According to O. Richard Norton's Encyclopedia of Meteorites 2002, 100-200km (abstract page for chapter 9) Rob
--- On Tue, 4/7/09, Meteorites USA <e...@meteoritesusa.com> wrote: > From: Meteorites USA <e...@meteoritesusa.com> > Subject: Re: [meteorite-list] Questions about accretion. > To: rob_mccaffe...@yahoo.com, "meteorite-list@meteoritecentral.com" > <meteorite-list@meteoritecentral.com> > Date: Tuesday, April 7, 2009, 5:40 PM > 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 Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
