[meteorite-list] There are no silly questions? -Vesta

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Wed Oct 20 09:19:43 2004
Message-ID: <6BA91B31.426203C7.0BFED528_at_aol.com>

Hola AL, Eric, Bernhard, Everyone,

Regarding the point mentioned that "only" spectra from Vesta suggest HED meteorites are from Vesta, I would like to briefly build in what I consider a practical sense, on AL's detailed comments:

McSween, AL comment that 1% of Vesta was ejected from an impact event... I would point out that the Asteroid belt has a lot of material, but Vesta, Ceres, Pallas, and Juno make up the majority of the mass in the trans Mars-Jupiter region. And Vesta being the only one that is practically a dead ringer, adds to this. Relative to the "1%", confirmed by topographic mapping of Vesta, a quick caculation of the Volume of Vesta shows that 1,500,000 1 km in diameter round asteroids would have resulted from this event, under the assumption that everything produced was in the 1 km diameter size.

And Not everything obviously is that big (though some are bigger), the majority would result in literally a Sagan (billions and billions) of pieces. It is no wonder that some of these make it Kirkwood Gaps and to Earth! Statistic is not the only thing in its favor, after looking at how many asteroids we think there are ... but the fact that I don't see mentioned is and perhaps is not in McSween (??) is that Be-Ne cosmic ray exposure dating brings the meteorites to traveling in space and arriving on Earth all to an Impact event some 38 million years old or less (Less: perhaps a Vestoid subsequent impact explains this)... (there may be something at 57 million yrs or so, also, don't recall at the moment)

That is quite convincing evidence! And then to consider that the HED's are from the same body (other evidence), and then to confirm that ol' Vesta has them all from reflectance spectra!!!

Saludos, Doug




AL Mitterling, Eric Olsen, and others have enthusiastically written:

Hi Bernhard,

My suggestion is to read Harry (Jr.) McSween's newest book "Meteorites and
Their Parent Planets" as it answers most of the questions you have addressed
with excellent reasons for our believe in what comes from where. A lot of
work has been done with Hubble and it has further verified our suspicions on
certain relations between meteorites and their parent bodies. I might add
that we think with a pretty good degree of certainty that the H class type
meteorites come from Hebe. There are others some more and some less certain.

Here is the not so short answer to two of your questions.

Bernhard Rems wrote:

> 1) HEDs are from Vesta.
>
> Fine. All of them? How comes that with 50.000+ known asteroids, all HEDs
> come from a single one? As far as I know, spectroscopic evidence points
> to Vesta, yes - but how large is the chance that HEDs do NOT come from
> Vesta.

An absence of meteorites consisting of recrystallized ultramafic rock
suggests the parent body hasn't suffered such a catastrophic collision that
it is no longer intact.
The mineralogy of eucrites provides a VERY distinctive reflectance spectrum
with a strong absorption band near 1 um attributable to pyroxene. This was
the first strong evidence we ever had that certain meteorites and Vesta were
probably related.

The brightness of this asteroid (or chips from Vesta, Vestoids) imply a very
limited chance that connection with any other asteroid is very low. No other
V-class asteroids have ever been discovered elsewhere in the solar system
making the correlation between the HED's and Vesta one that can be determine
with confidence.

No doubt the impact at Vesta's south pole (a 460 km impact crater) where some
1% of Vesta's mass was removed is the source of most of our Howardites,
Eucrites and Diogenites. The mapping of the surface by Hubble and the
spectral reflectance and signatures Make the odds of these meteorites coming
from anywhere else nearly 0. (source: "Meteorites and Their Parent Planets").

> 2) Meteorites have been ejected towards earth by collisions between
> asteroids.

As was said early that most likely very few meteorites came to us from direct
impact. Rather between the orbits of Jupiter and Mars are places that are
devoid of asteroids at all. These places are called Kirwood Gaps. These are
areas that have a common ratio of orbit with Jupiter. Examples are a 1:2 or
1:3 or 1:4 and there are many others combinations. When a collision happens
and a meteoroid falls into or near one of these gaps they begin an amazing
process of being perturbed. Each time they make an orbit Jupiter pulls them
out a little farther making a circular orbit an elliptical one. As this
cosmic, rhythmical orbiting continues over the eons it can be pulled out
futher become an Mars crossing, Earth crossing, Venus crossing and even
evidentially end up finding it's way to the sun, in the event it doesn't
collide with the inner planets. Probably this is the biggest source of our
meteorites. I have also read (heard) that the sun can heat the sides of
asteroids up causing them to "gas" also providing a means of them to move out
of a stable orbit and perhaps into one of the Kirkwood gaps.

--AL Mitterling
Received on Wed 20 Oct 2004 09:19:26 AM PDT