atmosphere

An atmosphere is basically any gas surrounding a celestial body such as a star or a planet. It is sometimes referred to as an envelope around the body. (For planets, the term envelope has also been used for the solid material above the core, a common usage in discussions of planet formation and early life, in which case the term is sometimes qualified to distinguish the intent, e.g., gaseous envelope.) A stellar atmosphere is essentially the external gas that we can see through, i.e., transmits visible light, but that of a planet or moon may include opaque portions, such as that of Venus or Jupiter, as well as the Earth's clouds. (The rest of this page is essentially about planet and moon atmospheres except that the comments regarding spectroscopy might apply to stars.)

A planet or moon atmosphere may be a significant portion of its mass, e.g., a gas giant, or nearly nothing, e.g., the Moon.

The body's atmosphere is of interest in itself as well as the hints it provides regarding the character of the body, and also as a factor in habitability and biosignatures. Emitted EMR and transmission spectroscopy are hints to its makeup, and any evidence of chemical mixtures that should not last, i.e., out of chemical equilibrium, indicates something replenishing the mixture, e.g., a reaction with the surface, or a emission from inside the planet, or life. Atmospheric models and climate models (e.g., one dimensional climate models or tailored general circulation models) are created to study and explain planetary atmospheres.

The Earth's atmosphere serves as a well-studied example of a planetary atmosphere. Its effects on astronomical observation (atmospheric windows and seeing) have motivated analysis also useful for understanding other atmospheres.

Atmospheric retention and its lack (atmospheric escape) are of interest, both to explain observations of a body's atmosphere (or its lack) and to establish that an atmosphere is being replenished, if it ought to be gone. Its temperature, the body's gravity, and the mass of the molecules determine if some molecules reach escape velocity and are lost: for example, hydrogen molecules are sufficiently light that the Earth's temperature assured they haven't remained. Photodissociation can break molecules into smaller units, affecting retention, as can gravitational separation, i.e., chemical differentiation, stratification of the atmosphere by molecular weight.

The phrase bulk atmosphere (and similar phrases, such as bulk planetary atmosphere) refers to the entire atmosphere, e.g., when speaking of its composition, it can indicate you're speaking of components of the entire atmosphere rather than what is at some particular portion.