[meteorite-list] Neutron freeing in large hypervelocity impacts

From: Chris Peterson <clp_at_meteoritecentral.com>
Date: Fri, 30 Oct 2009 08:36:26 -0600
Message-ID: <55948835F0564A4981286C3CA960A104_at_bellatrix>

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_webb at sbcglobal.net>
To: "E.P. Grondine" <epgrondine at yahoo.com>;
<meteorite-list at 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
Received on Fri 30 Oct 2009 10:36:26 AM PDT