[meteorite-list] Questions about accretion.

From: Jerry Flaherty <grf2_at_meteoritecentral.com>
Date: Tue, 07 Apr 2009 16:19:36 -0400
Message-ID: <55D1E7A0C9384CB78159674FFC037853_at_ASUS>

Just a smigen bigger than not enough?
----- Original Message -----
From: "Meteorites USA" <eric at meteoritesusa.com>
To: <rob_mccafferty at yahoo.com>; <meteorite-list at meteoritecentral.com>
Sent: Tuesday, April 07, 2009 12:40 PM
Subject: Re: [meteorite-list] Questions about accretion.


> 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
>
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Received on Tue 07 Apr 2009 04:19:36 PM PDT