According to ISRO the radioactive 40K nuclide present deep below the lunar surface, disintegrates and makes up the 40Ar gas.
Chandrayaan-2 currently orbiting the Moon has detected Argon-40 in the lunar exosphere according to ISRO. It has been able to do that with the help of its Chandra’s Atmospheric Composition Explorer-2 (CHACE-2) payload. Chandrayaan-2 has detected Argon-40 from an altitude of about 100 km. Argon-40 also known as 40Ar is an isotope of the noble gas Argon. Argon is third most abundant gas in the Earth’s Atmosphere.
ISRO states that 40Ar is a major constituent of the lunar exosphere. Argon-40 according to a statement by ISRO originates from the radioactive disintegration of Potassium-40. According to the space agency, the radioactive 40K nuclide present deep below the lunar surface, disintegrates and makes up the 40Ar gas. This then diffuses through the intergranular space making its way up into the lunar exosphere.
It also stated that scientists call the thin gaseous layer encompassing the Moon as the ‘Lunar exosphere’, due to the gas atoms present in it very rarely collide with each other, thus making up a very tenuous structure. The CHACE-2 payload is a neutral mass spectrometer-based payload, which according to ISRO can detect constituents in the lunar neutral exosphere in the range of 1-300 atomic mass unit (amu).
Chandrayaan-2 was able to detect 40Ar in the lunar exosphere by capturing the day-night variations of concentration. 40Ar is a condensable gas and reacts differently at various temperatures and pressures. It condenses during lunar night and after lunar dawn, the gas then again starts getting released to the lunar exosphere.
Source : The Indian Express