Chandrayaan-2 Orbiter Makes First-of-Its-Kind Observations on Global Distribution of Argon-40 in the Moon’s Exosphere

It has been nearly three years since the Indian Space Research Organisation’s (ISRO) lunar exploration mission Chandrayaan-2 took off from the Earth. While the Vikram Lander crash-landed on the Moon, the C2 Orbiter is still functioning fine, and continues to make amazing discoveries to this day.

Just earlier this year, in January 2022, the Chandrayaan-2 detected solar proton events caused by high-intensity solar flares — an especially noteworthy achievement considering most global missions failed to detect them. And now, before the excitement of this win could die down, ISRO’s lunar orbiter has made another thrilling find!

An instrument aboard the Chandrayaan-2 orbiter, named the Chandra’s Atmospheric Composition Explorer-2 (CHACE-2) quadrupole mass spectrometer, made some unprecedented observations regarding the dynamics of lunar exospheric species and on the radiogenic activities in the first few tens of metres below the lunar surface, ISRO reported on Tuesday, March 8.

CHACE-2 has detected the presence of Argon in the equatorial and mid-latitude regions of the Moon and provided insights into the global distribution of Argon-40 in the Moon’s tenuous exosphere.

The ‘exosphere’ is the outermost region of the upper atmosphere of a celestial body where the constituent atoms and molecules rarely collide with each other and can escape into space. The Earth’s Moon features a surface-boundary exosphere, and the lunar exosphere exists as a result of a dynamic equilibrium between several source and sink processes.

Argon is an inert, colourless and odourless noble gas. Noble gases serve as important tracers to understand the processes of surface-exosphere interaction, and Argon-40 (Ar-40) is one such important tracer atom to study the dynamics of the lunar exospheric species.

The vast majority of Argon occurs in its isotopic form of Argon-40, resulting from the radioactive disintegration of Potassium-40 (K-40) present below the lunar surface. And according to ISRO, once formed, it diffuses through the inter-granular space and makes its way up to the lunar exosphere through seepages and faults.

The CHACE-2 observations provide the diurnal and spatial variation of Ar-40 covering the equatorial and mid-latitude regions of the Moon.

Now, the detection of Argon-40 itself isn’t as noteworthy as the fact that the previous observations were limited to near-equatorial regions, as shown during the Apollo-17 and LADEE missions.

Despite the variations in temperature and topography, CHACE-2 data for the mid-latitude regions revealed for the first time that the change in Ar-40 number density with respect to solar longitudes is similar to that of the low-latitude regions.

Furthermore, there exists a steep latitudinal temperature gradient of the lunar surface. This is considered a gap area in the global dynamics of the lunar exospheric species.

In this context, Chandrayaan-2’s findings of Ar-40 up to the mid-latitude regions (−60º to +60º) play a significant role in bridging the gap in the knowledge of the element’s presence on the Moon. The observations reveal that distribution in Ar-40 has substantial spatial heterogeneity.

“The observed global distribution indicates that the interaction of Ar-40 with the surface is similar in low and mid-latitude regions. The CHACE-2 observations hint at a requirement for improvement in our understanding of the surface-exosphere interactions and source distributions of Ar-40,” according to the study.