Chemical processes are of major importance in determining the equilibrium distribution of ions in planetary ionospheres, even though photoionization and, in some cases, impact ionization are responsible for the initial creation of the electron–ion pairs. This is particularly apparent for the ionospheres of Venus and Mars because they determine the dominant ion species (Sections 13.2 and 13.3). The major neutral constituent in the thermosphere of both Venus and Mars is CO2, and yet the major ion is O+2, as a result of ion–neutral chemistry. Therefore, a thorough knowledge of the controlling chemical processes is necessary for a proper understanding of ionospheric structure and behavior. The dividing line between chemical and physical processes is somewhat artificial and often determined by semantics. In this chapter the discussion centers on reactions involving ions, electrons, and neutral constituents; photoionization and impact ionization are discussed in Chapter 9.

Chemical kinetics

The area of science concerned with the study of chemical reactions is known as chemical kinetics. This branch of science examines the reaction processes from various points of view. A chemical reaction in which the phase of the reactant does not change is called a homogeneous reaction, whereas a chemical process in which different phases are involved is referred to as a heterogeneous reaction. In the context of atmospheric chemistry, heterogeneous reactions involve surfaces and are significant in some of the lower atmospheric chemical processes (e.g., the Antarctic ozone hole), but do not play an important role in ionospheric chemistry.