[meteorite-list] More on meteorite temperature

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Fri, 1 Jul 2016 00:14:14 -0700
Message-ID: <000a01d1d368$2d033720$8709a560$_at_cox.net>

Hi All,

Posted this from work over 7 hours ago but seems not to have worked, so
resending from
home... <sigh> --Rob

- - - -

Hi All,

Playing Devil's Advocate, I decided to try coming up with a scenario that
attempts to maximize the
thermal equilibrium temperature of a chondritic meteoroid just prior to
encountering the earth's
atmosphere. The typical formula for computing the thermal equilibrium
temperature for an
object without an atmosphere is:
 
Te = [S0 * (1-A) / (4*epsilon*sigma)] ^ (1/4)
 
where the body is assumed to be spherical (the source of the 4 in the
denominator), S0 is the
solar constant (mean value 1361 W/m^2), A is the bolometric Bond albedo, epsilon
is the
meteoroid's emissivity, and sigma is the Stefan-Boltzmann constant (5.670 x
10^-8 W/m^2-K^-4).
A, in turn, can be estimated from the following equation:
 
A ~= q * pv
 
where q is the phase integral and pv is the visible albedo. Using Bowell's H, G
magnitude system,
we can compute q from:
 
q = 0.290 + .684*G
 
The commonly used value for the slope parameter, G, is 0.15, in which case:
 
q = 0.393
A = 0.393 * pv
 
For very dark asteroids (e.g. Trojan asteroids, Hildas, Cybeles), the albedo can
be 5% or lower.
However, most NEOs have semi-major axes less than 3 a.u. and albedos averaging
closer
to 20%.
 
The final missing value is the emissivity. For regolith, a range of 0.9-0.95 is
often mentioned.
However, emissivity and albedo work hand-in-hand (epsilon + pv ~= 1). So if
we're going
to choose an emissivity of 0.9, we should set the albedo, pv, to 10%.
 
So what is a typical equilibrium temperature for a spherical NEO with 10%
albedo, 0.9
emissivity, 1 a.u. from the sun?
 
A = .393*10% = .0393
 
Te = [1361 * (1-.0393) / (4*0.9*5.67 x 10^-8)]^0.25 = 282.9 K or about 49.6 F
 
So, cool, but certainly not freezing. How can we get a warmer answer? One way
is to pick the
time of year when the earth is closest to the sun (early January) and the solar
constant is
higher: about 1414 W/m^2. This raises the temperature in the above example to
285.6 K,
or 54.4 F. Still not warm, but warmer. Lowering the emissivity will help, too.
Let the albedo
increase to 20%, and set the emissivity to 0.8. With the perihelion solar
constant, the
equilibrium temperature is now up to 291.1 K (64.3 F). Lowering the emissivity
further
is probably not realistic for most earth-crossing asteroids, so we're at the
limit of what
we can achieve via S0 and emissivity.
 
However, there *is* a way to get a big increase in the equilibrium temperature
which
I'll cover in the next installment. --Rob
Received on Fri 01 Jul 2016 03:14:14 AM PDT