[meteorite-list] Cometary Meteorites

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Sun May 30 13:22:28 2004
Message-ID: <DIIE.000000270000224E_at_paulinet.de>

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

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Received on Sun 30 May 2004 01:22:26 PM PDT