Planetary atmospheres formed in a series of phases (Figure 9.2). At the time of formation, the young planets were initially enveloped by the remaining hydrogen and helium that filled the protoplanetary disk surrounding the Sun, and they were able to capture some of this surrounding gas.
Gas capture continued until the gaseous disk ultimately dissipated
(soon after formation of the planets) and the supply of gas ran out.
The gaseous envelope collected by a newly formed planet is its primary atmosphere.
Although the giant planets still retain most of their original primary atmospheres,
the terrestrial planets probably lost theirs soon after the Solar System’s protoplanetary disk was blown away by the emerging Sun, because of their relatively small masses and proximity to the Sun.