All celestial bodies are surrounded by gaseous envelopes, at least to some degree. When the gas is gravitationally bound to a parent body's nucleus it is called an atmosphere, whereas if the gas is not confined by gravity, such as at a comet, it is called a coma (Strobel, 2002). At one atmospheric extreme, such as Mercury or the Moon, the extremely tenuous atmosphere originating from the surface is referred to as a surface-bound exosphere, as the atmospheric atoms and molecules are much more likely to escape to space or to collide with the surface rather than collide with each other. At the other extreme, such as at the gas giants (Jupiter, Saturn, Uranus, Neptune), the rocky core about which the atmosphere is gravitationally bound is on the order of 0.1 planetary radii and gas constitutes the majority of the planet. A dense atmosphere is typically divided into two broad categories: the lower and upper atmospheres. The study of the lower regions (troposphere and stratosphere) forms the discipline of meteorology, while the study of the upper regions (mesosphere, thermosphere, exosphere) and their ionized component (ionosphere) forms the discipline of aeronomy.

Atmospheres play vital roles in planetary and satellite evolution, as they help to insulate the surface of a body from external influences. In particular, the upper atmosphere represents a key transition region between a dense atmosphere below and a tenuous space environment above. An array of complex coupling processes from below, such as waves, and from above, such as forcing by solar extreme ultraviolet (EUV) photons and energetic particles, means that aeronomy deals with the highly coupled system of neutrals, plasmas, and electromagnetic processes that link planets, moons, and comets from their surfaces to their magnetospheres, to the solar wind, and ultimately to the Sun itself (Mendillo et al., 2002).

Evidence of these coupling processes include various upper-atmospheric emissions, such as dayglow and nightglow, resulting from the absorption of solar photons, and aurorae, which are produced by the energy deposition of energetic particles from the space environment. Such emissions can be detected remotely, and have consequently allowed detailed study of the planets in the solar system. In addition to a host of ground-based observations, a number of spacecraft have also been used to study the giant planets.