Chandra’s Atmospheric Composition Explorer-2 (CHACE-2), a payload onboard India’s lunar exploration mission, Chandrayaan-2, has recently made an unprecedented discovery regarding the distribution of Argon-40, one of the noble gases nestled on the lunar surface. This news surfaced in the wake of India’s launch of Chandrayaan-2, its second lunar exploration mission after Chandrayaan-1, from the Satish Dhawan Space Centre in Sriharikota in July 2019.
Chandrayaan-2 Mission
The Chandrayaan-2 mission is marked by its groundbreaking 3-in-1 spacecraft structure, composed of an Orbiter, a Lander named Vikram (after Vikram Sarabhai), and a Rover named Pragyan (wisdom). The descent of the Vikram lander was successful until it was 2.1 km from the lunar surface. India would have joined the countries that have achieved a soft-landing, namely, the Soviet Union, the United States, and China, had this been accomplished. Despite this, the Orbiter is equipped with various cameras for creating intricate three-dimensional maps of the lunar surface, studying mineral composition and the lunar atmosphere, and assessing the presence of water.
Discovery Findings
The lunar exosphere gas revealed by CHACE-2 is thought to have originated from the lunar surface. It is distributed in Ar-40 with significant spatial variability, with notable enhancements (termed as Argon bulge) over the KREEP region (potassium, Rare-Earth Elements, and Phosphorus) on the South Pole Aitken terrain, an impact crater on the Moon’s far side.
Lunar Exosphere
Exosphere, the outermost layer of a celestial body’s upper atmosphere, allows atoms and molecules to collide infrequently and escape into space. Several processes such as thermal desorption, photo-stimulated desorption, solar wind sputtering, and micrometeorite impact vaporisation lead to the existence of a dynamic equilibrium in the lunar exosphere.
Significance of Discovery
The discovery is significant as noble gases, specifically Argon-40, are essential tracers to comprehend surface-exosphere interactions and study lunar exospheric species’ dynamics. Furthermore, the process will help understand radiogenic activities just below the lunar surface. Ar-40 arises from the radioactive decay of Potassium-40 located beneath the lunar surface. The observations by CHACE-2 offer the diurnal and spatial variations of Ar-40 covering the moon’s equatorial and mid latitude regions.
Noble Gases
Noble gases make up the seven chemical elements in Group 18 of the periodic table, including helium, neon, argon, krypton, xenon, radon, and oganesson. While these gases are typically colorless, odorless, tasteless, and nonflammable, recent studies have identified reactive compounds of xenon, krypton, and radon. As their atomic numbers increase, the abundance of these gases declines, with helium being the second most abundant element after hydrogen in the universe.
The result from this Chandrayaan-2 mission, although similar measurements were confined to the near-equatorial region of the Moon during the Apollo-17 and LADEE Mission 2014, exposes the presence of Ar-40 in the lunar exosphere. These findings flag the possibility of unknown or additional loss processes.
