It is, I think, very obvious to anybody with a reasonable understanding of
physics that we lack a good mechanism to explain how an impact- especially a
small one- can produce a lot of neutrons. There are, of course, possible
(but untested) suggestions, so I'm willing to allow for the small
possibility.
The point of my argument was not based on whether such a mechanism exists,
however, but on whether you can use the C14 calibration record to test this.
There are two very serious problems in doing this. The first is that we
don't actually have any well dated significant impacts to compare with the
dates in the C14 record. The second is that we have a cause-and-effect
issue: any impact large enough to produce enough neutrons (by whatever
hypothetical mechanism) to affect the global C14 levels is also big enough
to tweak the climate, which will also affect global C14 levels. So there's
really no way to tell if an impact associated spike or dip in the C14 record
is caused by neutrons or by some other effect.
The only way I can think of to test the neutron hypothesis would be to
actually go out and look for isotopic anomalies in meteorite and geological
samples. The C14 calibration record just isn't going to cut it.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "Sterling K. Webb" <sterling_k_w...@sbcglobal.net>
To: "E.P. Grondine" <epgrond...@yahoo.com>;
<meteorite-list@meteoritecentral.com>
Sent: Thursday, October 29, 2009 11:51 PM
Subject: Re: [meteorite-list] Neutron freeing in large hypervelocity impacts
E.P., Chris, Rob, List,
The problem is neutrons. "Difficulty coming up
with a mechanism which could cause a large
spike in neutrons," said Rob.
Neutrons, "free" neutrons that is, are produced
two ways. First, the nucleus of an atom can decide
to kick out a neutron and change its image (and
isotope). The energy of the evicted neutron varies
from one radioactive decay to another.
Some neutrons are released with a lot of energy;
others stroll along, obstructing joggers. If you think
I'm being whimsical, it's true. A so-called "thermal"
neutron moves about the speed of an old man in
carpet slippers.
But neutrons produced by neutron decay are
immune to the events of the world outside the
nucleus, so impact has nothing to do with them.
The other way of producing neutrons is called the
"spallation" method. Namely, whack an atom with
something, anything, real hard and knock a neutron
loose. Now, that sounds more like "impact," doesn't it?
A neutron can be "spalled off" by almost any particle
with enough energy to do the job. You can use electrons,
protons, muons, photons -- it really doesn't matter what
the hammer is made of, only how hard you whack the
nucleus.
So, the question of an impact (or an impactor) creating
neutrons (which will affect terrestrial isotope levels like
14C and 10Be) depends on mechanisms that can produce
energetic particles and are a product of the physical event
of the impact (and impactor).
Why do I keep throwing the impactor in there? Well,
think about a BIG object entering the atmosphere at
cosmic velocities (instead of a lousy 10-meter rock).
Say, a kilometer sphere of something (anything). The
leading area of that sphere has 31,415,926,536 square
centimeters and each and every square centimeter is
enveloped by a plasma that (unlike the re-entry plasma
of a small rock) can approach, achieve, or may exceed
50,000 degrees K.
At that temperature, a fair percentage of the plasma energy
is being emitted as X-rays. For about a meter "ahead" of
that plasma, the atmosphere is subject to x-ray photon
energies quite high enough to spall neutrons out of the
nuclei of atmospheric gasses and cause a cascade of nuclear
reactions and transmogrifications. (Even 20,000 or 30,000
degrees K is enough; anything over 15,000 K. will do.)
Small rocks never create that kind of heat, even at 40 km/s,
but a one kilometer object is essentially irresistible. Its
velocity is undiminished by the so-called "resistance" of the
atmosphere. Not only can the billions of quadrillions of air
molecules NOT get out of the way of that big s.o.b., their
frantic and chaotic attempt to wiggle free is exactly what
generates that high temperature plasma.
Now, if I wanted to spend all night curled up with a
calculator converting degrees K. to EV, estimating and
re-estimating x-ray production, I could -- nah, I couldn't.
Isn't what computers are for? Actually, Boslough's model
on the computers at Sandia predicts these high-temperature
plasmas, but I don't know if he calculated x-ray production
or its effect on the atmosphere or not... He calculated these
high-temperature plasmas in a small (34 meter) body, so
what would a 1000-meter body do? Considerably more...
http://www.lpi.usra.edu/meetings/lpsc1996/pdf/1068.pdf
"INTERACTING ATMOSPHERIC PLUMES FROM BOLIDE
SWARMS; M.B. Boslough and D.A. Crawford, Sandia National
Laboratories, Albuquerque, NM 87 185-0820"
Actually, a one-kilometer body would likely produce
a substantial isotopic productive effect if it merely
GRAZED the atmosphere good and deep. The final
impact also produces such plasmas but they are,
well, "quenched" by all the matter that envelopes
them and the temperatures thermalize downward
rapidly. It's possible that more isotope production
comes from the "entry" than the impact.
People suggested increases in carbon and beryllium
isotopes; my guess would be carbon isotopes (present in
the atmosphere) and not beryllium (not atmospheric).
We have nitrogen, oxygen, argon, carbon available in
the atmosphere (in decreasing order). Finding traces
of the decays is the problem. Carbon is only useful
because living things "fix" samples of carbon isotopes.
As for the continual variations in the carbon record,
we are only estimating which sources of variation in
radiocarbon isotopes account for which variations in
the record. If we are excluding a potential source
from consideration, naturally enough, it does not
"show up" in the record!
Whether it is possible to "filter out" abrupt events
and demonstrate this thesis of impacts producing
radiocarbon spikes, I cannot say. Willard Libby thought
he detected a "spike" from Tunguska, but his long-ago
analyis has been disputed (like everything else about
Tunguska).
Sterling K. Webb
______________________________________________
http://www.meteoritecentral.com
Meteorite-list mailing list
Meteorite-list@meteoritecentral.com
http://six.pairlist.net/mailman/listinfo/meteorite-list
