source

…the atmosphere is termed a source for the gas. Depending on the question under consideration, it can make sense to speak in terms of either an ultimate source—the process that delivered a component of the volatile inventory to Earth—or an immediate source—the process that sustains the abundance of a component…

The material from which the solar system formed is often described as a gas cloud or, at a later stage, a solar nebula. The cloud was rich in volatiles (termed primordial gases) and must have been the ultimate source of the atoms in the…