Xray fluorescence spectrometer

Having essentially no atmosphere, the Moon has little protection from the Sun's constant output of x-rays that wash over its day-lit surface and strike whatever gets in their way. When they strike certain elements, particularly those at the lighter end of the Periodic Table, they cause the atoms to re-emit or fluoresce x-rays in certain well-defined energies. Therefore, by comparing the spectral make-up of x-rays from the Sun with x-rays from the sunlit lunar surface, scientists could determine the composition of that surface's topmost layer. This was a particularly powerful technique because it was sensitive to those elements that formed the bulk of rocky planets, namely oxygen, silicon, aluminium, magnesium and iron. Apollo's x-ray spectrometer therefore consisted of two detectors, one of which was built into the SIM bay to receive x-rays from the Moon. The second was on the opposite side of the service module where it measured the x-ray flux from the Sun.

It wasn't long after the SIM bay was used for the first time on Apollo 15, that a combination of the signals from the x-ray spectrometer and the laser altimeter quickly revealed an important clue to the Moon's history. Scientists immediately noticed that a graph from the laser altimeter showing the surface elevation beneath the CSM bore a strong resemblance to another from the x-ray spectrometer showing the concentration of aluminium along the same path. The aluminium concentration declined over the low-lying maria. The significance of this lies in the fact that aluminium is a relatively light element. The discovery that its concentration was greater in the highlands strongly implied that, at one time, the Moon must have been largely molten to allow that element to rise to the top. This ran counter to one of the two popular theories about the Moon's genesis that were vigorously debated at that time. One school, dubbed the 'hot Mooners' believed that after the Moon accreted from the solar nebula, its interior had been sufficiently hot to allow thermal differentiation into a core and a mantle, and that it had later undergone substantial surface volcanism. The other school, the 'cold Mooners', thought that the Moon's interior had always been cold and that all surface features had been formed by impact, with the maria being splashes of melted rock from particularly severe impacts. Both schools had grasped elements of the truth. Current theories contend that very soon after its formation, the Moon's mantle was completely molten in what is descriptively called a magma ocean. Within this fluid mass, gravity allowed the various constituents of the magma to migrate up or down according to their weight, such that the fresh crust tended to have a high concentration of aluminium. The fact that strong evidence of this chemical differentiation is still extant today is testament to the extraordinary antiquity of the lunar surface when compared to Earth's surface.

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