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

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Mon Jul 31 09:56:12 2006
Message-ID: <20060731135549.60925.qmail_at_web36910.mail.mud.yahoo.com>

Hi Sterling, Marco, all -

I must be seeming like an incredible bore right now,
as I missed the first part of Sterling and Marco's
discussion, which appears to have been on super-nova
dust rather than cometary dust, and I hope I am not
tiring the list, but...

I just finished and have been out promoting a book on
impacts (both comet and asteroid) in the Americas:
"Man and Impact in the Americas", now available from
some bookstores through amazon. It covers the gross
evidence of impact. So...

As regards the processing of ice cores, the only
cometary markers I seem to recall (and this is
difficult since my stroke) were
1) an isotope of hydrogen which they used to identify
comets as the source of water on Mars - 3H?
2) whatever the hell signature isotopes the University
of Wurzburg team found in the Cheimgau comet core
fragments

My assumption is that this new ice core team would
have had to have put each of the ice core layers
through some kind of accelerator/collector bucket
device (huge electric bill!) to get their 3He
analysis.

As I mentioned before, I have been away, so please
excuse me for asking, but what is the date for the
BIT58 layer?

As Sterling points out, our solar system incorporated
matter from a nearby super-nova. Given the relatively
large size of the impact craters at Cheimgau, and the
small metallic fragments which were found there in
them, and these fragments isotopic analysis, I think
the U Wurzburg team actually managed to find and
demonstrate a comet impact and thus have comet core
samples.

It would appear that comets are generally composed of
aggregated cometissimals, each of which in turn have
been gravitationally differentiated, and have small
metallic cores.

good hunting,
Ed

--- "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>
wrote:

> 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.
>
>
>
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Received on Mon 31 Jul 2006 09:55:49 AM PDT