Comparative planetary science assumes that the science of Earth’s environment can be used to understand other planetary environments; similarly, observations of other planets can be used to broaden understanding of the terrestrial environment. Several aspects motivate a modern comparative study of planetary atmospheric electricity. Firstly, electrification appears to have a small effect on Earth’s climate. It follows from the Boltzmann distribution that most aerosol particles in an atmosphere are charged, with the charged fraction depending only on temperature (Keefe et al. 1959). The interaction between ions and aerosol particles could affect a planet’s radiative balance through electrical influences on aerosol particle changes, both on Earth (Harrison and Carslaw 2003) and elsewhere (Moses et al. 1992). Secondly, studying other planetary atmospheres could contribute to understanding the origins of life, in which lightning has been implicated (Miller 1953). Thirdly, as manned space missions to Mars appear possible (Bonnet and Swings 2004), there needs to be an assessment of the potential electrostatic hazards facing future space missions. Fourthly, the number of planets discovered outside our Solar System with atmospheres is increasing steadily (e.g. Seager and Deming 2010), and a comparative approach can be used to deduce some of their properties, as observations remain limited. Lightning and electrical processes are expected to take place in these planetary atmospheres just as they do in the Solar System, where lightning is relatively common [occuring in 4 ± 1 of 7 planetary atmospheres (Harrison et al. 2008)].

The innermost planet, Mercury, and almost all solar system moons will not be discussed further here, as they do not have atmospheres. Specifically, atmospheric electrification comprises lightning, which is caused by convection, and non-convective electrification. Non-convective electrification requires ionisation to produce electrically charged particles, which can originate from cosmic rays, radioisotope decay, or UV radiation. Cosmic rays are everywhere, so every planetary atmosphere can be expected to contain charged particles from ionisation, causing a slight atmospheric electrical conductivity in addition to any other sources of ionisation. Through interactions with the ions and electrons produced by cosmic ray ionisation, aerosol particles add complexity to atmospheric charge exchange. The latitudinal distribution of atmospheric ionisation by cosmic rays is related to a planet’s magnetic field, which modulates the incident cosmic radiation (Gringel et al. 1986). In addition, the strength of the solar wind controls the deflection of cosmic rays away from a planet, so the effectiveness of this ionisation modulation over the solar cycle is related to the planet’s distance from the Sun.

C.T.R. Wilson suggested that terrestrial atmospheric electrification was sustained by the existence of a global atmospheric electric circuit (Wilson 1920), resulting from the electric current flow generated by disturbed weather and ionisation in the weakly conducting atmosphere between the surface and the ionosphere, which are better conductors. The minimum parameters required for a “Wilson” global atmospheric electric circuit are the existence of an atmosphere bounded by a conductive ionosphere and surface, with a charge generation mechanism. Aplin et al. (2008) explain that the charge generation in the alternating current (ac) part of a planet’s global atmospheric circuit (mostly from lightning) can be identified by the presence of extremely low frequency (ELF) resonances in the surface-ionosphere waveguide. The existence of these “Schumann resonances” also demonstrates the presence of atmospheric charge generation and a conducting upper layer. Additionally, current flow is required to confirm the existence of a direct current (dc) global circuit. Both ELF signals and aspects of the ac global circuit are directly related to the occurrence of planetary lightning, which has been reviewed extensively (e.g. Zarka et al. 2008; Yair 2012), and will not be covered in this paper beyond its association with non-convective atmospheric electrification. Necessary conditions for a dc global atmospheric electric circuit to exist on a planet can be defined as:

  1. 1.

    Polar atmospheric molecules, to sustain charge.

  2. 2.

    Charge separation, usually generated by convection from meteorological processes, is required to form the dipole structure leading to electrostatic discharge. If there are no discharges, precipitation must carry charge to ground.

  3. 3.

    Evidence for conducting upper and lower layers.

  4. 4.

    Mobile charged particles, to provide current flow.

Assuming that all planetary atmospheres have some charge separation due to ionisation, three aspects have been selected to characterise electrical systems in planetary atmospheres.

  1. 1.

    If convective or other processes cause sufficient charge separation for electrical discharges, meteorological changes will cause a global modulation of the planetary electrical system.

  2. 2.

    Aerosols, if present, are linked to local atmospheric electrification through ion-aerosol interactions.

  3. 3.

    If ions are implicated in the formation or removal of aerosol particles, there is a potential link between cosmic ray ionisation and the planet’s atmospheric radiative balance.

This review begins with a brief discussion of atmospheric electrification at Earth, to introduce the relevant physical processes. Table 1.1 gives an overview of solar system planetary atmospheres to be discussed, and their electrical systems, in terms of the three principal aspects listed above. The book is organised moving outwards from the Sun from Venus to Pluto and concluding with a chapter on exoplanets. Measurements and theoretical predictions of ionisation and other electrical processes in each planetary atmosphere are summarised, and the likelihood of a global atmospheric electric circuit on each planet discussed. Two moons with atmospheres and probable electrical activity have been included—Saturn’s moon Titan, and Neptune’s largest moon, Triton. Pluto is also considered, even though it was “demoted” to a dwarf planet by the International Astronomical Union in 2006 (see http://www.iau.org/public/pluto/), due to its similar atmosphere to Triton.

Table 1 Summary of electrification in Solar System planetary atmospheres. Atmospheric constituents, surface temperature and pressure are from Lewis (1997)
Full size table