The atmosphere of Titan.

The discovery that Titan had an atmosphere was made by the identification of methane in the satellite's spectrum in 1944. But the abundance of this gas and the identification of other major constituents required the 1980 encounter by the Voyager 1 spacecraft. in the intervening years, traces of C2H2, C2H4, C2H6 and CH3D had been posited to interpret emission bands in Titan's IR spectrum. The Voyager infrared Spectrometer confirmed that these gases were present and added seven more. The atmosphere is now known to be composed primarily of molecular nitrogen. But the derived mean molecular weight suggests the presence of a significant amount of some heavier gas, most probably argon. It is shown that this argon must be primordial, and that one can understand the evolution of Titan's atmosphere in terms of degassing of a mixed hydrate dominated by CH4, N2 and 36Ar. This model satisfactorily explains the absence of neon and makes no special requirements on the satellite's surface temperature. The organic chemistry taking place on Titan today invites comparison with chemical evolution on the primitive Earth prior to the origin of life.