Distribution of Argon-40 on Moon

• 09 Mar 2022

For Prelims: Chandra's Atmospheric Composition Explorer-2 (CHACE 2), Chandrayaan 1 and 2, Noble gases Argon-40, Periodic table,

For Mains: Achievements of Indians in Science & Technology, Space Technology, Chandrayaan-2.

Why in News?

Recently, Chandra's Atmospheric Composition Explorer-2 (CHACE-2), a payload onboard Chandrayaan-2, has made the first-of-its-kind discovery on the distribution of one of the noble gases, Argon-40.

• India launched Chandrayaan-2, its second lunar exploration mission after Chandrayaan-1, from Satish Dhawan Space Centre, Sriharikota in July 2019.

What is the Chandrayaan-2 Mission?

• Chandrayaan-2 is an integrated 3-in-1 spacecraft consisting of an Orbiter of the Moon, Vikram (after Vikram Sarabhai) the lander and Pragyan (wisdom) the rover, all equipped with scientific instruments to study the moon.
• Failure of Lander: Vikram lander's descent was as planned and normal performance was observed up to an altitude of 2.1 km from Lunar surface in September, 2019.
  • A successful soft-landing would have made India the fourth country after the erstwhile Soviet Union, the United States, and China to do so.
• Orbiter: It is equipped with different kinds of cameras to create high-resolution three-dimensional maps of the surface.
  • It will study the mineral composition on the moon and the lunar atmosphere, and also to assess the abundance of water.
• Objective: The mission sequence involves meticulous planning of a series of Earthbound manoeuvres, trans-lunar injections, series of lunar burns, lander separation, lander descent, and touchdown and subsequent rover rolling out of the lander on the Moon surface.

What are the major Findings?

• The gas found in the lunar exosphere is believed to have escaped from the lunar surface.
• The CHACE-2 observations reveal that the distribution in Ar-40 has significant spatial heterogeneity.
• There are localised enhancements (termed as Argon bulge) over several regions including the KREEP [potassium (K), Rare-Earth Elements, and Phosphorus (P)] on South Pole Aitken terrain (impact crater on the far side of the Moon).

What is the Lunar Exosphere?

• '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 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: The exospheric atoms may be lost to space by the thermal escape (also known as the Jean's escape).
  • Photo-Stimulated Desorption: The atoms get ionised by photo-ionisation and charge exchange with the solar wind ions.
  • Solar wind Sputtering: The atoms can be swept away by the convective electric field of the solar wind.
  • Micrometeorite Impact Vaporisation: The impact of micrometeoroid is usually sufficiently energetic to cause vaporisation of the impacting particle as well as produce an impact crater of volume an order of magnitude greater than the impacting particle.
    • A micrometeoroid is orbital debris which is smaller than a grain of sand.
• Thus, the lunar exosphere exists as a result of a dynamic equilibrium between several source and sink processes.

What is the Significance of the Discovery?

• 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.
• It will also help decipher radiogenic activities in the first few tens of metres below the lunar surface.
  • 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.
• The CHACE-2 observations provide the diurnal and spatial variation of Ar-40 covering the equatorial and mid latitude regions of the Moon.
  • The uniqueness of this result from Chandrayaan-2 mission lies in the fact that although Apollo-17(1972) and Lunar Atmosphere and Dust Environment Explorer (LADEE Mission 2014) have detected the presence of Ar-40 in the lunar exosphere, the measurements were confined to the near-equatorial region of the Moon.
• The observations of Argon bulge by CHACE-2 are indicative of unknown or additional loss processes.

What are Noble Gases?

• Noble gases are a group of the seven chemical elements that make up Group 18 (VIIIa) of the periodic table.
• The elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), and oganesson (Og).
• The noble gases are colourless, odourless, tasteless, nonflammable gases.
  • However, recent studies have shown reactive compounds of xenon, krypton, and radon.
• The abundances of the noble gases decrease as their atomic numbers increase.
• Helium is the most plentiful element in the universe after hydrogen.\

PYQ

Q. Which of the following pairs is/are correctly matched? (2014)

Spacecraft                  Purpose

1. Cassini-Huygens :    Orbiting the Venus and transmitting data to the Earth
2. Messenger :     Mapping and investigating the Mercury
3. Voyager 1 and 2 :      Exploring the outer solar system

Select the correct answer using the code given below:

(a) 1 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Ans: (b)

Source: TH