Introduction

Most lightning discharges on Earth are produced by convective, precipitating clouds that contain water in both liquid and solid phases (Section 3.2). Apparently, convective clouds whose tops are below the freezing level, so that they do not contain ice, can also sometimes produce lightning (e.g., Foster 1950; Moore et al. 1960; Pietrowski 1960; Michnowski 1963; Rossby 1966). On Earth, lightning-like electrical sparks hundreds of meters long can be produced via charge generation and separation in volcano emissions, and sparks of meter length can occur in sandstorms (subsection 20.7.1). Further, electrical sparks having a length of the order of one meter or less occur in a variety of non-meteorological turbulent particulate media, such as in the material in grain elevators and in the mixture of water and oil present during the water-jet cleaning of oil tanker holds (Pierce 1974). Finally, transient luminous phenomena that may be associated with electrical discharges have been observed during earthquakes (subsection 20.7.2).

Based on the observed conditions for the production of lightning on Earth, the two requirements for the generation of lightning or lightning-like electrical sparks in the extraterrestrial environment are: (i) the interaction of particles of different types or of the same type but with different properties (for example, different temperatures), resulting in local charging,