By Mark Stewart
Abelson's Hypothesis tells us that the genesis of the primitive atmosphere was the result of out-gassing. I concur with the hypothesis that there is no geo-chemical evidence for a primitive methane-ammonia atmosphere and that there is much evidence against it including:
(A) It has been estimated an amount of ammonia equivalent to present atmospheric nitrogen would be destroyed in about 30,000 years due to degradation by ultraviolet radiation. That is a negligible amount of time compared to the vast geologic ages needed for the accumulation of reduced organic compounds.
(B) If large amounts of methane had been present in the primitive atmosphere, irradiation would have produced large quantities of hydrophobic organic compounds that would have been absorbed by sedimentary clays. The earliest rocks would then have contaminated a large proportion of organic compounds, but such simply is not the case.
(C) If the earth were unable to retain xenon of atomic weight 130, other light constituents such as hydrogen, methane, and carbon monoxide would have been lost as well.
It is thus concluded that the concept that the earth had a dense methane-ammonia atmosphere is not supported by geochemistry and it is contradicted by the scarcity of xenon and krypton in our present atmosphere.
Studies of the composition of volcanic gases have shown that water and CO2 are the major volatiles produced by out-gassing. A significant amount of reducing potential is also brought to the surface in the form of hydrogen. In this hypothesis the amount of reducing potential brought to the surface is estimated by making a balance sheet of the oxidized and reduced chemicals in the atmosphere, biosphere, and sedimentary rocks.
Most of the carbon in sedimentary rocks is present as carbonate. Part is present as reduced carbon. The amount of oxygen that would be required to convert this material to CO2 and H20 is estimated to be 235 x 10 to the 20th power g. It is suggested in this hypothesis that a probable explanation for this discrepancy is that an appreciable amount of the carbon released from volcanoes was in the form of CO. Assuming that the discrepancy in the balance sheet for oxygen was due to H2 and CO produced by out-gassing, in this hypothesis, it was then calculated that the amount of these gases which reached the surface was 19 x 10 to the 20th power g of CO.
It is further assumed in this hypothesis that one of the precursors that gave rise to biologically important compounds was hydrogen cyanide (HCN). A combination of gases that could give to hydrogen cyanide consists of CO, H2 and N2. The CO produced by out-gassing, however, would be removed by reaction with the water of the primitive oceans to produce formic acid, resulting in a very low pressure of CO. It is, therefore assumed that the H2 produced by out-gassing would react with CO2 to produce enough CO to raise its pressure to a sufficient level.
Further assumptions must be made, however, since CO, N2 and H2 will not react to produce hydrogen cyanide in the presence of water. In this hypothesis it is believed that on the primitive earth a very low temperature may have existed at an altitude no greater than 20 kilometers and that this low temperature which gave rise to hydrogen cyanide existed above this temperature minimum and that it constituted primarily of N2, CO, and H2.
While this hypothesis, on the basis of geo-chemical and cosmological data has effectively demolished the methane-ammonia atmosphere (also known as Urey's Hypothesis), this hypothesis has failed to produce equally convincing evidence for a postulated primitive atmosphere of CO, N2, and H2O. The critical feature of this hypothesis is the assumption that out-gassing produced such large quantities of H2 and CO as 19 x 10 to the 20th power g and 17 x 10 to the 20th power g, respectively. This must be recognized as speculation not supported by the facts. A certain reducing potential was required for the abiogenic origin of reduced chemical compounds, so a hypothetical source was proposed.
The main constituents of volcanic gases are H2O, CO2, and N2. The mean composition of volcanic gases from Halemaumau in Hawaii is, by volume: H2O = 68%, CO2 = 13%, N2 = 8% and the rest is mainly sulfurous fumes. In Urey's hypothesis it has been pointed out that the gases escaping from lava lakes of Hawaiian volcanoes are so highly oxidized that it is difficult to account for the high state of oxidation. The proposal that out-gassing produced vast quantities of H2 AND CO is thus not supported by known data but seems to be contradictory.
It should be emphasized that Abelson's Hypothesis, like all others, must appeal to hypotheses where data are lacking, and that, in the final analysis, no model of the primitive earth can be considered as anything more than a working hypothesis.
Mark Stewart A.D.
- The Primitive atmosphere by D. Denner Creation 3(3):23–27 August 1980