A chemical model for the lower atmosphere of Venus

Abstract

The chemistry of the postulated interaction of the atmosphere and lithosphere of Venus is discussed in detail and a zonal model capable of explaining some of the observed atmospheric characteristics is proposed. It is demonstrated that the upper limits in the quantities P CO/P CO 2 , P Hin2O,andP H 2 and the absolute values of T, P CO 2 ,andP(total) can be used to give a self-consistent picture of the atmosphere if hydrogen is assumed largely lost. The inferred surface values of T ∼ 700°K, P CO/P CO 2 ≤ 10 −3,andP H 2O ≤ 10 −3atm lead to the result that P H 2 ≤ 10 −5.05atm and indicate that metallic iron, free carbon, and hydrocarbons are not stable on the surface or in the lower atmosphere. The same is probably true of most hydrogen compounds. The upper limit in P CO/P CO 2 corresponds to the lower limit P O 2 ≥ 10 −27.1atm at 700°K, which is in the center of the magnetic field. The magnesium carbonate magnesite and the calcium-magnesium carbonate dolomite are found to be unstable in the presence of free silica at P CO 2 (surface) ∼ 10atm . Calcite may be stable in the presence of free silica, and there is a chance that the atmosphere may be approximately buffered by the wallastonite reaction in the range P CO 2 ∼ 1–50atm at the surface. These results agree with the previous conclusions of the writer (Mueller, 1963) and generally with the independent conclusions of Adamcik and Draper (1963) regarding magnesite and calcite. There is also a chance that some carbonates may be stored in the lithosphere in silicate rocks which are unsaturated with respect to SiO 2. However, simple carbonate dissociation equilibria are probably inapplicable to Venus because of the improbability of isolation of the carbonates from the silicates. Water in the lithosphere is probably confined to structurally bound H 2O in complex silicates such as amphiboles and micas. However, the escape of hydrogen should prevent the atmosphere from being buffered with respect to water. A detailed discussion of the iron compounds is presented, and it is concluded that FeCO 3 and Fe 2SiO 4 as well as other pure ferrous silicates should be unstable relative to the iron oxides and magnesium-rich silicates. These relations are used to set an upper limit on P O 2 corresponding to the magnetite-hematite boundary curve. The atmosphere has been divided into three zones: (I) a basal zone of chemical interaction with the lithosphere in which the observed molecular abundances are established; (II) a zone of frozen thermochemical equilibria that is observed in the spectra; and (III) an outer zone that imposes modifications on the spectra by the effect of photochemical reactions. A critique of previous views and speculations on the Venus surface is also presented and little agreement with these is found. In addition, it is proposed that chemical mass transport may be an important factor on Venus.