Organic synthesis in the atmosphere of Titan

Abstract

Titan, the largest satellite of Saturn, is one of the smallest known bodies with an atmosphere. Since the detection of an atmosphere on Titan by Kuiper, various models have been proposed for the structure of this atmosphere¹. These models were based on information from ground-based observations which, although advanced, failed to yield a sufficiently accurate and quantitative picture of the atmosphere. The IR, UV and radio science investigations on Voyager 1 have now provided much needed information on the composition, structure and dynamics of the atmosphere of Titan². The IR and radio science data indicated a mean molecular weight of 28 for the atmosphere ot Titan and it was thus concluded³ that the atmosphere consists predominantly of N₂. The minor components of the atmosphere as identified by the spectra from the IR Radiometer and Interferometer Spectrometer (IRIS) spectra include methane, acetylene, hydrogen cyanide, ethane, ethylene and possibly propane and methylactetylene³. The other IR bands observed in the spectra have not been firmly identified, although several other hydrocarbons and nitrogen-containing compounds are suspected. The minimum atmospheric temperature of ∼70 K was deduced to prevail near the 200 mbar level, which can act as a cold trap for methane and thus regulate and maintain a constant abundance of stratospheric CH₄ which has been estimated³ to be about 1% of N₂. The IRIS data also indicate that the stratospheric temperature near 1 mbar level is about 20 K colder in the north than at the equatorial and south latitudes but that it shows practically no diurnal and longitudinal thermal variation.