose basalts contain magnesium in the mineral pyroxene. Lower values of the Mg/Si ratio were observed in the plagioclase-rich highland rocks. (From Plate 10.5 of Lunar Sourcebook, Cambridge University Press, 1991.)