Atmosphere, Primitive Envelope
An atmosphere (or an atmospheric envelope) typically refers to the gas that is gravitationally bound to a planet or another planetary object. In relation to planet formation, atmospheres composed of hydrogen and helium are thought to form as a result of gas accretion by a growing planet embedded in a protoplanetary disk. For low-mass planets or even Moon- to Mars-sized planetary embryos, a relatively small amount of gas may be gravitationally captured into a primitive atmosphere. The atmospheres of full-size terrestrial planets are then made up of a combination of atmospheres from their constituent planetary embryos, as well as volatiles that may be degassed from the planetary interior or delivered to the planet via impacts of small bodies during final stages of accretion (late veneer). In that case a large range of atmospheric compositions is possible, and the atmospheres can consist of water and other volatile materials.