[meteorite-list] Questions about accretion.

From: Rob McCafferty <rob_mccafferty_at_meteoritecentral.com>
Date: Tue, 7 Apr 2009 15:02:15 -0700 (PDT)
Message-ID: <803012.79257.qm_at_web55201.mail.re4.yahoo.com>

According to O. Richard Norton's Encyclopedia of Meteorites 2002,
100-200km (abstract page for chapter 9)
Rob

--- On Tue, 4/7/09, Meteorites USA <eric at meteoritesusa.com> wrote:

> From: Meteorites USA <eric at meteoritesusa.com>
> Subject: Re: [meteorite-list] Questions about accretion.
> To: rob_mccafferty at yahoo.com, "meteorite-list at meteoritecentral.com" <meteorite-list at meteoritecentral.com>
> Date: Tuesday, April 7, 2009, 5:40 PM
> Thanks Rob! Great response. That pretty much sums it up for
> me and answers just about everything I was curious about in
> that email.
>
> You mentioned...
>
> "..If the rock is big enough, (which provides enough
> radioactive material to generate the heat AND enough lying
> over the middle to prevent the heat escaping, the body will
> melt..."
>
> How big is "big enough"?
>
> Eric
>
>
>
>
> Rob McCafferty wrote:
> > Hi Eric
> >
> > You are correct in thinking that electrostatics causes
> the initial clumping.
> > The early sun would have been extremely energetic and
> X-ray and UV radiation would produce electro static charging
> of small particles.
> > Once they begin to clump to a sufficient size, they
> will attract particles through gravity.
> >
> > The dynamics are as follows
> > An object with radius R will naturally sweep up any
> object within its radius (pi*R^2) but gravity will draw
> material from a greater distance S inside and outside its
> orbital path
> >
> > S=(R^2 + 2GMR/V^2)^1/2
> > M mass of body, V initial closing velocity of body and
> impactor
> >
> > Initially, you are correct, everything begins as a big
> clump of mixed material. Whether an iron core is formed will
> depend on the size of the initial clump of stuff. Heat is
> generated by radioactivity of short lived isotopes such as
> Al26. If the rock is big enough, (which provides enough
> radioactive material to generate the heat AND enough lying
> over the middle to prevent the heat escaping, the body will
> melt. Once this begins, the iron will migrate to the core as
> rock and iron don't mix. Iron, being denser, will sink.
> >
> > Accretion to differentiation is a very rapid affair,
> just a few million years. The almost identical ages of all
> asteroidal meteorites tends to confirm this.
> > My understanding is that this leads to the different
> classes of achondrites. These have been properly melted and
> lose their chondrules. The widmanstatten patterns in irons
> comes from the rocky material insulating the iron/nickel
> core allowing it to cool very slowly. Parent bodies forming
> in different orbits are likely to have differing
> constituents according the condensation model, hence
> different achondrite types.
> >
> > Chondrites may have come from smaller initial parent
> bodies, ones that weren't big enough to generate enough
> heat to fully melt. Higher petrographic types of chondrite
> (4-6) are samples that are progressively closer to the core
> and were heated more in bodies that were not properly
> differentiated. Petrographic type 3 are essentially the same
> material as the early solar system, mostly unaltered by
> heat, likely from near the surface of undifferentiated
> bodies. I don't see that all parent bodies would
> necessarily need 3-6 petrographic types. Small parent bodies
> may not reach the higher grades in the middle as they never
> got hot enough. Grade 6 seems to be the limit. If the parent
> body grew any bigger then it would melt producing a
> differentiated parent body.
> > I think petrographic type goes to 7 but I don't
> think any are actually given this grade (though I think it
> was NWA3133 that may have been discussed as a possible).
> > It is likley that H, L and LL meteorites come from
> different parent bodies possibly from different regions in
> the protosolar nebula.
> >
> > The relative rarity of petrographic type 3 ordinary
> chondrites may be due to them being removed first and
> subsequently removed from the system many aeons ago.
> >
> > Carbonaceous Chondrites are a whole different kettle
> of fish but I think I've said quite enough for now. I
> hope I've not made any glaring errors but if I have
> someone will put me right.
> >
> > Rob Mc
> >
> >
> >
> >
> >
> >
>
>
> -- Regards,
> Eric Wichman
> Meteorites USA
> http://www.meteoritesusa.com
> 904-236-5394


      
Received on Tue 07 Apr 2009 06:02:15 PM PDT