[meteorite-list] Questions about accretion.

[Jerry Flaherty]

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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