[meteorite-list] Questions about accretion.

From: Meteorites USA <eric_at_meteoritecentral.com>
Date: Tue, 07 Apr 2009 09:40:20 -0700
Message-ID: <49DB81F4.7030803_at_meteoritesusa.com>

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
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-- 
Regards,
Eric Wichman
Meteorites USA
http://www.meteoritesusa.com
904-236-5394

Received on Tue 07 Apr 2009 12:40:20 PM PDT