[meteorite-list] hot vs. cold meteorite falls

[Eric Christensen]

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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