Primary atmosphere
A primary atmosphere, often also called a primordial atmosphere or proto-atmosphere, is an atmosphere of a protoplanet that forms by accretion of gaseous matter from the protoplanetary disk. Gas giant planets including Jupiter, Saturn, have primary atmospheres. Primary atmospheres are very thick compared to secondary atmospheres like the one found on Earth. The primary atmosphere was lost on the terrestrial planets of the Solar System due to a combination of surface temperature, mass of the atoms and the escape velocity of the planet.
Formation and Composition
Protoplanetary disk formation
Primary atmospheres begin to form during the early stages of a solar system's development. As a star forms from a collapsing cloud of gas and dust, the remaining material flattens into a rotating disk around the star, known as the protoplanetary disk. This disk is rich in gases like hydrogen and helium, which are the most abundant elements in the universe.Accretion of gases
Planets start to form within this disk through the process of accretion. As dust and solid materials coalesce to form planetesimals and eventually protoplanets, these bodies begin to exert gravitational forces. The gravity of these growing protoplanets attracts surrounding gases from the protoplanetary disk. Larger planets, particularly those forming in the colder outer regions of the disk, are capable of attracting more substantial envelopes of gas, leading to the formation of thick primary atmospheres.Gas accretion processes can be influenced by mass and temperature of the protoplanet as well as chemical and physical conditions of the planetary disk. Accretion can be dependent on the host star's solar radiation and wind environment and the temperature and density of the surrounding gases in the planetary disk. A protoplanet will continue to accrete gases to its surface while resources are available. Its primary atmosphere can be lost however, due to the onset of atmospheric escape or outgassing of gases with low boiling points called volatiles. Atmospheric escape will occur after intense exposure to X-ray and UV radiation and stellar wind, driving gases to reach a terminal escape velocity. The secondary atmosphere can also be initiated from outgassed volatiles from the planetary core and mantle like water vapor and carbon dioxide or from volatiles contributed during collisions. The development of a proto-atmosphere is dependent on where it is formed within the protoplanetary disk or after it dissipates, resulting in the formation of a secondary atmosphere.