How did volcanic outgassing contribute to Earth’s earliest atmosphere?

Volcanic outgassing played a fundamental role in the formation of Earth’s early atmosphere by releasing volatiles that had been trapped within the planet's interior during its accretion and differentiation.12

As Earth formed, solid particles incorporated volatile components from the primordial gas cloud. During the planet's early history, intense heat caused the differentiation of Earth's layers, melting iron to form the core and creating the precursors to the mantle and crust. This process triggered the release of trapped volatiles through colossal-scale volcanic outgassing.132

The composition of this outgassed atmosphere was shaped by the chemical environment of the early crust. Initially, if outgassing products equilibrated with metallic iron, the atmosphere likely contained water vapour, carbon monoxide, and hydrogen sulfide. As the crust became more oxidized, these gases shifted toward water vapour, carbon dioxide, and molecular nitrogen. Notably, molecular oxygen was absent from these volcanic emissions. Over time, water vapour condensed to form the oceans, and carbon dioxide was consumed by weathering, leaving molecular nitrogen as the atmosphere's most abundant component.42