[meteorite-list] Re: Radioactive Meteorites
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:06 2004
Meteorites: not likely to find any hot ones, but
Comets: uhhh... well,
In the condensing solar nebula, water ice begans to form at the
so-called "snow-line," the distance from the sun (or proto-sun) where
temperatures drop to a lower enough level for water vapor to condense to
ice at very low pressures. In our solar system, this was somewhere in
the outer asteroid belt but closer to the sun than Jupiter.
At near-zero partial pressures this temperature for water is about
160-165 degrees K (depending on the exact low pressure level).
By a curious coincidence, the low partial pressure condensation
temperature of the uranic oxides is around 162-167 degrees K. Now, we
all know ice condenses best when it has a little nuclei of dust or other
solid particle to trigger condensation and allow the ice particle to
grow larger and larger, like the seed that starts a pearl in an oyster.
Could there be a class of uranium-rich comets?
Because we are talking about the era of solar system formation, 4.5
billion years ago, the percentage of U-235 (the good stuff) isotope to
U-238 (the quiet stuff) isotope would have been about 25%, instead of
the 0.7% of natural uranium today (many half-lifes later).
O, yeah, the degree of U-235 enrichment required for uranium
particles distributed in water to maintain a self-sustaining chain
reaction is only about 2%.
So, in such a comet, the reactive particles would self gravitate
toward the center of the comet as their heat softened the ices around
them. Eventually, such a body would reach a state of cyclical stability
in which warming at its core caused the reactive materials to expand and
slow the reaction, thus cooling the core, which would reverse the cycle
until it switched back to warming, and so forth. This principle of
semi-self-regulation of chain reactions is often employed in human
designed nuclear reactors. (Not often enough...)
With such a high initial concentration of U-235, a reactive system
would over time transform itself into a breeder reactor, or one which
would create more fissionable material than it consumed, thus giving it
an effectively eternal working lifetime.
There are a number of comets that have been observed to have huge
bursts of outgassing even when they're too far away from the sun to be
warmed enough to outgas. There is no "standard" explanation for these
Comet Schwassmann-Wachmann 1, possibly the largest cometary object
in the solar system outside of Chiron, with an orbit between Jupiter and
Saturn, undergoes brightenings of 6-9 or even once10 magnitudes on an
irregular and unpredictable basis with formation of a coma, high
brightness, and outgassing, then fades back to a quiet asteroid-like
body. It has been observed to follow this pattern for many decades and
all attempts to find any periodicity or relation to solar radiation have
So, just in case you think they're aren't enough really weird
objects in the solar system, try envisioning a flying natural nuclear
reactor about 100 kilometers in diameter
Received on Tue 30 Jan 2001 01:05:00 AM PST