javv-e donbâledâr, havâsepehr-e ~
Fr.: atmosphère de comète
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.