[meteorite-list] Re: Earth Originating Meteorites
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:06 2004
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
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
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
Received on Sun 28 Jan 2001 03:08:06 AM PST