[meteorite-list] COMETS AND CARBONACEOUS CHONDRITES

From: Larry Lebofsky <lebofsky_at_meteoritecentral.com>
Date: Thu Sep 21 22:12:13 2006
Message-ID: <1158891129.45134679c5545_at_hindmost.LPL.Arizona.EDU>

Hi Sterling:

Not a bad summary. However, do not know where you got the "heated above 50
absolute." Much too low. Just being in an orbit that takes them near the Earth
would warm them up to 100 c or so. Some clearly have not been heated much
above that, but at the same time, since they contain water of hydration, they
had to be warm enough to have had liquid water (clays are an alteration
product).

Until the Spitzer observations of Deep Impact, it was thought by many people
(but not all) that one would not find hydrated silicates in comets (too cold).
There is still some question about the Spitzer observations, but have not seen
anything is the Lunar and Planetary Science Conference last March.

Larry

Quoting "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>:

> Hi, E.P.,
>
>
> The truth is we really don't know what comets
> and asteroids actually are, or whether there's a real
> distinction between them, or if they are just keywords
> derived (mistakenly) from the two extremes of a
> continuous spectrum of bodies with every intermediate
> state fully represented.
>
> There are "comets" that "die" and turn into
> "asteroids," and there are "asteroids" that suddenly
> develop a coma and become "comets." But the
> two terms may not be a descriptions of two
> essentially different classes of bodies at all. After
> we sample and/or visit 50 or 100 of them, we'll have
> a much better idea...
>
> The association of carbonaceous chondrites with
> "comets" is supposed by many, but not ever demonstrated.
> No meteorite has ever been definitively linked to a comet.
> There are no "known" samples of cometary material. (We
> may have it, but if we do, we don't know it...) On the
> chance that CC's may be linked to cometary material
> or be similar to it...
>
> Here's a summary on Carbonaceous Chondrites
> (quickly ripped from the Net, not my data-leaky
> brain). The metal content runs from 50% for
> Bencubbinites, 15% for CH type, down to about
> 1% for other classes. Some classes have clearly
> never been warmed about 50 degrees absolute;
> some people have suggested that the CH class
> formed intra-Mercurially. Obviously, all carbon
> containing meteorites didn't start out in the same
> single nursery! Another indicator that the heresy
> that the early system was very well stirred might
> be true.
>
> Carbonaceous chondrites account for about
> 3% of all known chondrites. They are classified
> according to the proportion and size of the chondrules
> they contain (one rare subclass lacks chondrules).
> The average contents of CC's are: Carbon, 2.0%;
> Metals, 1.8%; Nitrogen, 0.2%; Silicates, 83.0%;
> Water, 11.0%. At most, they can be 20% water and
> can contain as much as 4% carbon. Carbonaceous
> Chondrites contain around 5% kerogen.
>
> The sub-classes are:
>
> CI chondrites, only a handful of which are known, are
> named for the Ivuna meteorite. They have very few
> chondrules and are composed mostly of crumbly,
> fine-grained material that has been changed a lot by
> exposure to water on the parent asteroid. As a result
> of this aqueous alteration, CI chondrites contain up
> to 20% water in addition to various minerals altered
> in the presence of water, such as clay-like hydrous
> phyllosilicates and iron oxide in the form of magnetite.
> They also harbor organic matter, including polycyclic
> aromatic hydrocarbons (PAHs) and amino acids,
> which makes them important in the search for clues
> to the origin of life in the universe. It remains uncertain
> whether they once had chondrules and refractory
> inclusions that were later destroyed during the formation
> of hydrous minerals, or they lacked chondrules from
> the outset. CIs have never been heated above 50?C,
> indicating that they came from the outer part of the
> solar nebula. They are especially interesting because
> their chemical compositions, with the exception of
> hydrogen and helium, closely resemble that of the
> Sun's photosphere. They thus have the most primitive
> compositions of any meteorites and are often used as
> a standard for gauging how much chemical fractionation
> has been experienced by materials formed throughout
> the solar system.
>
> CM chondrites are named for the Mighei meteorite
> that fell in Mykolaiv province, Ukraine, in 1889.They
> contain small chondrules (typically 0.1 to 0.3 mm in
> diameter) and similar-sized refractory inclusions.
> They also show less aqueous alteration than, and
> about half the water content of, CI chondrites. Like
> CIs, however, they contain a wealth of organic material -
> more than 230 different amino acids in the case of the
> famous Murchison meteorite. Comparisons of
> reflectance spectra point to the asteroid 19 Fortuna
> or, possibly, the largest asteroid, 1 Ceres, as
> candidate parent bodies.
>
> CV chondites are named for the Vigarano meteorite
> that fell in Italy in 1910. They resemble ordinary
> chondrites and have large, well-defined chondrules
> of magnesium-rich olivine, often surrounded by iron
> sulfide, in a dark-gray matrix of mainly iron-rich olivine.
> They also contain calcium-aluminum inclusions (CAIs) -
> the most ancient minerals known in the solar system -
> that typically make up more than 5% of the meteorite.
>
> CO chondrites are named for the Ornans meteorite
> that fell in France in 1868. They some similarities in
> composition and chemistry to the CV chondrites and
> may have formed with them in the same region of
> the early solar system. As in the CV group, CAIs
> are present but are commonly much smaller and
> spread more sparsely in the matrix. Also typical
> of COs are small inclusions of free metal, mostly
> nickel-iron, that appear as tiny flakes on the polished
> surfaces of fresh, unweathered samples.
>
> CK chondrites are named for the Karoonda meteorite
> that fell in Australia in 1930. They were initially thought
> to be members of the CV group but are now grouped
> separately since they differ in some respect from all
> other carbonaceous chondrites. Their dark gray or
> black coloration is due to a high percentage of
> magnetite dispersed in a matrix of dark silicates
> consisting of iron-rich olivine and pyroxene. This
> shows they formed under oxidizing conditions, yet
> there is no sign of aqueous alteration. Elemental
> abundances and oxygen isotopic signatures suggest
> that CKs are closely related to CO and CV types.
> Most CK chondrites contain large CAIs and some
> show shock veins that point to a violent impact history.
>
> CR chondrites are named for the Renazzo meteorite
> that fell in Italy in 1824. They are similar to CMs in
> that they contain hydrosilicates, traces of water, and
> magnetite. The main difference is that CRs contain
> reduced metal in the form of nickel-iron and iron
> sulfide that occurs in the black matrix as well as in
> the large chondrules that make up about 50% of the
> meteorites. A possible parent body is Pallas, the
> second largest asteroid. The CH and CB chondrites
> are so closely related to the CRs that all three groups
> may have come from the same parent or at least from
> the same region of the solar nebula.
>
> CH chondrites are named for their High metal content.
> They contain up to 15% nickel-iron by weight and are
> closely related in chemical composition to the CRs and
> CBs. They also show many fragmented chondrules,
> most of which, along with the less abundant CAIs, are
> very small. As with the CRs, the CHs contain some
> phyllosilicates and other traces of alteration by water.
> One theory suggests that the CHs formed very early
> in the solar system's history from the hot primordial
> nebula inside what is today the orbit of Mercury, later
> to be transported to outer, cooler regions of the nebula
> where they have been preserved to this day. Mercury
> may have formed from similar, metal-rich material, which
> would explain its high density and extraordinary large
> metal core.
>
> CB chondrites, also known as bencubbites, are named
> for the prototype found near Bencubbin, Australia, in
> 1930. Only a handful of these unusual meteorites are
> known. All are composed of more than 50% nickel-iron,
> together with highly reduced silicates and chondrules
> similar to those found in members of the CR group.
>
> C ungrouped chondrites (C UNGRs) fall outside the
> other groups and probably represent other parent
> bodies of carbonaceous chondrites or source regions
> of the primordial solar nebula.
>
>
> Sterling K. Webb
> ----------------------------------------------
> ----- Original Message -----
> From: "E.P. Grondine" <epgrondine_at_yahoo.com>
> To: <meteorite-list_at_meteoritecentral.com>
> Sent: Thursday, September 21, 2006 5:48 PM
> Subject: Re: [meteorite-list] 2003 EL61, IN PERSON
>
>
> > Hi Sterling -
> >
> > With Chiemgau under "challenge", the only evidence of
> > heavy elements in comets that I can easily point to is
> > the increased iridium at the KT boundary.
> >
> > I can't really comment on metals in carbonaceous
> > chondrite meteorites, and right now I would be most
> > interested in data from others on these.
> >
> > good hunting,
> > Ed
> >
>
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-- 
Dr. Larry A. Lebofsky
Senior Research Scientist
Co-editor, Meteorite                      "If you give a man a fish,   
Lunar and Planetary Laboratory               you feed him for a day.
1541 East University                       If you teach a man to fish,
University of Arizona                        you feed him for a lifetime."
Tucson, AZ 85721-0063                                     ~Chinese Proverb
Phone:  520-621-6947
FAX:    520-621-8364
e-mail: lebofsky_at_lpl.arizona.edu

Received on Thu 21 Sep 2006 10:12:09 PM PDT