An Etymological Dictionary of Astronomy and Astrophysics

The envelope of → gas and → dust around a → comet nucleus, also known as → coma. As the comet approaches the → Sun, the frozen materials → sublimate and give rise to an expanding atmosphere. The atmosphere is composed of dust, → molecules,
→ radicals, and molecular → ions
released from the inner coma with velocities ~ 0.5 to 1 km s^-1, well above the → escape velocity for the nucleus. The
→ chemical species observed in cometary spectra can be divided into several categories: (i) atoms and molecules related to → water (H, O, OH, OH⁺, H2O, H2O⁺),

(ii) carbon and related molecules (C, C⁺, CO, CO⁺, CO₂⁺, C₂, CH, CH⁺, HCO, H₂CO),

(iii) → nitrogen and related molecules (CN, CN⁺, HCN, CH₃CN, NH, NH₂, N₂⁺, NH₃, NH₄),

(iv) → sulphur and related molecules (S, CS, S₂, H₂S⁺),

(v) → metals (Na, K, Ca, Co, Cr, Cu, V, Fe, Mn, Ni).

For a typical average comet the neutral atmosphere is first seen when the heliocentric distance is d ≤ 3 → astronomical units.

See also: → cometary; → atmosphere.