Spherules found at the Spirit site.

Leafing through alt.sci.planetary, I read Henry Spencer's message of
Tue, 17 Feb 2004:

Many thanks for your information about meteoritic origin, Henry.

Other meteorites, on the other hand, clearly *have* been part of such
bodies, because their materials have been geologically altered. Current
opinion is that several super-asteroids, larger than anything seen today
except perhaps Ceres, formed and were later broken up by collisions.

This (super-asteroids) feels basically right to me, but if you think
about how much mass has gone missing from the asteroid belt, it looks
probable that many (even most?) such bodies were ejected from the
asteroid belt by resonances with Jupiter, rather than being broken up.
As Henry has explained in the past, such ejected bodies end up hitting a
planet (very likely Jupiter itself), being ejected from the solar
system, or being injected to the Sun.

But
the need for something to cause the breakup -- collisions are the only
viable method -- and the fact that meteorites with geologically-altered
materials are very much in the minority argue that condensation into large
bodies was never anywhere near complete.

Well, this argument doesn't quite persuade me of that conclusion. Is
there any evidence to deny one or more geologically-altered bodies
having been ejected basically intact from the asteroid belt? [I don't
think this invalidates Henry's argument about the inferred Martian
origin of the certain meteorites, though. I'm just picking up this
particular point, and heading off at a tangent.]

If I had to bet on this right now, I would plump for the largest bodies
in the asteroid belt having been about the size of Europa (3,000 km
diameter), which may be within the range envisaged by the super-asteroid
theories Henry refers to. But to pursue my line of argument to a more
tantalising limit, why shouldn't we posit a Mercury-sized body (5,000 km
diameter) formed in the asteroid belt, and it ended up somewhere else,
like inside Jupiter? If the early asteroid belt had as much mass as
Mars, you could have a Mercury-sized object and still have most of the
mass in smaller chunks.
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