A body such as a planet, a moon, an asteroid, or a comet, typically enveloped in a tenuous neutral gas, perturbs its surroundings in a flowing, magnetized plasma. The structure of field and plasma resulting from the interaction depends on properties of the plasma and of the body onto which the plasma flows. This chapter addresses the interaction of solar-system plasmas with a number of small bodies of the solar system. Size is taken as criterion, but we do not include small planets even though the biggest moons are similar in size to the smallest planet, Mercury. For reviews of planetary magnetospheres, see Chs. 10 and 13 in Vol. I.

Given that the Moon has no atmosphere, one might suppose that our discussion focuses on bodies that lack atmospheres, but that is not the case. Some moons, including small ones, are enveloped in neutral clouds that are not gravitationally bound but are, in many ways, similar to atmospheres. Other important properties differ from one body to another. Some moons have icy outer layers while others have outer layers of silicates and other non-icy materials; some have conducting regions in their interiors whereas others do not; and some are magnetized but others are not. Like the planets, the bodies that we discuss are spread throughout the solar system. Correspondingly, the plasma conditions vary from one to another. The focus on moons and comets is arbitrary, but the examples that we shall discuss are diverse and illustrate a large range of physical processes that can occur in the solar system. The interaction regions surrounding the small bodies of interest vary in global geometric configuration, in spatial extent relative to the size of the central body, and in the nature of the plasma disturbances. The objective of the discussion is to understand the physical processes that account for the observed features of the interaction regions.

Physics of large-scale processes in space plasmas

It is useful to start by discussing the physical principles that govern the behavior of the flowing, magnetized plasmas in which moons and comets are embedded. The plasma of the space environment is a partially ionized gas, usually dominated by the ionized component. The physical principles that govern the behavior of the flowing medium differ from those that describe a neutral gas.