Thermodynamic Equilibria in Planetary Atmospheres

The thermodynamic equilibrium composition of all possible combinations of the elements carbon, hydrogen, oxygen, and nitrogen at average pressures and temperatures have been calculated for the atmospheres of Earth, Venus, Mars, and Jupiter. All regions of the composition diagram consistent with the observed molecular abundances are located and discussed. The constituents of the terrestrial atmosphere considered as a closed system are in approximate thermodynamic equilibrium in spite of the continual addition of trace compounds by biological activity, lightning, radiation, and vulcanism. Under some circumstances, the presence of atmospheric organic molecules far in excess of their thermodynamic equilibrium proportions can be an indication of indigenous biological activity. The atmosphere of Venus seems likely to be in thermodynamic equilibrium. It then follows that the measured upper limits on the abundances of C114, NHl, and CO are all inconsistent with the possibility of elemental carbon or hydrocarbons in contact with the atmosphere, either on the surface or as clouds. The evidence for Mars is not inconsistent with thermodynamic equilibrium. The computations do not indicate whether small quantities of free Ol exist. The equilibrium abundances of all oxides of nitrogen on Mars are extremely low even if computed with a considerable excess of 01. For Mars and Venus there are no molecular species with a large predicted equilibrium abundance and spectroscopically accessible absorption features which have not already been identified. Under equilibrium conditions at low temperatures Jupiter also cannot contain significant amounts of any molecule not yet observed. However, at high temperatures such as would be produced by lightning discharges or in the Jovian thermosphere, simple hydrocarbons and cyanides, polycyclic aromatics, and a variety of nitrogen compounds would be expected. Some of these compounds are brightly colored, and it seems possible that such organic compounds contribute to the observed variable coloration of Jupiter.