[meteorite-list] Strange Asteroids Baffle Scientists

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Wed, 22 Aug 2007 21:17:42 -0400
Message-ID: <46CCE036.5020606_at_earthlink.net>

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
Received on Wed 22 Aug 2007 09:17:42 PM PDT