Atmospheres, Planetary

All nine planets, four moons, and most comets have atmospheres. These gaseous envelopes are remarkably diverse, although they are made largely from the same six elements-hydrogen (H), helium (He), oxygen (O), carbon (C), nitrogen (N), and sulfur (S). These are the most abundant elements in the Sun and stars. And with the exception of helium, they are the main elements in living things. Atmospheres differ in composition, mass, temperature, and degree of interaction with the underlying planet. At one extreme, the atmospheres of the four giant planets merge imperceptibly with the planetary interiors, which are made of the same gaseous mixtures as the atmospheres. Pressures in the interiors of these fluid planets reach millions of bars-millions of times the sea level pressure on Earth. At the other extreme, the atmospheres of the Moon and Mercury consist of individual molecules hopping around the surface, never colliding with each other because the density is too low. The intermediate cases are Venus, Earth, Mars, Pluto, Io (a satellite of Jupiter), Titan (a satellite of Saturn), and Triton (a satellite of Neptune). These atmospheres rest on solid or liquid surfaces and interact chemically with the solid planet on a variety of timescales. This article is organized around two main themes-chemistry and dynamics. The former involves composition and how it has evolved. The latter involves weather and climate. Table 1 gives the general properties of atmospheres around solar system objects.