[meteorite-list] Re: Comet: Talking Points, #1

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun Jul 30 19:00:06 2006
Message-ID: <004201c6b42b$e1c62350$64704b44_at_ATARIENGINE>

Hi, All,

    I never re-posted on this because I thought the
List was tired of the haggling (although there's
lots worse on here, i.e., eBay pillory is colossally
boring).

    Marco's missing the point. Of course, he found
a discreet layer event; from the kind of event that
leaves a discreet layer = larger particles. Wonderful,
good for you, but not the point. Tautologically irrelevant.

    The Harvey paper (the abstract of which I have
added as an appendix below) is talking about a layer
of either impact spallation products or from a very low
airburst, most are about the size of a sand grain. Very
large, coarse particles, not cosmic dust inflow, again,
not what I was talking about. Of course they found
the layer; you could see it with the naked eye.
Completely irrelevant to the discussion.
    There are a dozen or more papers on the attempt
to date the adjacent tephra layers by their argon ratios
but the argon picture is confused by argon released
from melt inclusions and other factors. The dates
suggested for the impact event range from 8000 years
to 874,000 years with great uncertainty = who knows?
    Harvey says: "Although direct evidence of an extraterrestrial
origin for this debris layer (such as the presence of cosmogenic
10 Be and 26Al) has not yet been obtained, the available data
strongly suggest that this sediment originated as meteoritic
spallation debris. This debris is distinct from other Antarctic
'cosmic dust' collections by virtue of its uniform, recognizable,
ordinary chondrite composition and the consistent relation
shown between grain size and texture. The BIT-58 layer
probably originated from a single transient event, the passage
and/or impact of a single large meteorite over the East
Antarctic icesheet."
    Interesting, but NOT WHAT I WAS TALKING ABOUT.

    To review:
1.
Large particles (airburst, etc.) = sharp discreet layer.
Small particles = smeared "enriched" layer
Fine particles = too smeared out in time to be detected
conventionally, may appear as a background level.
Very fine particles = forget about it, especially if
the material is unexceptional.

2.
Supernovae dust very, very fine. Most of what falls is
50 to 500 microns (IDP's, cometary particles, etc.),
but supernovae "dust" much finer than that, some is
better measured in nanometers instead. Portions of
it is even too fine to reflect light well, but much reflects
incoming sunlight very well or re-radiates it as outbound
IR = heat loss. Takes many months or years or more
to fall out. Compositionally unexceptional save for
"odd" isotopes like Fe60 or the result of long processing
in interstellar space. The Earth (and we) are made
out of supernovae dust, you know?

3.
Whatever is the cause of these events (534 AD, etc.)
are NOT merely statistical excursions of "everyday"
events, but some OTHER phenomenon, since, as Marco
points out, they do not produce the same results as
merely "scaling up" familiar events. They may persist
for a century or few, then vanish from the record for
a millennium (or more or less). I proposed an encounter
with supernovae dust globules. Small long-suspended
reflective particles are the most dramatically effective
in cooling the Earth and leave the least detectible
geological trace behind.

    Marco says, "What we are talking about here is
a significant flux of large meteoroids entering our
atmosphere and creating airbursts (given the lack
of impact craters), if this theory is correct."
    This, of course is exactly what I was NOT
talking about, was in fact arguing against, but Marco
pays very little (no) attention to what others are
saying.

    Marco says, "Not just fine dust. Dust in the range
of a few micron to up to half a millimeter. The fine dust
capable of blocking sunlight by being airborn for a
long time, is only part of the equation."
    But, of course, I was talking about events that
would consist of fine dust only. That was the whole
point, ignored of course.

    As for a cometary cause, well, there's a true
dilemma regarding cometary impact. There MUST
be impacts of cometary as well as asteroidal (meteoritic)
objects, just as interplanetary dust contains cometary
as well as asteroidal (meteoritic) materials, but on the
other hand, no one has ever successfully identified
ANY event as cometary. (By "successfully," I mean
"conclusively.").
    This argues some huge deficit in our knowledge:
comets are not what we think they are (?!!), or for
some reason we are never impacted by cometary
material (?!!), or they leave no traces (?!!), all of
which evoke a huge, "No, no, no..." There's
SOMETHING missing, though. Is there a fine-size
component in comets? There's a lot we just don't
know about comets.
    More than half a century ago, Urey "proved"
that aggregate bodies (comets as flying gravel
banks) were dynamically impossible. Whipple's
dirty snowball was instantly and universally accepted.
Now, we've concluded that asteroid Itokawa is
a flying rubble pile very much like a flying gravel
pile...Say What?
    EVERY asteroid and comet that we've gotten
a first look at is different from all the others. This
strongly suggests to me that we've got a lot to learn.
And our ignorance keeps comets alive as a candidate
for all sorts of things.

    As for the state of the art in ice core analysis,
we don't simply dissect 1000's of feet of core atom
by atom to find everything in there. We go in and look
for what we believe we might find and to look for that
only. We analyze gas in bubbles and ignore everything
else, if that's what we're after. Somebody else may search
only for embedded particles and analyze only them. And
so on... You can't look at everything at once. And most
importantly, you DON'T FIND what you DON'T LOOK
FOR... That's the most basic selection effect of all.


Sterling K. Webb
-----------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <epgrondine_at_yahoo.com>
To: <marco.langbroek_at_wanadoo.nl>
Cc: <meteorite-list_at_meteoritecentral.com>
Sent: Sunday, July 30, 2006 11:19 AM
Subject: Re: [meteorite-list] Re: Comet: Talking Points, #1


> Hi Marco, all -
>
> The lack of results from the new Europen Greenland ice
> cores is disturbing, given the gross physical remains
> from impacts of fragments of Comet Encke, the
> contemporary text accounts of climate collapses, and
> the tree ring evidence.
>
> --- Marco Langbroek <marco.langbroek_at_wanadoo.nl>
> wrote:
>
>> Sterling, did you ever see a cosmic dust particle
>> under the microscope, let alone have you searched
> for them?
>>
>> I did. I searched for and found cosmic spherules in
>> sediment samples from an archaeological excavation.
>
> Which archeological excavation was that, Marco? What
> were the results?
>
>> What we are talking about here is a significant flux
>> of large meteoroids entering our atmosphere and
>> creating airbursts (given the lack of impact
>> craters), if this theory is correct.
>
> Actually, no atmospheric burst are necessary,
> particularly in the case of comets, where there
> already is a dust stream.
>
>> If the skies of AD 540 dayly resounded with thunder
>> from meteoric airbursts, the enhanced dust influx
> due > to it should be visible.
>
> Yes, it should, and it is - in the tree rings. The
> question now is why it is not in the ice cores.
>
> Could this be due to processsing errors, in other
> words errors in technique? They have reported that
> isotopes were being used to measure sunspot activity
> as well, and could this have thrown off the dust
> measurements?
>
> I am waiting for further information on how the ice
> cores were processed, and the release of the raw data.
>
> good hunting,
> EP
>
<MARCO'S ORIGINAL POST IN FULL: APPENDIX 1>

Sterling, did you ever see a cosmic dust particle under the microscope, let
alone have you searched for them?

I did. I searched for and found cosmic spherules in sediment samples from an
archaeological excavation. (you see: I like experimentation too. When the
results of the SEM investigation on one of the particles done by a friend of
mine who studies cosmic dust as a profession came in, I did not open a beer
as I
don't like beer, but a good bottle of wine)

What we are talking about here is a significant flux of large meteoroids
entering our atmosphere and creating airbursts (given the lack of impact
craters), if this theory is correct.

As they disintegrate in the atmosphere they enrich it with dust. Not
just fine dust. Dust in the range of a few micron to up to half a
millimeter.
The fine dust capable of blocking sunlight by being airborn for a long time,
is
only part of the equation.

And such events leave their detectable mark in lake deposits, dune deposits,
deep sea deposits, ice deposits, peat deposits.

Here is such a case of a detectable dust layer in Antarctic ice (camouflaged
by
abundant tephra layers in the same ice deposit, and still then it has been
found). And this one the researchers believe was due to one, only one, big
meteor event over the area:

- Harvey, R. P. et al., 1995: A Meteoritic Event Layer in Antarctic Ice.
Meteoritics 30:5, p. 517

<ABSTRACT OF THIS STUDY POSTED BELOW: APPENDIX 2>

If the skies of AD 540 dayly resounded with thunder from meteoric airbursts,
the
enhanced dust influx due to it should be visible. And cosmic origin dust,
due to
not only its isotopic but also its petrological signatures, is recognizable
as
such, nothwithstanding all your blah blah. One of my friends made a career
out
of it.

- Marco
>> Dr Marco Langbroek
>> Dutch Meteor Society (DMS)
>>
>> e-mail: meteorites_at_dmsweb.org
>> private website
>> http://home.wanadoo.nl/marco.langbroek
>> DMS website http://www.dmsweb.org
>> -----
>>
Abstract
- Harvey, R. P. et al., 1995: A Meteoritic Event Layer in Antarctic Ice.
Meteoritics 30:5, p. 517

Where the East Antarctic ice sheet meets the Transantarctic Mountains, old,
deep glacial ice is tilted upward and exposed.Within this visible
cross-section of the ice sheet, layers of dark volcanic tephra serve as
stratigraphic markers and datable age horizons [1,2]. Systematic sampling of
these layers at a well-known meteorite collection site (the Allan Hills Main
icefield) has revealed a band consisting of unusually dark and rounded
particles, many of which are spheroidal. This debris layer (BIT- 58) extends
parallel to the stratigraphy of the ice established from the tephra bands,
and thus apparently marks a single depositional event. Several kg of ice
from two sites along this band were subsequently collected and melted,
yielding a few grams of sediment for further study. Microscopic examination
of sieved samples reveals that roughly 95% of the particles consist of a
singular olivine-rich hyaloclastic litholo gy; more that 40% of these are
spheroidal. The remaining 5% of the sediment consists of grains derived from
local bedrock exposures. Particles range in size from sub-micrometer to over
100 micrometers in diameter, with a strong mode around 85 micrometers
suggesting sorting by aeolian processes. However, preservation of delicate
particle morphologies such as small parasitic spheres suggests that
saltation and/or abrasion was limited. A representative group of particles
was mounted in epoxy and sectioned for subsequent electron microprobe
analysis. All particles show a mixture of three dominant phases; euhedral
and/or skeletal olivine, an Fe-rich glass mesostasis, and abundant Fe-Ni
opaques (mostly metals and sulfides). There is a strong correlation between
particle shape and the size of olivine grains: angular particles contain
larger, more distinct (presumably relict) grains, while the most spheroidal
particles are so fine-grained they appear homogenous at this scale.
Defocused beam major-element analyses of spheroidal particles show good
agreement with bulk H chondrite composition (Table 1). Euhedral olivine
grains also correspond to typical H-chondrite composition with Mg-rich cores
around 17% Fa zoned to rims of 24% Fa near contact with Fe-rich glass.
Opaques include some relatively exotic Ni-rich phases, such as a Ni3S2 /
gamma Ni,Fe assemblage. Although direct evidence of an extraterrestrial
origin for this debris layer (such as the presence of cosmogenic 10Be and
26Al ) has not yet been obtained, the available data strongly suggest that
this sediment originated as meteoritic spallation debris. This debris is
distinct from other Antarctic "cosmic dust" collections by virtue of its
uniform, recognizable ordinary chondrite composition and the consistent
relation shown between grain- size and texture. The BIT-58 layer probably
originated from a single transient event, the passage and/or impact of a
single large meteorite over the East Antarctic ice sheet. Ar-Ar dating of
the tephra layers that bracket the BIT-58 layer should yield a
well-constrained age for this event. References: [1] Dunbar N. W. et al.
(1995) Abstracts for IUGG XXI General Assembly, in press. [2] Dunbar N. W.
et al. (1995) Intl. Symp. Antarc. Earth Sciences VII, in press. [3]
Jarosewich E. (1990) Meteoritics, 25, 323-338. Table 1 shows a comparison
between average bulk major element composition of debris layer spherules and
H chondrite falls. +/- values represent sample standard deviation.
Received on Sun 30 Jul 2006 06:59:58 PM PDT