[meteorite-list] Giant Asteroid Fragment Makes Impact

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu May 11 17:52:20 2006
Message-ID: <001501c67545$231304c0$d75ee146_at_ATARIENGINE>

Hi,

    The rational for survivor fragments of an impactor is
that they are from the far back side of the impactor. The
transformation of the impactor's mass from a solid to a
plasma proceeds from the "front" or impacting surface.
A shock wave from this explosive vaporization preceeds
the actual transformation, traveling at the impact speed
of the body plus the rate of expansion of the vaporization.
If this shock wave speed exceeds the speed of sound in
the impacting body, the shock wave will fracture, pulverize
and even vaporize (if it's fast enough) the body of the
impactor ahead of it.

    What helps a fragment survive a large impact?

    Well, it helps if the impact velocity is really slow, slow
for an asteroid, that is. A body that reaches near the
Earth just faster the Earth's escape velocity will reach
the ground at about 11,500 m/sec. That's as slow as
you can get. If you make the impactor have a low internal
speed of sound, it will fracture more rapidly. It also helps
if you make the impactor elongated instead of spherical,
so that the far back tip has a chance of being blown off.

    In other words, you can jigger the models to make it
happen. But when you do, you immediately run into huge
problems that nobody likes.

    When you slow the impactor down to these low impact
velocities, below 15,000 m/sec, little vaporization occurs
and no fireball is created. Worse, little shock melting of
the target material occurs. But his lump was found 700
meters down in the magma! Sounds there was plenty of
melting going on... There's a problem.

    But Morokwong is a buried crater, not visible on the
surface. It is in fact only visualized by magnetic and
gravitational anomalies. It will need a lot more drilling
to establish the thickness of the magma layer the fragment
was in. But there is a lot of melt there; that's how it's been
dated to 145+/-1 million years by U/Pb ratios.

    The models say the transient crater is deep, but it would
shallow up dramatically from rebound and ends up as
an extremely shallow crater for its size. If there was
little shock melting, is it possible that rebound melting
occured? Or the release of local vulcanism? I don't know
if we know enough about this crater to be sure.

    You get slow internal speed of sound from low density
and poor consolidation. I don't know the density of the
fragment that was found but they mention the lack of
nickel-iron metal, so low density seems possible. But if
you have both low density and a slow impact velocity,
it takes a really big impactor to make a crater the size
mentioned for Morokwong (one press release says 70
km.; one says 100 km.). I found one study of core samples
that says the crater has to be less tham 80 km. This would
still require an 11,000 to 12,000 meter object at very low
densities and impact speed. "Normal" densities and
velocities for the impactor would make only a 4000
meter impactor necessary. A fast impacting iron (ruled
out by the fragment) would only have to be 2000 meters.

    I think the shock wave in a large slow impact in a
really big impactor might blow off some fragments.
The chunk flies up (luckily) on an almost-vertical
trajectory and drops back into the crater a few
minutes later and is enveloped in the rising magma.
Plop.

    The problem with models is that we can only test
their limits, not their accuracy. When you hear that
such and such a crater was made a certain kind, size
and speed of impactor, that only means that it fits inside
the envelope of the models, modified by what few clues
we can find on the ground, like extensive shocking that
suggests a high velocity... It doesn't mean we have a
measurement of the impactor.

    If only we had a 100 km crater that was only a million
years old (or less)! We could learn a lot about big impacts
from that! However, all the relevitely new ones are little
and all the big ones are old. The biggest newish ones are:
Zhamanshin, Kazakhstan Location: 48?24'N, 60?58'E
Diameter: 13.500 km Age: 900,000 +/- 100,000 years,
Bosumtwi, Ghana Location: 6?32'N, 1?25'W Diameter:
10.500 km Age: 1.30 +/- 0.2 million years. Both are
poorly investigated. Maybe there's something to be
learned there? There's definitely a lot to be learned
from this fragment!

    And lastly, there's no doubt what would be the most
expensive L chondrite of all time, though. Morokwong.
Be a while before we see it on eBay...


Sterling K. Webb
-------------------------------------------------------------
----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Thursday, May 11, 2006 12:26 PM
Subject: [meteorite-list] Giant Asteroid Fragment Makes Impact


>
>
> Public Relations Office
> Cardiff University
> Cardiff, U.K.
>
> May 11, 2006
>
> Giant asteroid fragment makes impact
>
> A first-ever discovery of a fragment from a giant meteorite which
> crashed to Earth millions of years ago could cause a re-think about
> asteroid collisions with our planet.
>
> Dr Iain McDonald of the School of Earth Ocean & Planetary Sciences was
> among the international research team who identified the 25cm sized
> fragment, found in a frozen magma pool at the bottom of the giant
> Morokweng crater in South Africa. The unique discovery, which has just
> been published in Nature, gives a direct insight into what was happening
> in the solar system 144 million years ago.
>
> The researchers found the fossilised meteorite fragment 766m below the
> surface whilst helping a company searching for copper and nickel in the
> giant Morokwong crater in South Africa. The international team comprised
> researchers from South Africa, America, Canada and the Universities of
> Cardiff and Glasgow.
>
> Dr McDonald, who led the UK component of the research team, said: "This
> was a huge stroke of luck, as had the borehole been sited just a metre
> away, it may have missed the object altogether. For the first time it is
> possible to hold in your hand an actual piece of a giant asteroid that
> hit the Earth. This intact fragment may tell us a lot more about the
> insides of asteroids than we currently know."
>
> Scientists have long believed that large asteroids or comets are
> obliterated by the enormous temperatures created when they collide with
> the Earth. Smaller impact craters of less than 4 kilometers diameter
> have been found to contain meteorite fragments. It was thought that any
> asteroid generating a crater larger than 4 kilometres in diameter would
> be completely destroyed, but the new discovery challenges that view.
>
> Morokweng is a very large crater of 70 kilometres diameter, and the
> fragment's survival suggests the asteroid struck the Earth at a lower
> speed than has been assumed in the past.
>
> Dr McDonald, who analysed the composition of the fragment, revealed a
> further twist to the story of the meteorite, of a type called an
> ordinary chondrite.
>
> He said: "Morokweng is no run of the mill meteorite. It shows some
> striking differences when compared with other known meteorites, such as
> the absence of iron-nickel metal. It appears that the Morokweng
> meteorite may have come from a very different part of the parent
> asteroid than other ordinary chondrites which currently fall to earth."
>
> To highlight its unique status, fragments of the asteroid have gone on
> display at the Science Museum's Antenna news gallery from Thursday 11th
> May.
>
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Received on Thu 11 May 2006 05:52:01 PM PDT