As temperature increases, so does the average speed of the atoms or molecules in a gas. Average speed declines with increasing atomic mass. A small fraction of the atoms in a gas move much faster than the average speed.
Atoms can't escape from the lower parts of the atmosphere of a planet because they collide with other particles after traveling a short distance. In the thin outer atmosphere, however, a rapidly moving atom is unlikely to collide and be reflected back by another atom before escaping. If the average atomic speed there is greater than about 1⁄6 escape velocity, all of that kind of atom will escape relatively quickly.
Density is the mass of gas per unit volume. Gas pressure is the force per unit area exerted by the gas. Gas pressure is proportional to the product of density and temperature.
Hydrostatic equilibrium occurs when the decrease of gas pressure with height balances the downward pull of gravity. Pressure falls gradually with height until the atmosphere blends into interplanetary space.
A blackbody is one that absorbs all of the electromagnetic radiation that falls on it. A heated blackbody emits electromagnetic radiation at the maximum rate possible for a body of a given temperature and size.
A blackbody emits electromagnetic radiation with a characteristic spectrum that depends only on the temperature of the blackbody. The wavelength at which the spectrum is brightest grows shorter with increasing temperature. The total rate at which a blackbody emits radiation varies with the fourth power of its temperature.
If the temperature of a body reaches the point where the body emits radiation at the same rate at which it absorbs radiant energy falling on it, the body is said to be in thermal equilibrium. The decline of solar radiation with increasing distance from the Sun leads to temperatures that decline with increasing distance. Highly reflecting bodies are cooler at a given distance than dark bodies, which absorb sunlight well.
Nearly all of the mass of an atom is contained in its nucleus, which is composed of neutrons and protons. The number of protons in a nucleus determines what element it is. Nuclei that are of the same element and differ only in numbers of neutrons are called isotopes.
In some nuclear reactions, matter is transformed into energy. This happens when the particles that react are more massive than the products of the reaction. The two types of nuclear reactions are fission, in which a massive nucleus splits to produce less-massive nuclei, and fusion, in which light nuclei combine to form a heavier one.
Radioactive decay occurs when an unstable isotope decays to form another nucleus. The rate of radioactive decay is described by its half-life, the time needed for half of the nuclei to decay. Radioactive decay is useful for determining the ages of material containing unstable isotopes because the decay occurs at a predictable rate.
For most solar system bodies, radioactive decays are the major source of internal heat. As the amount of radioactive material in the solar system has declined, so has the rate of radioactive heating. Solar system bodies also were heated by the release of kinetic energy as they accreted. However, accretional heating was significant primarily for larger bodies.
Conduction, convection, and radiative transfer are the processes that carry energy from place to place. Within a body, the rate at which heat flows is determined by the decrease of temperature from center to surface. Small bodies cool more rapidly than large ones.
Planetology is the comparative study of the Earth and other planets. Planetology emphasizes trying to understand an individual planet by accounting for the ways in which it is similar to and different from other planets. Different planets show how the same processes work in bodies of different mass, diameter, composition, and distance from the Sun.
