[meteorite-list] Strange Asteroids Baffle Scientists

[David Weir]

Sterling K. Webb wrote:

>     Now if they continue and find more (and more) unrelated
> basaltic small bodies, one of the questions that arises is "how
> big does a body have to be to differentiate?" We know (obviously)
> that a body as big as Vesta does, but how about smaller bodies?
> Some (by no means all) theorists believe that bodies as small
> as 100 kilometers can differentiate.
> 
>     How could they do that? Well, IF they formed early enough,
> they would have enough short-lived isotopes to really get
> cooking! At the supernova-mediated start of the solar system,
> there were lots of "hot" isotopes available but they decayed very
> rapidly. So, the question really is "how fast did planetesimals
> and planetoids form?"
> 

Sanders and Scott (2007) consider that any body with a diameter greater 
than 60 km which accreted within ~2 m.y. of CAI formation must contain 
enough 26Al to initiate internal melting. Most of the iron cores that 
exist are calculated to have formed within this time frame. These 
conditions are obviously minimums, yet they don't consider the 
heliocentric distance, a very important factor. Too far out from the Sun 
and accretion becomes too slow to accumulate enough 26Al to initiate 
melting before the planetesimal grows too large. We know that some 
differentiated meteorites were formed ~6 m.y. after CAIs (acapulcoites) 
or longer, perhaps reflecting an initial orbit close to the Sun. Another 
interesting question is where did the radiogenic elements come from and 
when did they arise. Some studies are showing a later injection of 26Al 
after CAI formation began, demonstrated by a generation of CAIs lacking 
26AL daughter isotopes. This could possibly affect the timing of the 
differentiation sequence. The most amazing thing to me is that we are 
even having this conversation... thanks to the cosmic theoreticians.

David