[meteorite-list] Cometary meteorites

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 25 Nov 2010 03:22:08 -0600
Message-ID: <00E914F2AFC94F98B970F413A28FAFB7_at_ATARIENGINE2>

Hi, E.P., Jason, List,

Jason, this is no criticism of you, because you are
indeed re-iterating orthodoxy, but the notion that
comets FORMED in the Kuiper Belt put of pure ice
(which you seem to imply) is absurd.

Let's define that zone as between 40 AU and 50 AU.
the area of such a zone is ~2800 square AU (a new
unit). Let's bias the assumptions in your favor by
saying that that zone is only 10 AU thick (even
though the inclination of object there demands
a much thicker "zone" of accretion.

That gives us 28,000 cubic AU's (another new unit).
Is that much volume? Only 8 x 10^23 cubic miles
(too old to use kilometers). OK, we start with a mere
800,000,000,000,000,000,000,000 cubic miles filled
with a sufficient density of ice and dust to accrete into
the Kuiper Belt objects, the ejected long period comets,
Pluto, Eris and the rest.

Who are you kidding?

Orbital velocities at 40 AU are slow -- 2800 m/s at Pluto
and slower still as you move out. How long does each
icy particle 5mm in diameter have to cruise slowly through
that 800,000,000,000,000,000,000,000 cubic miles before
it meets another icy particle and accretes?

Houston, we have a problem...

For comparison the accretion zone of the Earth, generously
defined, is about 20 cubic AU's. The density of the "nebula"
at 40 AU and 10 AU thick would be 1/160,000th of the peak
density at 1 AU and 0.1 thick.

Another problem is that water vapor at very low partial
pressures condenses directly into ice at about 160 degrees
K. For the Kuiper Belt to be warm enough to support water
vapor at that distance, early solar output would have to more
than 250 times the present solar output.

Oh, heck, let's worry about that later and just stick to
making accretion happen.

What variables can we manipulate to make accretion happen?
Well, there's density. Let's increase the density 100-fold!
Won't do it... How about 500-fold? Don't be silly... What if
we stretch time and allow a billion years for these objects to
accrete? And increase the density to insane levels?

Nope.

We have the same problem with Uranus and Neptune. No
reasonable model can account for them unless you have'em
take 800 million years to accrete AND have it happen closer
in and then migrate out. We even have a mild problem with
accounting for Saturn. Orbital migration is the only way out.

There are problems like this through the solar system. The
only way to account for iron asteroids (over 80 separate
compositions) in the Main belt is to explain them as having
formed very near to the Sun and get tossed out there. THAT
models successfully.

The Tossing Solution works to resolve the problems with
Uranus and Neptune which are after all mostly rocky bodies
with dense atmospheres. It explains Pluto, Eris, and Haumea,
the last two of which are as dense as our own Moon.

It's a theory. "Formed in place" is a theory, too, albeit a
much more ridiculous one. I even have a theory of my own
with Jupiter trying to spiral in closer to the Sun and tossing
everything in its way "out there" -- planets, comets, dwarves,
you name it.

But... they're all theories, and they're all inadequate. What
we really have is ignorance. That is a wonderful thing because
we retain the fun of finding out.

But there's no certainty about composition. The paper you
cite (which BTW suggests a formation nebular density they
place between Jupiter and Saturn, depending on which current
model you like) details the formation of the "ices," yes, but
that says nothing about whatever other components there
may or may not be present at the birth.

And that's the essential issue.

I seem to recall seeing lots of "rocky stuff" in that solar
neighborhood. How would a body that is only ices form there?
Are there any examples of purely icy bodies left behind there?
Well, yeah... Tethys, Mimas, Miranda, Rhea, Iapetus, Proteus
(maybe). All low enough in density to be just ices and nothing
else, most associated with low-density Saturn (which has a
plentiful supply of ices to make'em with, for some reason).
That's out of a hundred candidates.

Anything with a density of around 2.0 is half-rock, half-ice,
more or less (Ceres, Pluto, and a long list). It's the median
group and the most common, and there are some clearly rocky
bodies, though fewer. Roughly, it's a range of normal distribution
between ice and rock. IF comets formed in that Jupiter-Saturn
region, then they almost certainly have a similar distribution
of compositions.

Any certainty? No. It's a mystery. We'll just have to catch a few
comets and take'em apart to find out.


Sterling K. Webb
(with nothing better to do at three ayem)
----------------------------------------------------------------------------------
----- Original Message -----
From: "Jason Utas" <meteoritekid at gmail.com>
To: "Meteorite-list" <meteorite-list at meteoritecentral.com>
Sent: Wednesday, November 24, 2010 9:02 PM
Subject: Re: [meteorite-list] Cometary meteorites


E.P., All,

We know quite a bit about comets' interiors based on the knowledge
that we have of where and how they formed, to say nothing of the
chemical and isotopic data that we've collected from analyses of
samples. These aren't finer details - you're stating that high
abundances of heavier elements for some reason existed in the Oort
Cloud/Kuiper Belt region.

You might as well state that CI-chondrites likely formed at the core
of externally CV-chondrite bodies.
To a layman, that might kind of make sense. CV-chondrites have more
metal in them and are generally more dense and depleted in volatiles
compared to CI-chondrites. But that's not how things worked in the
solar nebula. CV-chondrites formed a little closer to the sun, where
there was less volatile material available for incorporation (and more
heavier elements), and CI-chondrites generally formed farther from the
sun. And when those chondritic bodies formed, they were generally
homogeneous.

In order to get complex structures like the ones you're talking about
(core, "crust," etc.), you require differentiation.

There were no large chunks of rock out there upon which comets might
"nucleate." Comets formed at the same time as all of the other
smaller bodies in our solar system, and there were no large masses of
rock out there.

This is basic stuff.

http://adsabs.harvard.edu/full/1983A%26A...122..171Y

Comets *are* composed of some silicate material, because the volatiles
did nucleate about those silicate minerals. The trouble with your
theory is that silicates that far from the sun never accreted into
larger solid bodies, and the silicate cores I'm talking about (that
we've seen from comets) are measured in microns. That's the sort of
material that was accreting out there when comets were forming.
Theoretically you could try to say that bodies that formed in the
inner solar system could have been gravitationally flung out and into
the fringes of our solar system, where they could slowly accrete icy
shells of their own, but I'm pretty sure that you're not trying to
suggest such a process.

> If Tunguska can be associated with Comet Encke, then the spherules
> there might be some indication, but only of what's left of that
> particular comet.

We don't even know if the Tunguska impactor was related to a comet in
any way. It could have been asteroidal. To say that the Tunguska
body is associated with Comet Encke is akin to saying that Mazapil is
associated with the Andromedid meteor shower. The evidence is
circumstantial, at best.

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-3T0T8W8-8&_user=10&_coverDate=03%2F31%2F1998&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1555037280&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=5a790f2b98b8acb2cdb55072b77d74c1&searchtype=a

And if the body was cometary, it says nothing of whether or not comets
have solid, rocky cores, since no sizable fragments have been
recovered and all of the spherules and microscopic fragments yet found
point towards the entirety of the body being composed of (assumed)
volatiles and microscopic siliceous grains.

> If I remember correctly, Moss Lake is also associated with Comet
> Encke.

I can only assume that you're referring to Lake Cheko. Those claims
were made and fairly promptly refuted by a number of sources - this
gives you the basics.

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3121.2008.00791.x/full

> Aside from that, there is the "KT fossil" meteorite.

The fragment found by Frank T. Kyte has been determined likely to be a
fragment of a carbonaceous chondrite. Which says nothing about
whether or not it actually came from the K-T impactor, or if the
impactor was a comet. Meteorites fall all of the time - they were
falling when the K-T boundary layer was forming as well. It could
well be a piece of an unrelated meteorite.

But since most recent studies have determined that the K-T impactor
was likely a carbonaceous chondrite (not a comet), your argument here
is kind of unrelated.

http://www.sciencemag.org/content/282/5390/927.abstract

>There are several large circular holes in the surface of the Earth,
>surrounded by impactites, which have not been fully examined yet and
>which are likely to also contain comet samples.

You'll have to forgive me if I don't take your word for the existence
of craters or samples of comets that can't be verified by real
academics...

> In any case, we're likely soon to learn a whole lot more about Comet
> Schwassmann Wachmann 3, with very large (and very fast) samples
> possible in 2022.

I found quite a few articles referring to the comet and its break-up,
but nothing stating that there would be any possibility of fragments
coming much closer than seven to ten million miles from earth. There
have been talks of increased meteor shower activity due to it, but
that's because the solar wind has a much greater effect on smaller
particles over shorter periods of time.

I just don't understand where you're getting these ideas from. What
you're saying that doesn't outright disagree with accepted science is
at best conjecture.

Regards,
Jason
Received on Thu 25 Nov 2010 04:22:08 AM PST