Aeronomic systems on planets, moons, and comets

Abstract

This chapter provides a basic introduction to the science of aeronomy and a comparison of aeronomic systems for planets, satellites, and comets in our solar system. The fundamental length scales of scale height, mean-free-path, and depletion length are introduced, along with the non-dimensional parameters, Mach number, the Jeans' λ ratio of gravitational potential energy to random kinetic energy, and the ratio of mean-free-path to scale height. These quantities are used to define exobase and the validity of the fluid approximation for a gas and to classify atmospheres, exospheres and comas. Planetary and satellite atmospheres are also classified by their chemical composition and photochemistry is addressed within these categories. The thermal structure of thermospheres is reviewed and the role of molecular heat conduction is emphasized. It is noted that solar heating is an insignificant thermospheric heat source on the giant planets. The general importance of wave heating is scrutinized, specifically the question is posed: why don't all planetary atmospheres have hot coronas?