Using the X-ray spectrometer onboard the Chandrayaan-1 spacecraft, researchers have gained the first unambiguous evidence of a high sodium abundance on the lunar surface1. The X-ray spectrometer also allowed them to detect the abundances of major rock-forming elements such as magnesium, aluminium, silicon and calcium. These measurements of the lunar surface chemistry will help astrophysicists piece together how different geochemical processes have shaped the evolution of the Moon.
By scanning the lunar surface, X-ray spectrometers provide a direct measure of the abundances of chemical elements. The X-ray measurements performed by the Apollo missions studied only 10% of the equatorial regions on the nearside of the Moon. Later X-ray measurements were affected by severe radiation-induced damage due to heightened solar flares.
To gain greater insight into the Moon’s surface chemistry, the researchers analysed data captured by the X-ray spectrometer onboard Chandrayaan-1, which recorded element-specific X-ray emissions with energies between 800 and 1,000 electron volts. The spatial resolution for a single spectral observation varied in the range 50 to 1,000 kilometres, depending on the spacecraft’s altitude and the integration time. To prevent radiation-induced damage to the spectrometer, these X-ray measurements were carried out during periods when the intensities of solar flares were weak.
The scientists found distinct X-ray signatures for a high sodium abundance on the lunar surface. They also detected high aluminium and calcium abundances but a low magnesium abundance in the highland regions of the Moon. The results suggest that there is an inverse relationship between calcium and sodium abundances.
The researchers say that the new insights the measurements provide into the lunar surface chemistry may reshape our current understanding of lunar evolution, which is based on the assumption that the Moon was partially molten in the past.
