[meteorite-list] COMETS AND CARBONACEOUS CHONDRITES

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu Sep 21 23:06:13 2006
Message-ID: <002b01c6ddf4$064790f0$cd2ce146_at_ATARIENGINE>

Hi, Larry, List, E.P.

    Just another brain-slip; should read 50 degrees C!
Hydration commences as nebular temperatures drop
below 120 degree C. Yeah, that's a warm and cuddly
50 C, not 50 K. Brr... At low pressures that's "steam,"
not "water," down to 160-165 K. after which it's "ice."
Like Mars, no ponds, no rivers, but oddly, the relative
humidity is always 100%. It's so muggy...

    I'm glad it was just a typo; means I don't have
to read chemistry... Normally, hydration means lots
of water, like magnesium sulfate brines on Europa
and Ganymede. Lots of meteorites have hydrated
minerals (amphibole), Martians (Nakhlites) have a
wide variety of hydrated minerals. E-class asteroids
have a spectral feature that has been interpreted as
hydrated minerals. And to modify the old saying,
"Where there's clay, there's water..."


Sterling K. Webb
------------------------------------------------------
----- Original Message -----
From: "Larry Lebofsky" <lebofsky_at_lpl.arizona.edu>
To: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>
Cc: "E.P. Grondine" <epgrondine_at_yahoo.com>;
<meteorite-list_at_meteoritecentral.com>
Sent: Thursday, September 21, 2006 9:12 PM
Subject: Re: [meteorite-list] COMETS AND CARBONACEOUS CHONDRITES


> 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
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Received on Thu 21 Sep 2006 11:05:56 PM PDT