[meteorite-list] Cometary Meteorites

From: Michael L Blood <mlblood_at_meteoritecentral.com>
Date: Sun May 30 13:38:52 2004
Message-ID: <BCDF6A1F.A700%mlblood_at_cox.net>

Hi Bernd and all,
        I had always heard Murchison was thought to be cometary material.
Does this (below - especially the last paragraph) indicate it has been ruled
out? (Inquiring minds [of limited intellect] want to know...)
        Best wishes, Michael



on 5/30/04 10:22 AM, bernd.pauli_at_paulinet.de at bernd.pauli@paulinet.de
wrote:

> Hello All,
>
> David wrote:
>
>> Material of cometary origin didn't get a mention.
>
> and:
>
>> ... is this thought to be because comets are largely composed
>> of volatile material - which seldom survives atmospheric entry?
>> David Entwistle
>
> Bob responded:
>
>> The answer to both of your questions is, "Yes!"
>
> In their invited review, H. Campins and T.D.Swindle conclude that comets
> do indeed yield macroscopic meteorites, which either have not been found
> or have not been recognized.
>
> The mineralogy of potential cometary meteorites would be dominated by
> highly unequilibrated anhydrous silicates with a nearly chondritic chemistry
> plus a high abundance of C and N.
>
> If an unknown process did produce extensive aqueous alteration in the
> meteoritic-cometary material, such meteorites they would resemble (or
> could even be) Cl carbonaceous chondrites.
>
> The authors do not expect cometary meteorites to have chondrules - nor CAIs.
>
> Cometary meteorites are most likely to come from the Kuiper belt, beyond
> 30 AU.
>
> Orbital considerations make the Oort Cloud comets (10 000 AU and beyond
> from the Sun) unlikely sources for meteorites.
>
> Asteroidal and cometary parent bodies capable of delivering meteorites
> to the Earth's surface would most likely have similar orbits.
>
> 3200 Phaethon has been identified as the parent body of the Geminid meteor
> shower
> and there are a number of arguments in favor of a cometary origin of 3200
> Phaethon.
>
> Cometary activity had been detected in minor planet 1979 on prediscovery
> plates
> taken in 1949. This is/was Comet Wilson-Harrington 1949 III and its
> identification
> (4015 W-H) confirms that some fraction of Earth-crossing asteroids have a
> cometary
> origin.
>
> The orbit of 4015 W-H orbit has the potential of delivering meteoroids to
> Earth at
> relatively low velocities.
>
> Although meteor showers are usually not accompanied by falls of meteorites,
> this
> is no valid argument against cometary meteorites. The entry velocity of most
> shower
> meteors is so high (~28 km/s) that even strong achondrite meteorites would not
> survive atmospheric entry.
>
> The fall of the Cl chondrite Revelstoke (and, of course, Tagish Lake) has the
> characteristics that the authors would expect for a "weak" cometary meteorite:
>
> - a fireball visually observed for hundreds of kilometers
> - atmospheric effects measured nearly 1500 kilometers away
> - less than a gram of friable black rock, dug out from one of several
> patches of dust-darkened snow on a frozen lake, was recovered
> - virtually all the material in the fireball was dispersed during atmospheric
> entry
> - Ni-rich magnetic dust was collected by dust collectors during the following
> few days.
>
> The Revelstoke fireball was as energetic as the Sikhote-Alin meteorite,
> which had left several craters, but, interestingly with regard to Tinguska,
> no craters were observed!
>
> Do we have cometary meteorites in our collections? As cometary meteorites
> are so primitive, they should have no chondrules. Now, there might be several
> such meteorites that are misclassified as achondrites. Are there any primitive
> achondrites with chondritic features? Yes, just think of acapulcoites,
> lodranites,
> brachinites, etc. ... but : these have igneous textures ... volcanism on
> comets?
> Ivuna and intense heat? ... No!
>
> The authors also state that *IF* cometary meteorites do contain chondrules,
> then C-rich, unequilibrated CO and CV, or ordinary chondrites might be good
> candidates:
>
> Mokoia (CV3), Colony (CO3.0), Kaba (CV3), Sharps (H3.4; gas-rich; xenolithic)
>
> The authors also examined xenoliths in certain chondrites and found a clast in
> the H4 Dimmitt regolith breccia that contains C-rich aggregates (up to 13 wt%
> C)
> with poorly graphitized C, magnetite, and anhydrous silicates (as expected for
> cometary material).
>
> Yes, I know ... everybody is waiting for Krymka (LL3.1). Of course, Krymka
> (and
> Supuhee, an H6 chondrite with similar exotic inclusions!) was one of the
> highly
> promising candidates for the authors. Not Krymka itself, but some rare,
> volatile-
> rich clasts with roughly chondritic chemistry (this led to the material in the
> clasts
> being referred to as "mysterite") and a fine-grained matrix identified as
> organic
> material. One Krymka clast was richer in noble gases than even CI chondrites !
>
> The authors conclude, and let me quote:
>
> We have not identified an individual meteorite that looks unequivocally
> cometary
> (i.e., none meet all the characteristics listed in Table 2), although some
> xenoliths
> in ordinary chondrite breccias come close. On the basis of studies of cometary
> fireballs, we should have collected approximately the same number of cometary
> meteorites as CI chondrites. In other words, given the rapid growth of the
> world's
> meteorite collections, we might be on the verge of collecting or identifying a
> cometary meteorite.
>
> Reference:
>
> CAMPINS H., SWINDLE T.D. (1998) Expected characteristics
> of cometary meteorites (MAPS 33-6, 1998, 1201-1211).
>
> *************************
>
> Best regards,
>
> Bernd
>
> To: bolidechaser_at_yahoo.com
> david_at_d-entwistle.fsnet.co.uk
> Cc: meteorite-list_at_meteoritecentral.com
> meteorobs_at_meteorobs.org
>
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Received on Sun 30 May 2004 01:38:23 PM PDT