"And the people that Miller refers to as 'considerable opinion', do you
think they were just making stuff up out of thin air?"
Of course not, by if you look carefully at what they say instead of reading
into their words what you want to believe you will see that they are
referring to the time period between 2 and 4 billion years ago, not the
period between 4 and 4.5 billion years, when Mason says the reducing
atmosphere would occur. Besides, they were speculating in the absence of
information, not extrapolating from information as Mason was. See below for
a more complete explanation.
"Fine, but this is indirect evidence."
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.
"Apparently we disagree on what constitutes evidence and what is
speculation."
Perhaps, but you didn't answer my question: what about what Mason said is
speculation, when everything is based on evidence?
Evidence is data; speculation is an attempt to seek answers to questions
based on the gaps in the data using imagination and inference. What Mason
did was extrapolation. Extrapolation is an attempt to seek answers to
questions based on the observed pattern of the data using logic and
deductive reasoning. Extrapolation is empirical, speculation is rational
(in the philosophical sense of being non-empirical). Both are important and
have their place in science, but calling someone's extrapolations
"speculation" simply because you do not agree with their conclusions is pure
rhetoric. Miller's "considerable opinion" was using their lack of
geological evidence of that period to speculate about what the atmosphere
was not like rather than using the evidence they had to extrapolate what the
atmosphere was like.
"'These speculations on chemical evolution, multiple origins of life, and
models of early environmental conditions in the atmosphere and oceans can
only be substantiated by the geological record.'"
I would expect geologists to say that, but are they saying that the only
evidence they will accept is evidence obtained from earth crustal rocks of
that period? When they know darn well that no rocks survive from that
period? That sounds like creationists saying they won't believe evolution
until we can show them a transitional form like a fish with frog legs, or a
half-lung/half-gill. Geologists should know better.
Kevin L. O'Brien