[meteorite-list] Grain Samples From Siberian Peat Bog Suggest Meteor Caused Tunguska Blast

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 11 Jun 2013 10:25:49 -0700 (PDT)
Message-ID: <201306111725.r5BHPnYn009127_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/rock-samples-suggest-meteor-caused-tunguska-blast-1.13163

Rock samples suggest meteor caused Tunguska blast

Grains from Siberian peat bog may be remnants of the biggest Earth
impact in recorded history.

Mark Peplow
Nature
10 June 2013

A meteor explosion in the atmosphere still seems the most likely cause
for the 1908 disaster that flattened a forest in Siberia - and a new
analysis of rock fragments appears to support that conclusion.

They came from outer space. Fragments of rock retrieved from a remote
corner of Siberia could help to settle an enduring mystery: the cause of
the Tunguska explosion.

On 30 June 1908, a powerful blast ripped open the sky near the
Podkamennaya Tunguska river in Russia and flattened more than 2,000
square kilometres of forest. Eyewitnesses described a large object
tearing through the atmosphere and exploding before reaching the ground,
sending a wave of intense heat racing across the countryside.

At an estimated 3 to 5 megatonnes of TNT equivalent, it was the biggest
impact event in recorded history. By comparison, the meteor that struck
the Russian region of Chelyabinsk earlier this year 'merely' packed 460
kilotonnes of TNT equivalent.

Numerous scientific expeditions failed to recover any fragments that
could be attributed conclusively to the object. Hundreds of microscopic
magnetic spheres have been found in the 1950s and 1960s in Tunguska soil
samples, but there is continuing debate about whether they are the
remnants of a vaporized meteor. "There's really not much out there, and
nothing that's definitively Tunguska," says Phil Bland, a meteorite
expert at Curtin University in Perth, Australia.

The lack of samples has allowed wild speculation about the cause of the
event, with some of the more esoteric explanations invoking antimatter
and black holes. But most geoscientists think that part of an asteroid,
or perhaps a comet, broke away and fell to Earth as a meteor.

Now, researchers led by Victor Kvasnytsya at the Institute of
Geochemistry, Mineralogy and Ore Formation of the National Academy of
Science of Ukraine in Kiev say that they have found a smoking gun. In
what Kvasnytsya describes as the most detailed analysis yet of any
candidate sample from the Tunguska event, the researchers conclude that
their fragments of rock - each less than 1 millimetre wide - came from
the iron-rich meteor that caused the blast. The study was published late
last month in Planetary and Space Science [1] .

"If these are Tunguska fragments, it could end any doubt that it was an
asteroid impact," says Gareth Collins, an Earth-impact researcher at
Imperial College London. "We would have convincing proof that this was
an extraterrestrial event, and it would rule out a comet."

Under pressure

Kvasnytsya says that Ukrainian scientist Mykola Kovalyukh, who died last
year, collected the fragments in 1978 from a peat bog close to the
epicentre of the blast. Research on the fragments in the years following
their discovery found that they contained a form of carbon called
lonsdaleite, which has a crystal structure somewhere between graphite
and diamond, and forms under extreme heat and pressure. But the grains
also contained less of the dense metal iridium than is typically found
in meteorites - the meteor fragments that are actually recovered on the
ground - so researchers had concluded that they were terrestrial rocks
altered by the impact. The findings, published in the 1980s in Russian,
went largely unnoticed by Western scientists at the time.

Kvasnytsya and his colleagues decided to take a closer look at the
fragments using a battery of modern analytical techniques. Transmission
electron microscopy showed that the carbon grains were finely veined
with iron-based minerals including troilite, schreibersite and the
iron-nickel alloy taenite. This patterning and combination of minerals
is very similar to that in other iron-rich meteorites. "The samples have
almost the entire set of characteristic minerals of diamond-bearing
meteorites," says Kvasnytsya.

"An iron-rich, stony asteroid fits with our understanding of Tunguska,"
says Collins. Over the past 20 years, several modelling efforts have
concluded that a stony asteroid was the only culprit that could have
produced the effects reported on the ground. [2][3] However, a small
 but significant minority of scientists still backs the comet
hypothesis, he adds.

Another one bites the dust

"They've got some interesting stuff here," but the team does not yet
have conclusive proof, says Bland. The low levels of iridium and osmium
in the samples are "a red flag" that raises doubts that the fragments
originated in an asteroid, he says, and the peat sediment in which the
samples were found has not been convincingly dated to 1908. "We get a
lot of meteorite material raining down on us all the time," adds Bland.
Without samples of adjacent peat layers for comparison, "it's hard to be
100% sure that you're not looking at that background".

Confirming the meteor claim so long after the event will not be easy.
Geoscientists will take some convincing, not least because the subject
attracts so much speculation. In May, a paper posted to the preprint
server arXiv [4] claiming to have found pebbles from the Tunguska
meteor was quickly dismissed by field specialists.

Kvasnytsya'steam hopes to do further tests on the grains, including
measuring the ratios of key isotopes of helium and xenon, which could
provide more evidence of the rocks' extraterrestrial origin.

Journal name:
    Nature
DOI:
    doi:10.1038/nature.2013.13163


  References <javascript:;>

   1. Kvasnytsya, V. /et al/. Planet. Space Sci.
      http://dx.doi.org/10.1016/j.pss.2013.05.003 (2013).

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   2. Chyba, C. F., Thomas, P. J. & Zahnle, K. J. Nature 361, 40???44 (1993).

          * Article <http://dx.doi.org/10.1038/361040a0>
          * ISI
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   3. Boslough, M. B. E. & Crawford, D. A. Int. J. Impact Eng. 35,
      1441???1448 (2008).

          * Article <http://dx.doi.org/10.1016/j.ijimpeng.2008.07.053>
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   4. Zlobin, A. E. Preprint available at http://arxiv.org/abs/1304.8070
      (2013).

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Received on Tue 11 Jun 2013 01:25:49 PM PDT