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(meteorobs) The Heating of Meteorites (long)



In the book Meteorites by F. Heide and F. Wlotzka it states:

"We are all familar with the resistance of air, particularly when we
experience higher velocities, such as traveling on a motorcycle. It depends on
the cross-sectional area of the moving body, but more importantly on the
velocity. Beyond the velocity of sound (330m/s) the air resistance increases
with the square of the velocity i.e., it increases by a factor of 4 with the
doubling of the velocity. Meteorites with higher entrance velocities,
therefore,  will be more strongly decelerated in the atmosphere than ones with
smaller velocities. The atmosphere has then an equalizing influence on the
different entrance velocities of meteorites.

Figure 12 (I'll scan it if anyone wants to see it) presents a plot of the
velocity and the height of the photographically determined path of the
Innisfree meteorite. The entrance velocity of 14.5km/s was essentially
constant for the first seconds as the meteorite decended from 70 to 35 km,
however, it was then rapidly reduced to 5 km/s in the following 15 km. At a
height of  about 20 km the meteorite had reached its (previously mentioned)
retardation point. Its cosmic velocity has been overcome, and the meteorite
continues to fall only under the influence of its weight and the resistance of
air. The uniform end velocity with which the meteorite decends is, when
compared to its entrance velocity, exceedlingly small. It is so small, in
fact, that the meteorite no longer gives off a luminous gas cloud and friction
no longer melts its surface. Light phenomena cease at the retardation point.
Observers who find themselves near the landing point of the meteorite see it
fallfrom the heavens as a dark body.

...Up to now there has never been a fire caused by an iron meteorite. By
investigating the interior structure of meteorites one can determine that most
of them could not have been strongly heated since their formation, certainly
not to  melting temperatures. Evidence for heating is found only in the few
milimeters thick 'heat affected zone' directly under the fusion crust. This is
due to the fact that the heating is very rapid and lasts only a short time.
Heat requires time to penetrate into the interior, and the heating period
lasts only a few seconds. The thin melt layer on the surface cools rapidly
during the decent and solidifies to a fim crust well before the meteorite
reaches the Earth."