At 10:22 PM 11/15/98 -0700, Kevin wrote:
>Greetings Brian:
>
>"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.
No, this is false. Further, you have a bad habit of jumping to
conclusions, i.e. your statement that I'm reading in what I
want to believe. Will you stop? I'm also curious why
you claim that I will find something that is clearly not there.
Can you explain?
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.
==============================================================
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.
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.
>
BH:==
>"Apparently we disagree on what constitutes evidence and what is
>speculation."
>
KO:===
>Perhaps, but you didn't answer my question: what about what Mason said is
>speculation, when everything is based on evidence?
>
OK, first let's be clear about words. Anticipating possible confusion
I specified previously what I meant by speculation. Here's what
I wrote:
"Now, just as there is more than one type of "opinion" there is
also more than one type of speculation. So, I'm not talking here
about wild speculations but reasonable speculations based
upon reasonable models etc." -- BH
Now, you define speculation somewhat differently below and present
your argument as if that's what I meant when I said speculation,
exactly what I was trying to avoid. Now, you are free to use the
word however you wish and I appreciate your clarifying what you
mean, but please don't attribute that meaning to what I said.
Now, you say everything is based on evidence. Fine. I hadn't
thought to throw that in previously since, from my point of
view, a speculation not based on evidence is a wild speculation
not a reasonable speculation. :)
Below you introduce another word, extrapolate. I like this
word and feel that it captures better what I had in mind
when I said speculate before. My professional expertise is
as an experimentalist. Speaking with this cap on I would say
that I would always consider any extrapolation as speculation.
So, I say "better" above only because speculation may have
negative connotations that I did not intend.
So rather than "...reasonable speculations based upon reasonable
models" I would be happy to say instead "extrapolations based
upon available evidence and data using reasonable models."
But I'm afraid we still may have potential confusions over
words since you appear to mean something different by extrapolate
than I do. For example, your statement that "Extrapolation is
empirical" causes me great pain and suffering :). The very
nature of extrapolation seems to me to be to go beyond the data.
True, extrapolation will involve patterns in the data and logic
and all that but it will also make a prediction about a situation
not contained in your data set. This is also clearly the case
in the results I've described above. Available evidence is
combined with accretion models to make a prediction of how the
earth formed and what the atmosphere was like.
Oh, I almost forgot to answer your question :). Due to possible
negative connotations of "speculate" I'll use extrapolate as
I have defined it above. So, what I would formerly have
considered speculative about Mason is any prediction based
on extrapolation using models. For example, that the Earth's
atmosphere was highly reducing prior to 4 billion years ago.
KO:===
>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.
Once again jumping to conclusions. I thought it would be obvious
that the authors I have been referring to using Miller's
words "considerable opinion" are also trying to predict what
the early Earth's early atmosphere was like and consequently are
also speculating.
Let me add also that I've changed my views somewhat after learning
some details about the models being used, in particular the
level of uncertainty which seems to be present. Consistent with
my good experimentalist upbringing :) I really can no longer say
that I would support any of models and predictions based thereon
at present. This is not in any way a criticism. I think this
should be an exciting field to follow over the next few years,
particularly as more data becomes available from mars.
[...]
Brian Harper
Associate Professor
Applied Mechanics
The Ohio State University
"He who establishes his arguments
by noise and command shows that
reason is weak" -- Montaigne