[meteorite-list] Questions about accretion.

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sun, 5 Apr 2009 13:45:07 -0700
Message-ID: <GOEDJOCBMMEHLEFDHGMMIEALDMAA.mojave_meteorites_at_cox.net>

Hi Eric,

I'll take a stab at a few of your questions:

> How long does the formation of meteoroid bodies and larger asteroids take?

This is not an easy question, as there were many processes at work during
the early solar system -- some constructive (gravitational/electrostatic
clumping), some destructive (high velocity impacts between clumps), and
the time it would take to form, say, a 100-km sized body would depend on
the initial quantity of dust in the pre-solar nebula. I don't know how long
planetary scientists believe it took to form 1-km-sized bodies, but it
was at least hundreds of thousands of years, probably longer. But when
do you "start the clock"? When what became the solar system was just a
molecular cloud, when the protostar formed, or tens of millions of years
later when the protostar transitioned from T-Tauri stage to main sequence
burning?)

Whichever you choose, once you have asteroids a kilometer or so in size,
barring collision with other such bodies they would continue to accrete at
a rate of centimeters per year. So it would still take more than a million
years to grow from 1-km to 100-km size.

> How does the iron migrate to the core?

Through the combination of porosity, heat and gravity. If you start
with a glass of finely crushed ice and let it melt, the water doesn't
stay put in the ice matrix -- it settles to the bottom (since water
is denser than ice).

> Do all "large" asteroids consist of an iron core surrounded by
> lighter materials further towards the asteroids surface?

Yes, beyond a certain size nearly all should. One way to create an
exception might be to have a large, already-differentiated asteroid
get impacted by a smaller one in such a way that its iron core
remains intact, but a portion of the outer rocky shell is blown
off. Any large fragments of the original differentiated asteroid
would then be depleted in iron/nickel.

--Rob
Received on Sun 05 Apr 2009 04:45:07 PM PDT