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Re: Vesta questions


My questions were prompted by reading
 
http://www.seds.org/ftp/images/clementine/mpeg/viewers/msdos/animations/space/Vesta.txt>

where Ben Zellner of Georgia Southern University said: "Vesta has
survived essentially intact since the formation of the planets..."

Since the Earth-Moon system is considered the product of a collision
between two objects that reformed, and since some planetary
satellites are thought to have broken up and re-formed by collisions,
I thought that most of the matter in the solar system had undergone
countless collisions, breakups and reformations in the history of the
solar system.

> >a) how small can an object be and still undergo differentiation?

>About 175 km in diameter.  Vesta is much larger than that.  The lava
>flows on Vesta happened early on before its interior cooled off.

I find this astounding, since I thought it was gravitational forces 
that provided much of the heat. I guess it was really the radioactive
matter decaying that produced the heat, then.

This radioactive decay comes from the source that produced the dust
that formed the solar system, right? Neutron-rich heavy elements from
an exploding supernova. I would have thought the half-life of this
matter wouldn't have lasted long enough to heat up accreted bodies
in the later solar system.

How was the 175 km figure arrived at?

>>b) I though most asteroids came from collisions between larger objects
>>   and that very few are still in their original composition. 
>
>The "larger objects" you are referring to are the larger asteroids, and
>Vesta is in fact one of the larger objects in the asteroid belt.  
>Were you expecting all asteroids to be exactly the same size?  They
>come in various sizes, shapes and compositions.

No. I was recollecting an old article from Scientific American that
modelled the early solar system from many small-bodied collisions.
I was thinking of the age far before the major planets were formed.
Of course, my recollection could be faulty. And I haven't kept up with
the advances in planetary sciences of the last few years.

I think we are talking about two different eras - I was thinking of
the time the solar system was accreting from dust - and collisions
caused as many breakups as it did build-ups. Maybe this old model has
been discarded? Do scientists now think the planets we now see were
pretty much fully-formed like that in the early solar system? That is,
there were 9 major bodies that had condensed when the Sun ignited and
blew away the dust and gases, and that the smaller bodies caused the
cratering?

-- 
    Jim Hurley         mailto:hurleyj@arachnaut.org
 Arachnaut's Lair    http://www.arachnaut.org/ >
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