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
---------------------------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <[email protected]>
To: <[email protected]>
Sent: Thursday, October 29, 2009 10:42 PM
Subject: [meteorite-list] Neutron freeing in large hypervelocity impacts
Hi Rob -
Yes, I have read through all that before, but the spike that gets to
me is that huge spike in the INTCAL98 data right around the time of
the Barringer impact.
I don't think the neutron release is related to what hits, or what is
hit, but rather just the total impact energy. I wonder what the big
ones like Chicxulub or Shiva or Zamanshin will show. If part of the
impact energy in the form of infra-red is concentrated in a small
enough region, then releases could occur.
Take the IR measured from Tunguska for example, then scale massively
and localize to points. Do we hit freeing energies?
Speaking of Beryllium, the protons released at the same time as the
neutrons should be causing spikes in 10Be as well.
In closing, I have been wrong before, and reserve the right to be
wrong both now and in the future.
E.P. Grondine
Man and Impact in the Americas
( a damn fine book, really, despite all its flaws)
"Matson, Robert D." <[email protected]>
Subject: Re: [meteorite-list] Odessa
E.P. wrote:
Take a look at the INTCAL98 14C calibration chart. Major spikes
appear
to map to impacts.
"Spikes" in the C14 calibration chart can be caused by a number of
factors (including measurement uncertainty/error). But the main cause
of
variability in the production rate of atmospheric C14 is simply
variation in the flux of cosmic rays. Cosmic ray intensity is
modulated by both the strength of the earth's magnetosphere and the
sun's solar wind, neither of which is constant.
"From the other side of the equation, atmospheric C12 is ALSO
modulated
by earthly processes (e.g. volcanic eruptions, ocean temperature
changes)which can produce regional anomalies in the samples used to
build the radiocarbon calibration curves.
"There is no evidence that large impacts can cause nuclear reactions
that
release neutrons. There isn't sufficient energy or fissionable
material,
so I have difficulty coming up with a mechanism which could cause a
large spike in neutrons. I suppose if an impactor had an anomolously
high beryllium content and it happened to hit an earth location with
rich
uranium deposits, then you could get a small neutron hiccup. But
siderites are very low in berrylium (< 10 parts per billion), so
that's a no-go on Odessa. Even chondrites typically have only a few
hundred parts per billion.
--Rob
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