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

Jeff Grossman jgrossman at usgs.gov
Fri Oct 2 08:33:15 EDT 2009 

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

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