I have read the papers in question; none of them mention the period prior to
4 billions years but all talk about the period after. Since you insist that
they do talk about the period prior to 4 GYA when in fact they do not I had
to conclude that you were reading into them what you wanted to see. The
only alternative is that I misunderstood you, but your comments above seem
to suggest otherwise. Did I misunderstand you?
>
>KO:====
>>Besides, they were speculating in the absence of
>>information, not extrapolating from information as Mason was. See below
for
>>a more complete explanation.
>>
>
>No, wrong again Kevin. I have not checked item by item but it seems
>to me that everyone agrees on what the facts are. The disagreement
>comes during the extrapolation process. The critical extrapolations
>are based on models, one of the most important of which are
>models describing how the earth formed. There seems to be universal
>agreement on an accretionary model, however there is more than
>one such model. For an explanation of the importance of the model
>wrt to the earth's early atmosphere I will turn to Kasting (1993).
>
>======begin=========
>Theories for how the atmosphere and ocean formed must begin
>with an idea of how the Earth itself originated. We are now
>reasonably certain that terrestrial planets formed by accretion
>of solid materials that condensed from the solar nebula(2).
>Any primary, captured atmosphere (if one existed at all) must
>have been lost, as evidenced by the pronounced depletion of
>rare gases in Earth's atmosphere compared to cosmic abundances (3).
>The present, secondary atmosphere was generated from volatile
>compounds contained within the solid planetesimals from which
>the earth formed. Thirty years ago, it was believed that the
>earth formed relatively slowly, with a cold interior, and that
>most of its volatiles were originally trapped inside the planet
>(4,5). As time passed, the earth's interior was heated by
>radioactive decay, and the trapped gases were gradually released
>by volcanic outgassing. These volcanic gases would have been
>highly reduced [containing H2, methane (CH4), and ammonia
>(NH3)] until the Earth's core formed, after which time they
>would have been similar to modern volcanic gases (containing
>H2O, CO2, and N2, with traces of H@ and CO) (5).
>
>More recent models of planetary accretion (6-8) suggest that
>the Earth formed in 10 to 100 million years and that its interior
>was initially hot as a consequence of large impact events,
>including one that may have formed the moon [see papers in
>(9)]. The Earth's core probably formed simultaneously with
>accretion (10,11); thus, metallic iron could have been removed
>from the upper mantle, and volcanic gases could have been
>relatively oxidized starting as early as 4.5 billion years
>ago (Ga) (12,13). .... <<there's much more but I'm tired of
>typing :)>>
>--Kasting, J.F. 1993. "Earth's Early Atmosphere," <Science> 259:920.
>==============================================================
>
Then Kasting has changed his mind, because in a 1997 Science article he
appears to take the idea of an early reducing atmosphere as a given. He
even quotes some evidence that strongly suggests the early mantle was much
more reduced than it is now.
>
>Another major factor is the composition of the Earth, as indicated
>in the last part of your description below. As an indication of
>the uncertainty still remaining in this area I will quote a little
>from a book review appearing in a recent issue of <Science>. The
>book being reviewed is <The Earth's Mantle> edited by Ian Jackson,
>Cambridge University Press, 1998. The review was written by
>David Stevenson. It opens as follows:
>
>=====================================
>"In many areas of science, the "big" questions remain unanswered
>even though we have much knowledge and a plethora of models.
>The study of the mantle, which constitutes 70% of the Earth's
>mass, is a good example. Here the important remaining questions
>include: What is the mantle made out of? Where did the material
>come from as the Earth formed? How was it altered during and
>after delivery? ...."
>
><<and then later in the review--BH>>
>
>"In the first, and by far the longest, chapter Hugh O'Neill and
>Herbert Palme summarize the bulk composition of Earth and its
>relationship to meteorites. They discuss elaborate models for
>the materials from which the Earth accreted (different compositions
>for late-arriving impactors, for example). One cannot help
>but suspect that this problem is insufficiently well constrained
>to be solvable at present.(Needed are more lab data, including
>those from high temperatures and pressures.) The chapter is,
>however, an excellent source of information on the topic."
>-- David J. Stevenson, "The World Between Crust and Core,
><Science> 281:1462.
>=================================================================
>
>Another recent study may end up throwing considerable doubt
>on a number of accretionary models:
>
>Bertka, C.M. and Fei, Y. 1998. "Implications of Mars Pathfinder
>Data for the Accretion History of the Terrestrial Planets",
><Science> 281:1838.
>Abstract: Accretion models of the terrestrial planets often
> assume bulk compositions with nonvolatile element abundance
> ratios equivalent to those of C1 carboneceous chondrites.
> The moment of inertia factor of Mars reported by the Pathfinder
> team is inconsistent with a bulk planet C1 Fe/Si ratio or Fe
> content which suggests that C1 chondrite accretion models are
> insufficient to explain the formation of Mars and other terrestrial
> planets. Future planetary accretion models will have to account
> for variations in bulk Fe/Si ratios among the terrestrial planets.
>
All these sources say is that we don't have all the answers yet; they do not
say that the planetesimals that made the earth were radically different from
those that would produce a reducing atmosphere. We know they could not be,
otherwise the earth would have a radically different chemical composition.
>
>BH:==
>>"Fine, but this is indirect evidence."
>>
>
>KO:==
>>Hardly. We know the mantle is mildly reducing (it still outgasses a ratio
>>of methane to carbon dioxide of about 1% [JA Welhan, Chem. Geol. 71, 183
>>(1988)] from hydrothermal vents) and that it was more strongly reducing in
>>the past based on the thermodynamic analyses of diamond inclusions [JF
>>Kasting, DH Eggler, SP Raeburn, J. Geol. 101, 245 (1993)] and studies of
>>metal-silicate partition coefficients of siderophile elements [MJ Walter
and
>>Y Thibault, Science 270, 1186 (1995); K Righter and MJ Drake, Earth Planet
>>Sci. Lett. 146, 541 (1997)]. We also know that the mantle had degassed
over
>>75% of its volatile gases by 4 billion years ago, based on the fact that
the
>>atmospheric Ar-40/Ar-36 ratio is only 295.5, which is what the mantle
ratio
>>would have been 4 billion years (the current mantle ratio is greater than
>>10,000 [M Ozima, Geohistory: Global evolution of the earth, Springer,
>>Berlin (1987)]). Since is known that the earth was created by accreting
>>planetesimals, in order to make an earth with its basic chemical
composition
>>the planetesimals would have had to be very similar to meteorites composed
>>of 98% high-iron chondrite (stone) and 2% C1 (carbonaceous) chondrite (20%
>>water, 3-5% C. Since these kinds of meteorites have a direct 1:1 relation
>>between the relative abundances of their less-volatile elements and the
>>corresponding abundances in the solar atmosphere, they are believed to be
>>the most primitive material known in the solar system and to have
condensed
>>directly out of the solar nebula. If you heat these meteorites up to 1500
K
>>you get predominantly reducing gases. If the earth was made from these
>>planetesimals and heated until the surface was molten, then degassed over
>>75% of its trapped volatiles gases by 4 billion years from a strongle
>>reducing mantle, then you will get a reducing atmosphere. This is not
>>speculation, this is extrapolation based on known evidence using deductive
>>reasoning.
>>
>
I have snipped the remaining discussion of extrapolation vs. speculation
because that is mostly a matter of a difference of opinion. What is
important is the evidence, and the evidence I have presented strongly
supports the existence of a reducing atmosphere prior to 4 GYA, whereas the
evidence that Brian presented shows that the atmosphere was largely neutral
with significant traces of oxygen after 4 GYA. This is perfectly consistent
with an abiotic scenario that combines atmospheric and seafloor vent
generation of biomolecules, thermal copolymerization and solid state
generation of catalytic macromolecules, and the subsequent evolution of
proto-life by 3.8 GYA. At which point the composition of the atmosphere
would no longer be important.
Kevin L. O'Brien