[meteorite-list] Extraterrestrial Impact Recreated in the Laboratory

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Tue Jun 29 18:38:28 2004
Message-ID: <36.5beb6243.2e13495f_at_aol.com>

>>??????

>>I've seen video of this type of test being performeded numerous times on
>>various TV programs on places like The Discovery Channel, etc.

>>So unless I missed something here, what was so special about this test?
>>Craig

Hola Craig, I think the "special" aspects you could be missing is that, the
"test" is far more rigorous than a video on The Discovery Channel! It seems
that the Dutch group is quite experienced with integrating mathematical
"singularity" events into coherent explanations, which bridge observations to theory.
Like asking "how does a bubble really pop and can we predict the volume or
pitches of the sound wave produced?". While most of us are happy modeling that
with some chewing gum or bubble bath, it sounds like these curious physicists
probably can get into the almost philosophical question of how the weakness
develops, at what limit it opens up, and where, and how that affects the
trajectories of the material that goes flying.

Keeping in mind that the holy grail of this sort of work ought to be
scalability:

In the impactor case of this study, they chose to model the surface of the
experiment impact site based on aerated (carefully fluffed) uniform sized
particles ("sand").
  The reason for this, it seems, was to create a "solid" to be impacted which
maximized the relative amount of energy the incoming iron impactor had. In
other words, rather than create a super accelerated Hulk smashing projectile
hitting compacted (higher stored potential energy) earth, in which events are
apparently experimentally much more difficult and less reliable to set up on
reasonable timescales and energies, it was easier to create a situation where
the impact site had a lot less energy. They determined that the granulated
impacted materials act somewhat like water and could be mathematically explained
by common engineering fluid flow (Euler) equations, which reduced into a
special case called a Rayleigh-type equation.

This was an intriguing result, in the authors' opinions, as it is
theoretically scalable. And scalability is what all of these videos wish they could do.
But the fact remains - no one has ever been able to record a major impact
event, so a new scalable model has the potential to give us much insight. The
authors claimed, further, that a liquid (predictable) model is general not
accepted, and these impact events are too easily written off as not reproducible -
to random -

So being a nice combination of experimental-theoretical physicists, they more
rigorously developed the major ideas on the blackboard. It looks like their
major result was that the iron tunnels into the site, and after mathematically
modeling the tunnel created which collapses, and then solving for that point
of first contact in the collapse, they derived equations from this simple
experimental design which agreed with the observations of the creation of a
"splash" and more importantly two "jets": one forward into the tunnel, and the other
exactly reverse, which implications were not covered well in the post (see
next paragraph).

They then further derived what sort of patterns should be created ... and
scaled them up as permitted by their clever model. Their major result here was
that the peripheral splash didn't cause most of the material thrown out as
(tektites if you believe, or ejecta from earth to become earth meteorites). That
honor was shown to be from the reverse jet created upon the not turbulent
closing of the tunnel. There was a further observation not mentioned in the
initial abstract posted here, that I think is very significant. That is that for a
liquid like water, no matter what the entry angle, the resulting jet goes
vertical. But in this case, unlike water, it went exactly backward along the
entry trajectory.

Furthermore, they were able to characterize what layers (call them sediments)
ended up where. So the suggestion is that using this type of model to look
for different impactites as these predictions go could be a good tool to
understand what happened in the big time events.

I am sure I got some of it backwards, so I am copying this post to Dr. Detlef
Lohse, the first author of the work, who I hope can offer further comment on
the work for the meteorite collecting community. I think it is an excellent
approach, though my main question would be on the reasonability of the
assumption of using aerated sand. Could the compressibility of the chosen substrate
could create fluid-like behavior in something initially that was much less
compressible, based on the modeling?


Saludos
Doug Dawn
N. 25.4? W. 100.2?
Mexico


In a message dated 6/29/2004 12:50:03 PM Eastern Daylight Time, Ron Baalke <
baalke at zagami.jpl.nasa.gov> writes:

>
>
>http://physicsweb.org/article/news/8/6/12
>
>Extraterrestrial impact created in the lab
>Belle Dume
>Physics Web
>22 June 2004
>
>Scientists in the Netherlands have successfully recreated a
>small-scale meteoritic impact in the laboratory for the first
>time. The novel yet simple experiment, devised by Detlef Lohse
>and colleagues at the University of Twente, involves dropping a
>small steel ball onto the surface of a sand bed. The results
>could shed more light on the processes occurring during
>large-scale impacts on Earth and other planets in the solar
>system.
>
>Lohse and colleagues first prepared a sand bed, around 25 cm
>thick, from fine sand grains measuring on average 50 microns
>across. The sand was "decompactified" by blowing air through it
>and then allowed to settle in an extremely loose-packed structure,
>so that it essentially behaved like a fluid. Next, the scientists
>dropped a steel ball, with a diameter of 2.5 cm, onto the sand
>from various heights and angles while taking images with a
>high-speed digital camera.
>
>The Twente team observed a series of well-defined steps: on impact,
>sand is blown away in all directions to form a crown-shaped splash.
>The ball then penetrates the sand and creates a void, which then
>collapses under the influence of the hydrostatic-like pressure of
>the sand. This pressure subsequently ejects sand grains into the
>air to form jets (see figure). Using numerical simulations the
>scientists developed a theory to explain how the void collapsed.
>
>"We have shown that the impact of an object on loosely packed
>granular material can be well described by a simple, fluid
>dynamical continuum model. So in our system sand behaves like
>water!" team member Devaraj van der Meer told PhysicsWeb. "This
>is very surprising since it has often been argued that, in general,
>no continuum description of granular materials is possible," he
>added.
>
>"There is a striking similarity with the large-scale impact of
>meteors and other celestial objects on the surface of the Earth --
>for example the Chixulub impact crater in Yucatan, Mexico, thought
>to be responsible for the extinction of the dinosaurs -- and our
>experiment," said van der Meer. "Our scaled-down granular
>experiments under laboratory conditions possibly capture the
>essential features of these crucial events in the history of our
>planet."
>

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