The correct option is Argon.
Explanation
The evolution of the Earth's atmosphere is primarily attributed to the process of degassing. This involves the release of gases and water vapour from the Earth's interior during the cooling and differentiation of the planet. This endogenic process was the primary source of the primordial atmosphere.Analysis of Primordial Gases
According to standard geological principles, the early atmosphere formed by degassing largely consisted of the following constituents:
- Water Vapour ($H_2O$): The most abundant volatile released.
- Nitrogen ($N_2$): A primary component released from the interior.
- Carbon Dioxide ($CO_2$): Released in significant quantities.
- Methane ($CH_4$) and Ammonia ($NH_3$): Present in the early atmosphere, contributing to its reducing nature before the advent of free oxygen.
Argon ($Ar$), specifically the isotope Argon-40, is a radiogenic gas produced by the radioactive decay of Potassium-40 in the Earth's crust and mantle. While it is the third most abundant gas in the current atmosphere, it accumulated gradually over billions of years. It was not a primary constituent of the initial primordial degassing event in the same capacity as the volatiles listed above.
Key Takeaway
The primordial atmosphere formed through degassing was dominated by water vapour, nitrogen, carbon dioxide, methane, and ammonia. It was distinct from the modern atmosphere as it lacked significant free oxygen and the current concentrations of radiogenic Argon.