Just a smigen bigger than not enough?
----- Original Message -----
From: "Meteorites USA" <[email protected]>
To: <[email protected]>; <[email protected]>
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
______________________________________________
http://www.meteoritecentral.com
Meteorite-list mailing list
[email protected]
http://six.pairlist.net/mailman/listinfo/meteorite-list
______________________________________________
http://www.meteoritecentral.com
Meteorite-list mailing list
[email protected]
http://six.pairlist.net/mailman/listinfo/meteorite-list
