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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