Also important is to consider that the body in space may well have been
a good fraction of a meter (or more) across. But a meteorite producing
body didn't just ablate, it most likely fragments. And the small
fragments very, very rapidly drop below the speed necessary to sustain
ablation. So what we have left is fragments that are probably a bit
below freezing being blasted for several minutes with very cold air.
Small fragments means not much volume, but lots of surface area, so the
heat transfer is pretty efficient.
It's hard to imagine a scenario where a meteorite is warm on landing.
The interior will be cold, and the outer few millimeters might be near
ambient, simply because of the warmer air encountered over the last
minute or so of dark flight. But within a minute I'd expect the outside
to get colder again because of transfer to the cold interior.
Chris
*******************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
On 6/28/2016 10:10 PM, Rob Matson via Meteorite-list wrote:
Hi Elton,
Any body arriving from space is at least -60�c and closer to -120�c to -180�c
based on
some black body studies of asteroids-- IIRC
The temperature for a typical earth-crossing asteroid with a chondritic
composition is
actually likely to be warmer than this -- perhaps -20 C. Depends on how "black"
the
original meteoroid was. Equilibrium temperatures for irons are quite a bit
warmer.
The radiative cooling during dark flight is probably calculable and a missing
factor in
estimating the state of heat content upon landing.
Not just a missing factor -- perhaps the dominant factor. 3-5 seconds of
ablation is nothing
compared to 2-8 minutes of freefall through atmospheric temperatures as low as
-70 C. Basically you have a frozen, baked Alaska situation: pre-atmosphere, a
cold body
through and through. Then (in the case of non-irons), you expose this
low-thermal-
conductivity mass to a brief blast of extreme heat that boils off the exterior
almost as
fast as the heat can be conducted to the cold interior. Bur almost as soon as
it starts, it's
over. You have a thin crust of hot material surrounding the still ice-cold
interior. And for
the final act, you refreeze the exterior for a time period 20 to 100 times
longer than
the ablative phase. For stony meteorites, there just isn't enough time to raise
the
bulk temperature of the body.
So I disagree with this statement:
"An immediately-recovered, newly-fallen silicate/stony meteorite is
usually--but briefly
"hot/uncomfortably warm" to the touch. The rind is very hot but lacks much heat
reservoir."
As long as there is an extended period of freefall through the atmosphere (a
very
reasonable assumption for non-cratering events), atmospheric cooling will
always win out
for a stony meteorite. --Rob
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