Argon-40 in Moon’s Atmosphere

In News

• Recently, Chandrayaan-2 made the first observations of distribution of Argon-40 in the Moon’s atmosphere.

Key Findings

• Lunar exospheric species: 
  • These observations provide insight on the dynamics of the lunar exospheric species, as well as on the radiogenic activities in the first few tens of metres below the lunar surface.
• Number density: 
  • CHACE-2 observations reveal:
    • an increase in the number density of Ar-40 near the sunrise terminator, 
    • a decrease through the dayside, 
    • a secondary peak near sunset terminator and 
    • a night-side minima. 
  • This is the typical behaviour of a condensable gas.
• Mid latitudes: 
  • For the mid latitude regions, CHACE-2 observations showed for the first time that the variation in the number density of Ar-40 with respect to solar longitudes are similar to that of low latitude regions, despite the differences in temperature and topography.
• Spatial heterogeneity: 
  • It also reveals that the distribution in Ar-40 has significant spatial heterogeneity. 
• Argon Bulge:
  • There are localised enhancements (termed as Argon bulge) over several regions including the KREEP [potassium (K), rare-earth elements, and phosphorus (P)] and South Pole Aitken terrain.
  • The observations of Argon bulge by CHACE-2 are indicative of unknown or additional loss processes, Moonquakes or regions with lower activation energies, which call for a better understanding of the surface-exosphere interactions and source distributions of Ar-40.

Exosphere

• Definition: 
  • ‘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.
• Earth’s moon: 
  • It features a surface-boundary-exosphere. 
  • For Moon, different constituents in the exosphere are fed from the surface by a variety of processes, such as:
    • thermal desorption, 
    • solar wind sputtering, 
    • photo-stimulated desorption, and 
    • micrometeorite impact vaporisation.
• Exospheric atoms: 
  • The exospheric atoms may be lost to space by the thermal escape (also known as the Jean’s escape). 
  • Also, the atoms get ionised by photo-ionisation and charge exchange with the solar wind ions. 
  • Subsequently, they can be swept away by the convective electric field of the solar wind. 
  • Some of these atoms/ions can also be deposited back on the lunar surface. 
  • Thus, the lunar exosphere exists as a result of a dynamic equilibrium between several source and sink processes.

CHACE 2

• The Chandra’s Atmospheric Composition Explorer-2 (CHACE-2), a quadrupole mass spectrometer onboard Chandrayaan-2 mission, has made the first-of-its-kind observations of the global distribution of Argon-40 in the tenuous lunar exosphere
• CHACE-2 was a sequel to the CHACE experiment on the Moon Impact Probe (MIP) of Chandrayaan-1 mission.
• It draws heritage from the Mars Exospheric Neutral Composition Analyser (MENCA) experiment aboard the Indian Mars Orbiter Mission.

Chandrayaan-2

• It was launched in July 2019 and was scheduled to be an effort aimed at landing a rover on the Lunar South Pole.
• It was sent abroad by a geosynchronous launch vehicle, the GSLV-Mk 3.
• Its lander Vikram carshed and prevented rover Pragyaan from successful travel on the surface of the moon.
• Had the mission been successful, it would have been the first time a country landed its rover on the moon on its maiden attempt.

Argon-40

• About:
  • Noble gases serve as important tracers to understand the processes of surface-exosphere interaction, and Argon-40 (Ar-40) is such an important tracer atom to study the dynamics of the lunar exospheric species
• Origin: 
  • Ar-40 originates from the radioactive disintegration of Potassium-40 (K-40) present below the lunar surface. 
  • Once formed, it diffuses through the inter-granular space and makes its way up to the lunar exosphere through seepages and faults.
• Observations by CHACE 2:
  • It provides the diurnal and spatial variation of Ar-40 covering the equatorial and mid latitude regions of the Moon.
• Uniqueness of this finding: 
  • Although Apollo-17 and LADEE missions have detected the presence of Ar-40 in the lunar exosphere, the measurements were confined to the near-equatorial region of the Moon

Source: TH