Sterling- I think you underestimate the effect of convective heat transfer
during cold flight. A fist-sized meteorite might fall for a good three to
five minutes through -40°C air, at around 100 m/s. That is long enough for
the entire stone to equilibrate to that temperature. In the last minute or
so of flight it will generally be in warmer air, and will therefore start to
warm up- but probably not to equilibrium. The critical point here is that
the meteorite will not maintain an interior temperature similar to its
temperature in space. The exception would be a larger stone that remains
hypersonic to a lower height, and therefore spends less time in dark flight.
We don't really care what the temperature was for the parent's millions of
years in space. For any given distance from the Sun, it shouldn't take more
than a few days to reach equilibrium, and any meteorite can be assumed to
come from a parent that was at 1 AU for that long. So the only real variable
is emissivity.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "Sterling K. Webb" <[email protected]>
To: "Meteorite List" <[email protected]>
Cc: "Chris Peterson" <[email protected]>; "Bernd Pauli"
<[email protected]>; "Larry Lebofsky" <[email protected]>;
<[email protected]>
Sent: Tuesday, November 23, 2010 1:18 PM
Subject: Re: [meteorite-list] Temperature of meteorites
Some points for the debate:
The rapid flight through the atmosphere is very brief --
1-2 seconds. This is not much time to change the
temperature of the stone.
The rate at which the friction-generated heat is
transferred to the interior of the stone is determined
by the thermal conductivity of that rock, and rock's
thermal conductivity is very low, so low that virtually
none of the heat will affect temperatures deeper
than a few millimeters or a centimeter into the stone.
Most of that heat generated by friction on the outer
surface goes into melting rock which is then is removed
from the meteorite by on-going ablation. The molten
material stripped from the stone takes that heat with it
as it becomes the particles in the trail (which have their
own thermal evolution that does not affect the stone).
Only a small fraction is "wasted" by warming the stone
itself.
That said, thermal equilibrium of the stone is likely
achieved (or nearly) within a very short time once it
lands. Its temperature will be more-or-less whatever
it was before it encountered this obstructive planet.
Apart from some rough treatment of the surface, the
stone's temperature is the same as it always was.
So, what temperature WAS the meteoroid in the many
thousands or millions of years that it orbited the sun?
That depends on what its orbit was, or more precisely,
WHERE its orbit was and its emissivity and reflectivity
and so on. Take a look at the following chart of Meteoroid
Temperature vs. Solar Distance, supplied by MexicoDoug:
http://www.diogenite.com/met-temp.html
It is a model derived from fairly complete and reasonable
assumptions, which were discussed on this List long ago:
http://six.pairlist.net/pipermail/meteorite-list/2005-January/007521.html
This is the first of three parts; follow the links for #2 and
#3. Those with more factors to include are welcome to
refine the model, I'm sure.
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