[meteorite-list] Observed lunar meteorite impacts hit 100

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 24 May 2008 02:23:32 -0500
Message-ID: <018401c8bd6f$12bc9ef0$fd2ce146_at_ATARIENGINE>

Chris wrote:

> Comets originate in the outer edge of the
> Solar System...

With the caveat that I am not jumping on Chris in particular,
that statement about comet formation is both common and
wrong.

Icy bodies form where ice forms. Ice forms wherever the
original material of the so-called "solar nebula" cools enough
for its water vapor to turn into ice. That point has to be close
enough in to the Sun for there to be enough material to produce
sufficient ice, to be close enough for water vapor to exist in
the first place, and so forth.

All these factors depend on your theory and model, but
there is reasonable agreement between the 57 varieties.
This point is sometimes called the "snow line." It was long
thought to be just inside the orbit where we find Jupiter
now, or about 4.5 AU. More recently, 2.7 AU, or the middle
of the Asteroid Zone, has been calculated. Last year there
was a paper that argues for 1.6 to 1.8 AU, or just outside
the orbit of Mars. Clearly, it wasn't "far out."

Out in icy-body territory, like the Kuiper Belt, the amount
of water vapor would be far less than 1% of what would be
found at Jupiter (not enough to form the bodies we find there).

Besides which, it may have never been warm enough out
there for water vapor to exist (that depends on your model).
Out in the Oort Cloud, where we are assured there are
trillions of "comets," no comet could ever have formed.

Icy bodies formed where ice particles formed, then accreted
to each other to produce icy planetesimals that accreted to form
larger... and so forth, essentially from inside Jupiter's orbit to
somewhere well inside Saturn's orbit. It's clear that Saturn does
not have a high ice content (density 0.90), so icy bodies did not
form even that far out in any great numbers.

So, one primary characteristic of "comets," or of small icy
bodies, is that they could not have formed where we find them
now. They therefore have a refugee history that involves their
re-location. You have to suspect that big bully Jupiter, of course.

That's assuming the big planets formed where they are now, but
another possibility is that the major planets formed further out and
spiraled in by nebular friction, but another theory has some of the
big planets (Uranus and Neptune) forming closer in and moving
out, as if the whole thing wasn't confusing enough already.

If icy bodies ("comets") have a history of being transported
around the solar system, it is not a history we presently know
much, if anything, about, and one we can hardly even guess at.
It's likely a tangled history, which makes assumptions about
their composition less than certain.

As for the apparent spectral composition of "icy" bodies, the
problem there is that most of such a body's volatiles will be
found on their cold surfaces, regardless of whether their volatile
composition is 20% or 80%. Then, there's the sublimation of
ices that leaves dust behind, so that in a suitable thermal regime,
a body could evolve to have a rocky core, an icy mantle and
a rockdusty crust. What a marvelous disguise!

It's a messy problem. We'll just have to go there and check'em
out! No substitute for the geologist (planetologist? cometologist?)
on the ground, drilling, taking seismic profiles, whacking things
with those neat little hammers, whatever.


Sterling K. Webb
---------------------------------------------------------------------------
----- Original Message -----
From: "Chris Peterson" <clp at alumni.caltech.edu>
To: <Meteorite-list at meteoritecentral.com>
Sent: Thursday, May 22, 2008 9:32 AM
Subject: Re: [meteorite-list] Observed lunar meteorite impacts hit 100


There are clearly two very distinct populations of objects, which have
very different properties. Comets originate in the outer edge of the
Solar System, and ices account for a significant proportion of their
entire mass. Very few ever make it to the inner system, and when they
do, they can usually be identified by their high eccentricity orbits.
Asteroids are differentiated rocky or iron bodies that were formed or
trapped in orbit between Mars and Jupiter.

It is quite correct to distinguish between the two types of bodies. The
confusion comes from the likelihood that some comets have ended up in
asteroidal orbits, and have lost their volatiles. And also, that
gravitational perturbations have put some asteroids into more comet-like
orbits. It isn't that these aren't very different objects, but that in
some cases we are uncertain about an object's true classification.
Additionally, we know little about composition. A burned out comet may
or may not be similar to an asteroid from a mineralogical standpoint.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message -----
From: "Mark Ford" <mark.ford at ssl.gb.com>
To: <Meteorite-list at meteoritecentral.com>
Sent: Thursday, May 22, 2008 3:42 AM
Subject: Re: [meteorite-list] Observed lunar meteorite impacts hit 100

> Good point Larry.
>
> But I can't understand why people are still carefully distinguishing
> between comets and Asteroids?, I think by now we can assume they are
> basically one and the same, and not some exotic different species. To
> me
> it's just that some rocks are more 'wet and oily' than others...
>
> I'd find it very very hard to believe there are no pieces of comet in
> our collections.
>
> Best,
> Mark Ford
-list

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Received on Sat 24 May 2008 03:23:32 AM PDT