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

From: Chris Peterson <clp_at_meteoritecentral.com>
Date: Tue, 28 Jun 2016 22:48:18 -0600
Message-ID: <57735312.9030206_at_alumni.caltech.edu>

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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Received on Wed 29 Jun 2016 12:48:18 AM PDT