[meteorite-list] hot vs. cold meteorite falls

[Finbarr Connolly]

One of the eye-witnesses to Barwell, Joseph Grewcock -

https://www.bbc.com/news/av/magazine-35054625

described the meteorite as being 'red-hot'. He originally thought it had
fallen off a lorry, so had no reason to be 'psychologically tricked'. The
number of similar reports connected to other falls, makes me believe you
are correct.

Regards,
Finbarr.

On Mon, Mar 22, 2021 at 9:30 PM Eric Christensen via Meteorite-list <
meteorite-list at meteoritecentral.com> wrote:

> There was a recent discussion on a social media forum about a stone from
> the recent Punggur fall being warm enough on impact to melt a synthetic
> bedsheet.  I followed the discussion with interest but don't have an
> account on that platform - so wanted to post here.  The original poster
> also referenced the other recent Indonesian fall (Kolang), where a finder
> reported the stone felt as if it had been "cooked with sunlight".  There
> are many other references to freshly fallen meteorites being warm or hot to
> the touch, or sometimes cold to the touch.  The oft-repeated rebuttal is
> that meteoroids come from the icy void of space where they must be
> extremely cold, and that any brief heating experienced during the luminous
> ablative phase will dissipate during the few minutes of dark flight through
> the atmosphere.  Also, that the human brain will trick surprised finders
> into misinterpreting "very cold" for "very hot".  It seems to me that
> there's an obvious error in this argument - the initial condition of a
> meteoroid being very cold is not (necessarily) true.  In fact the opposite
> can be true - meteoroids (or asteroids) can actually be very hot prior to
> Earth impact.  "Cooked with sunlight" is an extremely good description.
>
> Consider figure 1 from Delbo and Harris "Physical properties of near-Earth
> asteroids from thermal infrared observations and thermal modeling",
> published in 2002 in MAPS:
>
> https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2002.tb01174.x
>
> The sunlight side of a model asteroid at 1 AU has a temperature of about
> 400 Kelvin = 127 C = 260 F.  The side facing away from the sun will be
> cooler; how much cooler will depend on the thermal inertia of the body,
> pole orientation, rotation speed, etc.  There may be steep temperature
> gradients across an asteroid at impact time, or it may be relatively
> equilibrated.  Most meteorite droppers should fall into the latter
> category, being small (sub-meter), fast rotators, and regolith free.
>
> How much heat is gained during ablation, and retained during dark flight,
> ought to depend on the thermal inertia of the meteorite.  Metal-rich
> meteorites or those with low porosity ought to retain more heat, and be
> less efficiently cooled during dark flight.
>
> So - are fresh meteorites hot or cold on impact?  I think the answer is,
> "it depends!".  One could even contrive a set of circumstances where an
> asteroid with a large thermal gradient drops two meteorites of equal sizes
> right next to each other, coming from different parts of the asteroid,
> where one lands hot and the other lands cold.  Tarp-melting hot?  I don't
> see why not.  Cold enough to form frost?  Sure.  Hot enough to ignite a
> grass fire?  No.
>
> Regards,
>
> Eric Christensen
>
>
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