[meteorite-list] Cali chondrite fell extremely cold!

[Chris Peterson]

> We all agreed that meteorites
> will be cold (or at least very cool) when they land. The effects of
> heating due to entry and fusion crust formation should be "gone" in 
> tens
> of seconds (while they are still falling). So their temperature on the
> ground should reflect their ambient temperature in space.

Hi Larry-

While I think the conclusion is correct, the reasoning is too simple.

A typical meteorite-producing meteoroid (let's call that an object not 
more then a few meters across, and probably much less) will have spent a 
considerable length of time at about 1 A.U.- weeks if in a highly 
eccentric orbit, and months or years otherwise. It will have reached an 
equilibrium temperature associated with 1 A.U. That temperature is easy 
enough to calculate. While there's no way of knowing just what the 
surface characteristics were like for the parent of any given meteorite, 
we can use the material properties of meteorites, or the measured 
optical properties of asteroids, to come up with a reasonable range, 
from which we calculate a temperature on the order of 30°C ± 50°C 
(cooler for stones, warmer for irons). Not surprising, since that's not 
far from the range for a rock sitting on the surface of the Earth.

That's not the whole picture, however. Most meteorites are small 
objects, measured in centimeters and 10s or 100s of grams. They normally 
become small while still quite high, and then spend several minutes 
falling through ~-40°C air at 50 or 100 m/s. That kind of airflow 
results in extremely efficient convective heat transfer. I think that in 
the case of most small meteorites (the sort people are likely to pick up 
immediately after a fall), the passage through the atmosphere has 
actually cooled the object down below what it was in space.

Run that around during your next asteroid lunch and let me know what you 
all think.

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: <lebofsky at lpl.arizona.edu>
To: "Charlie Devine" <moonrock25 at webtv.net>
Cc: "Robert Woolard" <meteoritefinder at yahoo.com>; 
<meteorite-list at meteoritecentral.com>
Sent: Monday, July 30, 2007 5:07 PM
Subject: Re: [meteorite-list] Cali chondrite fell extremely cold!


> Hi All:
>
> My two cents.
>
> We had are usual "asteroid lunch" today (discuss them and other 
> related
> topics, not eat them). I brought up this topic. Among others there was 
> me,
> an asteroid person (used to do them modeling of asteroids and, yes, 
> they
> are cold inside since the "typical" asteroid probably spends much of 
> its
> time further from the Sun than the Earth); a meteorite person, Tim
> Swindle; and an impact person, Jay Melosh. We all agreed that 
> meteorites
> will be cold (or at least very cool) when they land. The effects of
> heating due to entry and fusion crust formation should be "gone" in 
> tens
> of seconds (while they are still falling). So their temperature on the
> ground should reflect their ambient temperature in space.
>
> By the way, they had never heard of the (very good) suggestion that a
> person touching a very cold meteorite may actually think that the 
> burning
> sensation is heat and not cold.
>
> Larry