Asked by: Alex Round, London
The volume or thickness of a planet’s atmosphere depends on its temperature, its composition and the planet’s escape velocity. The escape velocity depends only on the mass of the planet. Although planet masses can change (due to accretion of material from space for example), these changes are so tiny compared to the total mass of the planet that the escape velocity remains essentially constant.
The temperature of planetary atmospheres can, however, change significantly due to changes in solar irradiance for example. Higher average temperatures would result in a larger atmosphere. Composition changes, which can occur due to geological processes or the transference of gases in and out of the atmosphere, also affect the size of the atmosphere. A smaller average mass of the atmosphere’s constituents would result in a larger atmosphere. So, planetary atmospheres can become larger (or smaller) over very long timescales due to environmental, geological and biological processes.