2
$\begingroup$

This answer to Would a gold-containing meteor cause it to “rain” gold? says (in part):

Meteors, when in space, are very very very very cold. Their travel duration in Earth's atmosphere is very short, just a couple of seconds. Actual melting only occurs at the very most outer layer of the meteorite. This is not sufficient to heat up the interior of the meteorite to any significant degree. In fact, some meteorite falls have found to have frost on them when found soon after landing, because they condense the moisture in the air around them!

The "very very very very" might be overstated, but let's see if they could be below the freezing point of water.

The surface temperature of an asteroid can be applied to roughly spherical objects of arbitrary size, there's no scale in the derivation; it should work for centimeter-sized objects as well as it would for a planet sized rocky object sans atmosphere.

$$T^4 = \frac{L_0 (1-A)}{\epsilon \sigma 16 \pi a^2 }$$

Here $L_0$ is the total solar luminosity of 3.827×1026 W and $\sigma$ is the Stefan-Boltsman constant 5.67×10-8 W/m2/K4. $a$ is the distance from the Sun, let's assume that the meteor has been near 1 AU long enough (weeks) to reach equilibrium temperature. $\epsilon$ is the asteroid's infra-red emissivity and a value of 0.9 is given as typical.

The ~visible light average albedo $A$ of asteroids is the big variable here. They are usually pretty dark but there is a lot of variation and measurements are often based on other assumptions. I'll calculate the equilibrium temperature for a few, but as one can guess 1 minus a small number is close to 1, and after you take the fourth root, it's very close to 1!

Albedo    Temperature
(vis)       K     C
 0.05     282     8
 0.10     278     5
 0.2      270    -3
 0.5      240    -33

So for a meteor to be even barely below the freezing point of water it would have to have an above-average albedo.

It seems to me that after tens of seconds to minutes there is not enough time for the transient heat pulse on the skin to do more than melt the surface, but the radiative heating from the plasma created for larger meteors can be quite intense, so they sometimes "explode" or breakup due to stress.

Nonetheless the core temperatures of larger meteorites reaching the surface could very well reflect the temperature in space.

So a meteorite becoming "frosty" soon after landing is plausible if it were of a higher than average albedo.

Question: Have "frosty meteorites" ever been observed soon after landing? Are there photos?


Related:


Asteroid albedos: graphs of data by Wm. Robert Johnston last updated 25 March 2017

above: From Asteroid albedos: graphs of data by Wm. Robert Johnston last updated 25 March 2017 below: From NYTimes' A 22-Million-Year Journey From the Asteroid Belt to Botswana "A fragment of the asteroid 2018 LA, found in the Central Kalahari Game Reserve in Botswana in 2018.Credit...P. Jenniskens/SETI Institute"

This is what got me thinking about this topic

A fragment of the asteroid 2018 LA, found in the Central Kalahari Game Reserve in Botswana in 2018.Credit...P. Jenniskens/SETI Institute

For comparison: from this answer to Who discovered “Egg Rock”? The Curiosity rover or people?

...Cropped section of image of "Egg Rock" from redplanet.asu.edu/?p=21047 showing the spots where Curiosity's ChemCam laser has ablated material.

Egg rock on Mars showing the spots where Curiosity's ChemCam laser has ablated material.

Spoiler alert:

People did!

$\endgroup$
6
  • 2
    $\begingroup$ Have you read this? $\endgroup$ – Gimelist Apr 30 at 10:26
  • 1
    $\begingroup$ @Gimelist have now! :-) $\endgroup$ – uhoh Apr 30 at 10:47
  • 1
    $\begingroup$ Something that would be required for a frosty or wet meteorite would be for the atmosphere at the landing site to be have a high humidity & for the dew point conditions to be satisfied. Any dew or frost that would form on the meteorite would be derived from the moisture in the atmosphere. $\endgroup$ – Fred Apr 30 at 11:01
  • $\begingroup$ @Fred I bought my friend a dry/wet bulb thermometer for fun and science, but it's pretty boring because here the humidity is always high and the two never differ much. (we were both disappointed) Here it seems that anything below 0 C frosts up instantly, so I assumed that any water at all in the atmosphere would be sufficient, but I guess that's not true? At say 20 C there's some nonzero relative humidity where a -3 C surface would not accumulate any frost (given no wind, no sunshine, etc...)? $\endgroup$ – uhoh Apr 30 at 11:07
  • 1
    $\begingroup$ Whirling hygrometers are a lot more fun. They have to be rotated to function properly - if only for 30 to 60 seconds. ;-) $\endgroup$ – Fred Apr 30 at 11:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.