[meteorite-list] Hot vs. Cold again...

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Tue, 28 Jun 2016 21:10:02 -0700
Message-ID: <003601d1d1bc$1d22c5d0$57685170$_at_cox.net>

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
Received on Wed 29 Jun 2016 12:10:02 AM PDT