As we emphasized back in Chapter 1, atmospheres are not static. The mass and composition of an atmosphere evolves over time, as a result of a great variety of chemical, physical, and biological processes. Now it is time to survey those processes in greater detail, and to put numbers on them to the extent possible in the limited space available in this chapter.
Throughout the following we will need to refer to some constituents of a planet as volatiles. These are “not rocks” – things that can become gases to a significant extent. The concept of a volatile is relative to the temperature of a planet. On Earth, water is a volatile but on Titan it is basically a rock, as is CO2, though N2 and CH4 remain as volatiles even at the low temperatures of Titan. On Earth, sand (SiO2) is a rock, but on a roaster – a hot extrasolar Jupiter in a close orbit – it could be a volatile.
For planets in which some atmospheric volatiles exchange with a condensed reservoir, as in the case of Earth's ocean and glaciers, the whole atmosphere–ocean–cryosphere system is best treated as a unit for many purposes, and we will refer to this as the volatile envelope. In other cases, the portion of the volatile envelope which resides in the atmosphere plays a distinguished role.