Chandrayaan-2 detects noble gas isotope from altitude of 100 km

Indian Space Research Organisation (ISRO) announced on Thursday that a specialised payload aboard the Chandrayaan-2 orbiter has detected Argon-40 in the gaseous envelope around the Moon. The payload named Chandra’s Atmospheric Composition Explorer-2 (CHACE-2) has made the discovery from an altitude of around 100 km from the lunar surface.

CHACE-2 is a spectrometer capable of scanning the thin gaseous envelope around the Moon called the lunar exosphere with a very high mass resolution. The primary objective of this instrument is to analyse the composition and distribution of Moon’s exosphere.

Argon-40 is one of the forms of the noble gas Argon and is an important constituent of lunar exosphere. The presence of Argon is well established, and even the rock samples from NASA’s Apollo missions also contained traces of Argon. The radioactive decay of rocks deep below the lunar surface release Argon-40 which escapes to the lunar exosphere through seepages and faults. The detection of this isotope in the early days of the orbiter mission demonstrates that the instrument is functioning efficiently onboard Chandrayaan-2.

As part of its early operation, CHACE-2 has detected Argon-40 in the lunar exosphere from an altitude of ~100 km, capturing the day-night variations of concentration. Being a condensable gas at the temperatures and pressures that prevail on the lunar surface, Argon-40 condenses during the lunar night. After lunar dawn, the Argon-40 starts getting released to the lunar exosphere,” explains a statement from ISRO on Thursday.

Argon is considered to be highly unreactive and is hence classified as a noble gas. It is one of the most abundant elements in the lunar exosphere as measured by the sensor instruments placed by the Apollo astronauts as well. Back in 1972, the Apollo-17 (last human landing on Moon) attempted to study the lunar atmospheric composition using a mass spectrometer. It detected Argon-40 and revealed that its abundance increases at times of high seismic activity as the fractures and faults allow it to escape.