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.
creation@bright.net
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