Agree with Craig.. The simpler tests
using a drop of milk in a puddle produced much better information. You could
see the concentric rings being created (complex ring structure crater) as the
central mass sank, upwelled, and sank again. By using the fluid dynamics math
on the liquid, and extrapolating to the fluid effects that sand and rock would
have, you could understand how the concentric rings were frozen in time, when
the energy level dropped low enough so that the material stopped behaving like
a fluid, thus producing the multiple rings in the crater, and a domed central
uplift in the exact center.. I do not see this behavior modeled well enough in
this new test, to call in unique.
CharlyV
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED]
Sent: Tuesday, June 29, 2004 6:38
PM
To:
[EMAIL PROTECTED]
Cc: [EMAIL PROTECTED]
Subject: [meteorite-list]
Extraterrestrial Impact Recreated in the Laboratory
>>??????
>>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."
>
