[meteorite-list] What to look for if large impacts liberate neutrons - part 1 of 2

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sat, 29 Dec 2007 16:00:47 -0800
Message-ID: <GOEDJOCBMMEHLEFDHGMMEEFKDHAA.mojave_meteorites_at_cox.net>

Hi All,

While this subject is clearly related to meteorites, I suspect that
most list members are bored to tears over the topic -- possibly due
to the math, and also due to the emotional content of some of the
volleys that have been tossed back and forth. This is a shame,
because E.P.'s hypothesis, however farfetched, is a scientifically
interesting one, and more importantly, a testable one. You just
have to be a little creative in developing a good test.

I don't want to put words in E.P.'s mouth (so E.P. please correct
me where I'm wrong), but I think his idea goes something like
this:

In a large impact event, there may be sufficient energy available
(either kinetic or thermal) to alter [increase] the isotopic
abundance of carbon-14 in the atmosphere. If this is the case,
then carbon-dating of artifacts or other objects contemperaneous
with such an impact will result in computed ages which are much
younger than their true ages. E.P. suggests that thermal neutron
capture by atmospheric nitrogen would be the mechanism by which
C14 is boosted:

14 1 14 1
  N + n --> C + p
 7 0 6 1

This, by the way, is the same nuclear reaction that generates C14
"naturally": in this case the neutrons are byproducts of cosmic
ray bombardment.

Cosmic ray production of carbon-14 is not constant, but oscillates
with the solar cycle. When the sun is very active, fewer galactic
cosmic rays (GCRs) reach earth's atmosphere, and carbon-14 production
decreases. When the sun is quiescent (as it has been for the last
few years), the GCR rate goes up as does the amount of C14.

In addition to temporal variability, C14 production is also latitude
dependent (geomagnetic latitude). More than twice as much C14 is
produced at the poles than at the equator. However, since the C14
is in the form of atmospheric carbon dioxide, and CO2 is a well mixed
gas, it's not that important ~where~ on earth C14 is produced since
it soon averages out geographically.

This latter factor is important when considering the expected
effects from a sudden boost in C14 at a specific location. The
C14-enriched carbon dioxide wouldn't stay at the bolide impact
location -- it would get distributed globally over the course of
several weeks. So you shouldn't expect just a localized boost
in carbon-14 -- there should be global effects as well.

For solid objects containing nitrogen close to the impact location,
you might expect a small boost in C14, provided the mean free path
for fast neutrons is great enough to reach the ground. But if you
are trying to look for a nuclear-event signal in ground objects,
I wouldn't bother with carbon-14 -- there are far better "markers"
for such an event. (continued in part 2)
Received on Sat 29 Dec 2007 07:00:47 PM PST