[meteorite-list] Temperature of meteorites

Wed Nov 24 09:03:18 EST 2010

Good morning list. This is actually a good topic for debate and is not so
easily answered by simple application of constants. There are several
variable factors here that require scrutiny before being able to determine
whether the temperature of any given meteorite would / could be hot upon
hitting the earth.

Thermal conductivity: This is dependent upon the composition of the
meteorite and specifically in the metallic percentage of the subject. The
internal temperature of a meteor during ablation would be relative to its
thermal conductivity, its size and its melting point. If the melting point
of an iron /nickel meteor is 1700 degrees, it would obtain a higher core
temperature than a stone of the same size with a melting point of 1200
degrees under the same flight conditions.

Elevation where ablation is discontinued: During ablation a meteor should be
heating through its dominant thermal conductivity. When ablation stops the
meteorite should begin to cool through the same conductivity process in
reverse. This relates to the amount of time any meteorite is exposed to the
effects of heating and cooling. 

Velocity: This factor is related to the rate of fall and is not necessarily
a constant as is commonly repeated. The meteor has a velocity when entering
our atmosphere. Atmospheric drag is imposed on the body but is influenced by
the type of fall (ie. Stabilized flight or tumbling) and by the body
becoming more streamlined (oriented) through the effects of ablation.
Comparing two falling objects, higher velocity occurs for greater weight and
lower drag coefficient. An oriented meteorite would have a lower drag
coefficient and therefore have a higher velocity than a non oriented
meteorite of the same weight. This is relative to both the heating and
cooling of any given meteorite because of the amount of time (exposure) to
these conditions.

Flight path of the fall in latitude: The influence here is related to both
air temperature and height of the Troposphere. The temperature of any given
altitude is directly related to the temperature at sea level at the same
latitude. Using a constant of -3 degrees for every 1000 feet rise in
elevation, it can be conceived that the temperature at 30,000 feet is
different depending upon the season and the location. The troposphere has a
lower ceiling over the poles than at the equator and the sea level
temperatures in these locations are very different which would produce very
different temperatures at 30,000 feet. Remember also as the meteorite gets
closer to earth the air temperature is rising in the Troposphere, again by
the same factor in reverse (+3degress per 1000 feet). Again the velocity of
the meteorite would determine the amount of time the body is heating or
cooling.

I think it is conceivable under the right circumstances that a meteorite may
be warm or hot to the touch when impacting earth. ( I have not built a model
using these factors but please feel free)

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Chris
Peterson
Sent: Tuesday, November 23, 2010 2:14 PM
To: Meteorite List
Subject: Re: [meteorite-list] Temperature of meteorites

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" <sterling_k_webb at sbcglobal.net>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Cc: "Chris Peterson" <clp at alumni.caltech.edu>; "Bernd Pauli" 
<bernd.pauli at paulinet.de>; "Larry Lebofsky" <lebofsky at lpl.arizona.edu>; 
<meteorite-list at meteoritecentral.com>
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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