The global map was produced using a large-area X-ray spectrometer on board
ISRO’s Chandrayaan 2 orbiter has used the Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS) instrument to produce a global map of sodium abundance on the Moon. This is the first map of its kind ever made. There are two types of sodium atoms on the surface of the Moon, one that is loosely bound to the surface and those that are part of the minerals. Subtle meteorite impacts and high-energy particles from the solar wind free loosely bound sodium atoms, transporting them into the lunar exosphere. There is also a diurnal variation of sodium abundance, which tracks with day-night cycles and exposure to ultraviolet radiation from the Sun. The process also explains how the supply of sodium atoms in the lunar exosphere is constant.
The high regions of the Moon are the regions that make Earth’s satellite so bright and reflect light from the Sun. The Apollo missions brought back samples from the lunar highlands composed primarily of silicate material. The Luna and Chang’e missions have also returned samples from the lunar surface, widening the range of compositions of lunar regolith, the remains of an ancient lunar crust. A variety of minerals found on the Moon are richer in calcium than sodium, a substitute. However, the samples returned are from particular regions of the Moon, and do not paint a picture of the global distribution. The element has not been a target of remote sensing observations because sodium does not have a telltale signature in visible or infrared wavelengths.
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This is where the class instrument manufactured by the UR Rao Satellite Center in Bengaluru was perfectly positioned to fill in the scientific knowledge gap. The high sensitivity and performance of the instrument allowed for a clean signature of the sodium line.
Atoms are rarely found in the Moon’s intelligent exosphere. The exosphere extends for several thousand kilometers from the surface. There is a faint sodium glow around the Moon, which is the same color as a sodium vapor lamp. The findings pave a way for the investigation of surface-exosphere interactions, which could be used for modeling similar interactions on other planetary bodies with intelligent atmospheres within the Solar System, such as Mercury. Such models may also apply to distant exoplanets.
A paper describing the findings is published in The Astrophysical Journal Letters.