[meteorite-list] Re: Earth Originating Meteorites

From: David Freeman <dfreeman_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:06 2004
Message-ID: <3A7445BB.EDEE5604_at_fascination.com>

Dear Kelly,
I was just sitting here thinking this same senario myself. Any thoughts of
what a vesuvian meteorite would look like? More of a melt-like igneous
Post sure stretched the wrinkles on my brain.
Best Wishes,
Dave F.

Kelly Webb wrote:

> Hi, All,
> Dave Freeman wrote:
> > It was evident to me that the coupling factor...cushioning of initial
> acceleration seemed to be a very > determining factor in the survival of
> the ejecta.
> There are basically two issues involved in getting chunks off a
> planet. One is accelerating the chunks gradually enough to keep from
> crushing, melting, or vaporizing them.
> Sheer acceleration is not a problem for a terrestrial rock. 1,000
> gee's -- enough reach escape velocity in 1.2 seconds -- is nothing to a
> good rock. Imagine you have a big set of identical stone blocks, one
> inch cubes. Could you stack them 1000 blocks high without bursting the
> bottom block? (Ignore that tricky balancing problem for now.) Sure.
> In the case of the Washington Monument, the tallest free-standing
> stone structure ever built, each bottom imaginary one-inch cube of stone
> has 6611 one-inch cubes of stone standing on top of it, a total static
> load equivalent to 6612 gee's (escape velocity in 1/6 second.). It's
> holding up pretty good.
> The second issue is getting the rock through the earth's atmosphere
> without melting or vaporizing it. The key there is atmospheric blowout.
> The current models say that a big enough impactor, when it vaporizes on
> impact, blows out a tunnel back through the atmosphere through which
> debris can escape without encountering significant air resistance.
> A big enough explosion can produce an ejection plume, no doubt about
> it. On March 1, 1954, the U.S. had a Pacific Ocean nuclear test
> officially called Castle Bravo. Back in the lab, the gadget was simply
> known as Shrimp, a boosted fission (fission-fusion-fission) device
> employing tritium and lithium-6 deuteride. Shrimp turned out, due to
> unforeseen factors, not to be a shrimp at all. Intended to be a couple
> of megatons (2 to 4) and small enough to be stuffed into a B-47, it ran
> away to over 15 megatons. In the films of the test, you can clearly see
> one and two ton chunks of coral reef (which started out directly UNDER
> the explosion) flying straight UP the ejection plume for 10 or 12 MILES
> until the plume failed.
> This is all pretty puny compared with a big impactor, which I
> believe pre-creates its own ejection plume. Take a one kilometer rock.
> It takes no more than 7 seconds to traverse the height of the
> atmosphere. Every 40 milliseconds or so, one cubic kilometer of
> atmosphere is displaced by the frontal surface of the rock. About 25% of
> the converted kinetic energy goes into soft x-rays with a range of one
> or two meters in air. The air is compressed 1000 to 5000 fold to a
> density almost as great as the rock of the impactor and is heated to
> 100,000 degrees K. or more. This is not the stuff we normally call air
> anymore, guys.
> It is now a plasma, more like what you'd find in the lower corona of
> the sun than anything on earth. The rolloff eddies roil it into toroidal
> shapes which generate ring-shaped magnetic fields which interact with
> the plasma to expand up to 50 to 70 times the diameter of the impactor
> until their expansion force is balanced by the atmospheric pressure
> outside. Inside, the atmospheric pressure is close to nil. If there were
> time, the plasma would radiate its energy away and cool until electrons
> and ions recombine and the "tunnel" would collapse. But the time scale
> here is only a few seconds.
> Then, the impactor does what impactors do, it impacts, vaporizes,
> and the plume races up the already largely evacuated blowout tunnel. The
> standard computer modelling of impacts shows this material reaching and
> exceeding the escape velocity of the planet with no difficulty. Since
> the highest velocity impact material is already a plasma itself at
> 50,000 to 80,000 K., it serves mostly to keep the blowout tunnel open by
> insuring that it doesn't cool off too quickly. Eventually cooler
> material will follow: debris, free stones, dust, gravel, and everything
> not absolutely screwed down real good. The last, coolest material,
> mostly atmosphere, finally collapses the blowout tunnel by cooling it
> down to below 10,000 K.
> So, that's my heretical idea. Rocks don't have to be BLOWN off
> planets; they can just as easily be virtually SUCKED off planets!
> Interestingly enough, this mechanism would be more effective in
> denser atmospheres, like Venus, than it would be in thinner atmospheres,
> like Mars. Which is why I hold out hope for somebody finding Venusian
> meteorites someday.
> Kelly Webb
Received on Sun 28 Jan 2001 11:15:56 AM PST

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