How did outgassing help form Earth’s earliest atmosphere?

Earth’s earliest atmosphere was formed through the process of planetary differentiation, during which the planet’s interior became hot enough to melt and reorganize into distinct layers, including the core and mantle.12

As the planet accreted, it incorporated volatile components that had been trapped within the solid particles of the primordial gas cloud. During differentiation, these trapped volatiles were released from the interior through outgassing, a process occurring on a colossal scale that expelled gases to the surface.1

Unlike a captured atmosphere, which would have consisted of primordial gases, this outgassed atmosphere was composed of substances that were chemically "sticky" enough to be retained in the rocky material of the forming planet. The primary constituents included water vapour, carbon dioxide, and molecular nitrogen, while free oxygen was notably absent. As the planet cooled, the water vapour condensed to form the oceans, and the remaining acid gases reacted with crustal minerals, leaving nitrogen as the most abundant atmospheric gas.134