[meteorite-list] Clue to Antarctica Space Blast

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 5 Mar 2010 16:49:17 -0600
Message-ID: <09EE4FC09A584EB8A4ECB308F71FBBA6_at_ATARIENGINE2>

Hi, Paul, Greg, List

In February, 2006, SWRI published this paper in Science
announcing the discovery of an new recent asteroid family
associated with the asteroid 1270 Datura:
http://www.boulder.swri.edu/~bottke/Reprints/Nesvorny_Vok_Bottke_Science_2006_Datura_breakup.pdf

A summary of its conclusions:
    "The newly identified family is a group of six 1- to
2.5-km-diameter asteroids, whose orbits are very
tightly clustered near the inner main belt of the
~10-km-diameter object 1270 Datura...
    The result (of hundreds of numerical simulations)
shows that the Datura cluster is 450 +/- 50
thousand years (ky) old (Fig. 1), considerably
younger than other known asteroid families...
    We estimate that the disrupted parent body was ~15 km
in diameter. Apparently, a substantial fraction of the
parent body's mass was ejected to space as fragments
ranging in size down to micrometer-sized dust particles.
The production of these particles implies that the
Datura cluster may be a source of some of the
material in the circumsolar (zodiacal) dust cloud...
    We estimate that micrometer-sized Datura particles
migrate by radiation effects from 2.235 AU to 1 AU in
only ~2000 years. Therefore, a wave of micrometer-sized
Datura particles may have reached Earth only a few
thousand years after the formation of the Datura cluster.
Signs of this event may be found by analyzing tracers
of extraterrestrial dust in deep ocean sediments and
Antarctic ice cores."

I wonder if these researchers were following up on that
last suggestion? At any rate, the minimum amount of
dust found in these separated cores would require
(they say) 3,000,000,000 kg of dust to reach the ice surface.
The characteristics of the dust show it was not blown in,
but was deposited at one time from above.

This amount of dust is 100 times greater than the
"normal" annual cosmic dustfall for the entire planet.
This is too much to be accounted for by dust bands
drifting in from the orbits of the Datura family. No, it
would require a "chunk" to be delivered all at once.

The impactor would have to be bigger than this minimum
amount of 3 million tons. To account for this amount of dust
would require a sphere of silicate about 130-135 meters
in diameter. If the dust is found further afield in Antarctica
or if not all of the body reached the ground as dust (it never
does), you are probably talking about an impactor at least
a quarter-kilometer in diameter.

As an illustration of this problem, everyone seems to agree
that Tunguska was an airburst of an object that had to
weight at least 50,000 or 100,000 tons, and yet no convincing
traces of the "impactor" can be demonstrated. This suggests
that if three million tons of Datura-type dust reached the
ice surface, the "impactor" could have been much, much
larger than three million tons.

Interestingly, ALL the news stories are ignoring the most
amazing part of this story. There are TWO dust layers BOTH
found 2000-3000 km apart. One is at 481,000 years ago
and the other is at 434,000 years ago. Both produced big
traces at both locations. So, we are talking about TWO
big impactors in less than 50,000 years.

One of the questions worth asking when talking about
recent big impactors is "Anymore at home like you?"
If, as Bottke says, the parent body was about 15 km
and we have a 10 km body and six 1- to 2.5-km-diameter
asteroids surviving from its breakup, they account for
only 40% of the parent body. 60% of the original body
is unaccounted for.

Did all 60% get turned into dust? Or are there some
undetected sub-kilometer bodies left over? 60% of that
parent body is enough to make up 16,000 500-meter
bodies. Even if 90% was turned to dust, that leaves
10% or enough for 1600 500-meter bodies. 1600 such
bodies is more than enough potential impactors. So is
1% or 160 500-meter bodies. Of course, in the real
world there is a variety of sizes created by a collision,
but there seems to plenty of rocks to go around.

Two massive airbursts over Antarctica in 47,000 years
is pushing at the limits of shrugging things off as random
chance. Antarctica is a mere 2.5% of the Earth's land
surface. If two such bodies fell on it in a short window
of time, we are dealing with a "shower." Shouldn't
random chance suggest 40 times as many impacts
for the rest of the Earth in that same 47,000 years?

Has anyone gone into the freezer and pulled the old
Greenland ice cores for 400 to 500 kyears ago? Can this
type of analysis be performed on Greenland cores? They
appear to only go back 500,000 years, are less complete
at this age, and Greenland has only 14% of the area of
Antarctica. I can't find any studies that show cosmic
dust in the ice at that age (or that looked for it).

The discovery of a fresh asteroid family has generated
spectral studies of their surfaces, mostly to gauge the
degree of "space weathering." Here's the result of optical
spectroscopy using the large Gemini telescopes:
http://www.gemini.edu/node/11082

They say:
    "Optical spectroscopy from the Gemini telescopes has
revealed a relatively uncommon type of asteroid in the
main-belt for the first time... They found that spectra of
asteroids in the newly discovered Datura family have a
deep absorption feature near 0.8 microns which classifies
them as ?Q-type? asteroids. This spectral feature is produced
by silicate material, in particular olivine and pyroxene. Most
interestingly, the spectra of Q-type objects are well-matched
to the most common type of meteorite found on Earth
called an ordinary chondrite (OC)."

The full paper on the spectra is available at:
http://adsabs.harvard.edu/abs/2008A%26A...486L...9M

OC's are only "ordinary" because they're common in
this geological era. 75% of all current falls are OC's. There
is a logic to the suggestion that geologically recent falls
come from the most recent asteroidal breakups. Are today's
ordinary chondrites common because they all came from
the youngest families like Datura? Some think so. (The
Datura spectra is an almost perfect match for the OC
FAYETTEVILLE):
http://www.sciencedaily.com/releases/2008/07/080710103903.htm

This odd because Fayetteville is a breccia of two very
different materials, and there is lots of argument about
it. Perhaps it dates from the breakup and one of the
materials is that of the disrupting body and the other
is from the Datura family parent body (or Datura itself).
Nobody knows (yet).

It all makes me wonder if there is any other evidence of
numerous impacts elsewhere on Earth between 400,000
and 500,000 years ago?


Sterling K. Webb
-------------------------------------------------------------------------------
----- Original Message -----
From: "Paul Heinrich" <oxytropidoceras at cox.net>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Wednesday, March 03, 2010 9:57 PM
Subject: [meteorite-list] Clue to Antarctica Space Blast


Greg wrote:

?Interesting

http://news.bbc.co.uk/2/hi/science/nature/8547534.stm

Clues to Antarctica space blast By Paul Rincon
Science reporter, BBC News, The Woodlands, Texas?

The paper is:

Engrand, C. B. Narcisi, J. r. Petit, E. Dobrica, and J. Duprat,
2010, Isotopes of EPICA ? Dome C Extraterrestrial Dust Layers:
Constraints on the Nature of the Impactors. 41st Lunar and
Planetary Science Conference, held March 1-5, 2010 in The
Woodlands, Texas. LPI Contribution No. 1533, p.1981

http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1981.pdf
http://adsabs.harvard.edu/abs/2010LPI....41.1981E

Related papers are:

1. Misawa, K., M. Kohno, T. Tomiyama, T. Noguchi, T.
Nakamura, K. Nagao, T.Mikouchi, and K. Nishiizumi, 2010,
Two extraterrestrial dust horizons found in the Dome Fuji
ice core, East Antarctica. Earth and Planetary Science Letters.
vol. 289, no. 1-2, pp. 287-297.

http://dx.doi.org/10.1016/j.epsl.2009.11.016
http://www.ssl.berkeley.edu/cosmochem/publications.html

The PDF for the above paper can be downloaded from
the link for this paper at:

http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%235801%232010%23997109998%231578540%23FLA%23&_cdi=5801&_pubType=J&view=c&_auth=y&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=0377f75ed4c6eabaca0119b87f30f76c

2. Narcisi, B., J. Robert Petit, and B. Delmonte, nd, Extended
East Antarctic ice-core tephrostratigraphy Quaternary Science
Reviews, Article in Press, Corrected Proof

http://dx.doi.org/10.1016/j.quascirev.2009.07.009

3. Narcisi, B., J. R. Petit, and C. Engrand, 2007, First discovery
of meteoritic events in deep Antarctic (EPICA-Dome C) ice cores.
de la Recherche Scientifique & Universit? Paris Sud, Orsay, France)
Geophysical Research Letters. vol. 34, no. 15, CiteID L1550

http://adsabs.harvard.edu/abs/2007GeoRL..3415502N
http://www.agu.org/pubs/crossref/2007/2007GL030801.shtml

A summary paper about the icecores:

Wolff, E. W., C. Barbante. S. Becagli, and many, many others,
2010, Changes in environment over the last 800,000 years from
chemical analysis. Quaternary Science Reviews. vol. 29,
pp. 285?295.

http://www.climate.unibe.ch/~stocker/papers/wolff10qsr.pdf
http://www.climate.unibe.ch/~stocker/publications10.html

Yours,

Paul H.

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Received on Fri 05 Mar 2010 05:49:17 PM PST