Upper Atmospheres

Abstract

The rarefied, upper part of planetary atmospheres (for the Earth, above ≈ 100 km) is characterized by mean-free paths bigger than the scale height (and consequently lacks thermodynamical equilibrium), by a strong influence of solar radiation and a close contact with outer space. Complex photochemical reactions take place; as a consequence, the composition changes with height and a weakly ionized gas is present with a complex dynamics. This gas forms the ionosphere, in contact with the magnetosphere outside. In the ionosphere the dispersive refractive index deeply affects the propagation of electromagnetic waves; at lower altitudes, refraction in the neutral atmosphere has important consequences on astronomical observations. In this chapter we also briefly discuss the evolution of planetary atmospheres. In the giant planets the atmospheres are a continuation of their interiors and coeval with them; they probably have not undergone great changes since their formation. In terrestrial planets, atmospheres have been affected by four factors, which make their geological evolution complex and uncertain: the early, strong depletion of light gases (hydrogen and helium, in particular) in the solar nebula; the degassing of planetary mantles as a consequence of the heat produced in the accretion phase; volatile delivery by different carriers from different sources in the solar system; and the continuous escape of gas, especially of lighter molecules, to outer space.