Jeff brought up an important point on which that I would like to
elaborate. A few chondrules may sport a peculiar feature that may
have important constraints on how those few chondrules formed. The
problem is in generalizing. I may feel that those few chondrules are
a Rosetta Stone that have preserved important features that were
erased in the vast majority of chondrules because of secondary
processing. Another researcher may feel that these peculiar features
are just anomalies that have little bearing on the general problem.
There may be no objective way around this other than finding
additional examples or fitting the observations into a coherent model.
Let me give a specific example. Sasha Krot and I reported that a
handful of chondrules have fine-grained matrix-like rims that are
loaded with microchondrules. No one disputes this observation. I
believe that these microchondrule-bearing objects are rare surviving
representatives of a general process relating to chondrule formation.
My inference is that, in many cases, after formaton, chondrules were
surrounded by dustballs and that some highly localized flash-melting
mechanism, perhaps akin to a lightning bolt, zapped the
chondrule-dustball assemblage and caused melting of the chondrule
surface (thereby forming droplets that solidified into
microchondrules). Other researchers are skeptical that the handful of
chondrules with microchondrule-rich rims have anything to say about
the general chondrule-forming process. My idea fits into my scenario
of how chondrule formation occurred. Those who favor the model in
which chondrules were produced in large-scale events involving gas
dynamic shock waves would tend to dismiss these microchondrule-bearing
chondrules as being a sideshow. Who is right? It depends on who you
ask!
Quoting Jeff Grossman <jgross...@usgs.gov>:
There are all kinds of controversies involved with this subject.
The observations themselves are rarely the problem. It's how the
observations are interpreted that leads to the controversies!
Turning observations into constraints on chondrule or CAI formation
always involves interpretation. The first steps in forming these
constraints from observations of the natural systems usually
involve: (1) Arguing that the observations are of primary features
(as opposed to features formed by metamorphism, alteration, shock,
weathering, or other late processing) and, (2) arguing that the
observations do not reflect some kind of anomalous or unusual
special case, but are instead applicable to the general problem. If
you can do that, then you have a primary constraint, namely that the
chondrule- or CAI-formation process has to be able to result in the
observed feature. So, following some of what Alan said, he argues
that chondrule formation must be able to produce the observed rims.
I think nearly all researchers would agree with this statement.
Where things get really messy is the next stage of the process. For
example, one might make a list of all the processes that could
conceivably make these rims and try to rule out all but one. Alan
has argued that they could only form by multiple heating events, and
therefore declare that another constraint on chondrule formation is
that it must be able to melt the objects multiple times. But his
arguments to get to that constraint are complex and might be
questioned by other workers. Another worker might look at the same
data and conclude that the rims could form by multiple accretion
events of material splashed from asteroidal impacts. It is these
secondary constraints that are often highly controversial.
Built on top of these constraints, as well as constraints provided
by astronomical observations, physics, and chemistry, are models for
the early solar system. If the constraints on which the models are
built are really good, then the models may eventually approach
reality. But these models are houses of cards. If the constraints
are weak, they will fall. When you ask for a definitive answer to
how chondrules or CAIs form, you are asking for the definitive early
solar system model. At this time, there isn't one.
Jeff
At 04:16 PM 10/1/2009, Alexander Seidel wrote:
Sorry, I should have added: the observable facts don´t seem to be
the problem here, but all the *CONSTRAINTS* on them involved, right
Jeff?
Alex
-------- Original-Nachricht --------
Datum: Thu, 01 Oct 2009 21:57:35 +0200
Von: "Alexander Seidel" <g...@gmx.net>
An: Jeff Grossman <jgross...@usgs.gov>, meteorite-list@meteoritecentral.com
Betreff: Re: [meteorite-list] CAI and chondrules
From my layman´s perspective and point of view: isn´t it
interesting to note that there is still **so much** controversy
over chondrule formation, those little round objects which are so
evident and very clearly visible in many of the meteorites in our
collections, while at the very same time all the basic physical
conditions and evolutionary laws even on small timescales seem to
be quite well understood? But then again all the many empirical
facts obviously still have to come under serious scrutiny to
finally have, at best, sort of a "generally accepted truth" emerge:
a mainstream theory of chondrule formation that will be agreed upon
by most scientists - one of these days.
Fascinating, especially in this era where many "basic" things seem
to be understood! I´m excited to learn more about this, as time
goes by... :-)
Alex
Berlin/Germany
-------- Original-Nachricht --------
Datum: Thu, 01 Oct 2009 14:52:08 -0400
Von: Jeff Grossman <jgross...@usgs.gov>
An: meteorite-list@meteoritecentral.com
Betreff: Re: [meteorite-list] CAI and chondrules
I didn't say "we don't know cr*p"... I said there
was not a definitive answer. We know a lot about
the timing, materials, and physical conditions
needed to make chondrules and CAIs, and people
have used these to make models for their
formation. But many of these constraints are
under scrutiny right now, and some pretty basic
things are still controversial. What this means
is... ok, we don't know cr*p. But progress is being made.
Jeff
At 02:39 PM 10/1/2009, Sterling K. Webb wrote:
>Hi, Carl, Francesco, Jeff, List,
>
>Wrong, Carl. A good solid "We don't know
>crap about this yet; it's all up in the air"
>is the best kind of definitive answer.
>
>There are probably a half-dozen scenarios
>for how this data came about and there's few
>reasons to choose any one over the others.
>
>I was glad to hear Jeff confirm it.
>
>
>Sterling K. Webb
>-----------------------------------------------------------
>----- Original Message ----- From: "Carl 's" <carloselgua...@hotmail.com>
>To: <meteorite-list@meteoritecentral.com>
>Sent: Wednesday, September 30, 2009 5:49 PM
>Subject: Re: [meteorite-list] CAI and chondrules
>
>
>
>Ahhh! Good questions, Francesco. I had been
>wondering about CAIs and chondrules myself. None
>of the books I've read explains,how?
>
>Thanks also for your answer, Jeff. While not
>definitive, it seems to be the only answer at this time. Thank you.
>
>Carl
>
>
>Jeff Grossman wrote:
>>...
>
>Definitive answers to what caused the formation of CAIs and chondrules
>are not known.
>
>_________________________________________________________________
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
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