[meteorite-list] Fwd: Re: CAI and chondrules

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Fri, 02 Oct 2009 08:33:15 -0400
Message-ID: <OF3A00B9A0.535A3956-ON85257643.0045335A_at_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" <gsac at gmx.net>
>An: Jeff Grossman <jgrossman at usgs.gov>, meteorite-list at 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 <jgrossman at usgs.gov>
> > An: meteorite-list at 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" <carloselguapo1 at hotmail.com>
> > >To: <meteorite-list at 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
Received on Fri 02 Oct 2009 08:33:15 AM PDT