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Chandrayaan-2 observes Argon-40 in lunar exosphere


BENGALURU: A payload onboard India’s second moon-orbiting mission Chandrayaan-2 has made the first-of-its-kind discovery on the distribution of one of the noble gases, Argon-40, to study the lunar exosphere as well as the surface from where this gas is understood to have escaped.

The finding is significant considering manned lunar missions are being planned by countries like the USA to set up human bases on the Moon — a mission currently scheduled for 2025 — and there is an increased interest in studying the lunar surface and its atmosphere, the exosphere being the topmost of its layers.

The Chandra’s Atmospheric Composition Explorer-2 (CHACE-2), a quadrupole mass spectrometer on Chandrayaan-2 mission made the observations to provide an insight into the dynamics of the lunar exosphere, and on activities of gases emerging through radioactive decay in the first few tens of metres below the lunar surface, India’s premier space agency, the Indian Space Research Organisation (ISRO) said on Tuesday.

Noble gases serve as important tracers to understand the processes of surface-exosphere interaction, and Argon-40 (Ar-40) is an important tracer atom to study the dynamics of the lunar exosphere. Ar-40 originates from the radioactive decay of Potassium-40 (K-40) present below the lunar surface. Once formed, it diffuses through the inter-granular space in the lunar surface dust and makes its way up to the lunar exosphere through seepage and faults.

The CHACE-2 observations provide details on the spatial variations in Ar-40 through the lunar day covering the equatorial and mid-latitude regions of the Moon. The importance of Chandrayaan-2’s CHACE-2 observations is that although NASA’s Apollo-17 (the last of the manned lunar mission carried out in 1972) and Lunar Atmosphere and Dust Environment Explorer (LADEE) — a robotic mission that orbited the Moon and impacted its surface on April 17, 2014, to gather information on lunar atmosphere and determine whether dust is lofted into the lunar sky — detected the presence of Ar-40 in the lunar exosphere, the measurements were confined to the near-equatorial region of the Moon.

Knowledge gap to be bridged by Argon study: ISRO

AS there is a steep latitudinal temperature gradient of the lunar surface, it was a gap area to study the pan-lunar dynamics of Moon’s exosphere, which is a temperature-driven process. In this context, CHACE-2’s observations on Ar-40 up to the mid-latitude regions play a significant role to bridge the gap in the knowledge, ISRO said.

This was possible because Chandrayaan-2 has a polar orbit while the other two had orbits in a line closer to the Moon’s equator. CHACE-2 is a sequel to the CHACE experiment on the Moon Impact Probe (MIP) of Chandrayaan-1 mission and draws heritage from the Mars Exospheric Neutral Composition Analyser (MENCA) experiment aboard the Indian Mars Orbiter Mission. The results of the findings are published in the journal Geophysical Research Letters.