Venus Lithosphere Research Articles

Venus: Atmospheric and lithospheric composition

Viewed in terms of possible chemical equilibrium between the atmosphere and lithosphere of Venus, it seems that our knowledge of the composition of the upper atmosphere (above the clouds) may contain useful information regarding the presence or absence of particular mineral assemblages on the surface. It also is possible in principle to determine the surface temperature and pressure at which such equilibrium occurs, and thus to place limits on the surface conditions independent of radar-astronomical and spacecraft observations. The geochemical and observational aspects of three possible models for the surface are discussed.

Geochemistry of Venus and the Interpretation of the Radar Data

A geochemical study of the interactions of the atmosphere and lithosphere of Venus predicts that a substantial mass of electrically conducting clouds may be present on the planet. Furthermore, the existence of a substantial abundance of gases capable of strong microwave absorption also seems probable. The possible effects of these phenomena on the microwave properties of the atmosphere are discussed in a qualitative manner, and it is concluded that, although models with surface pressures ranging from perhaps 30 to 140 bars may be reconciled with the radar data, models near the lower end of this range are more appealing from the standpoint of atmospheric chemistry and geochemical thermodynamics.

Effect of Temperature on the Strength and Composition of the Upper Lithosphere of Venus

PLANETARY probes have confirmed temperatures of the order of 700 K for the surface of Venus1,2. Given the similarities of the radii and densities of Venus and Earth, it is a reasonable assumption that the two planets have similar overall compositions3. Although the upper lithosphere of Venus should therefore have evolved in a manner roughly parallel to Earth's upper lithosphere4, it is interesting to consider what modifications may have been imposed on this evolution by the high temperatures. I shall consider particularly the effect on magmatic differentiation, isostatic adjustments and surface relief.

A chemical model for the lower atmosphere of Venus

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.