[meteorite-list] COMETS AND CARBONACEOUS CHONDRITES

[Larry Lebofsky]

Hi Doug:

Tell this to the astronauts in their space suits.

I wish I still had access to my old thermal model programs so that I could 
give you real answers, but I will do my best.

If you look up the surface temperture of the day side of the Moon, you get 107 
degrees C. However, the "noon" temperture is well above 120 C (130 C ?).


The mean and high temperture of an object is dependent on:

Distance from the Sun

Its reflectance (how much sunlight it absorbs)

How fast it is rotating

The size of the particles that make up the material (sand vs. rock)

The lower the albdeo, the more sunlight you absorb, the hotter you get.
The faster you rotate and the rockier your surface, the more heat you dump out 
the night side, so the lower your highest temperture.

The Moon's day/night cycle is 29 days (slow) and its reflectance is 12%, so it 
gets fairly hot at noon.

A typical NEO will rotate much faster, but if a C asteroid, it will have a 
much lower albedo (maybe 5% or 6%, but that really is not that much more 
energy since the absorbed energy is 88% vs 95%). Still, the asteroid will 
reach average "daytime" tempertures very close to 100 degrees C. The interior 
will be cooler (insulated), but will still be warm depending on the object's 
mean distance from the Sun.

If anything is hard pressed to get above freesing at the Earth's distance, why 
does it get so hot on the surface of the Earth in the summer even though the 
Earth reflects 30% of the light that hits it?  Go stand outside in July and 
tell me you are cold!

Remember that the volatiles (water) are lcked in the minerals themselves 
(clays) and can withstand vacuum and moderate heating with being lost to space.

Larry
Quoting MexicoDoug <MexicoDoug at aim.com>:

> Hello Larry,
> 
> In the case of carbonaceous chondrites, I believe your inference that "Just
> being in an orbit that takes them near the Earth would warm them up to 100 c
> or so" is way too high, and that the right number in direct Sunlight hovers
> around freezing (0 degrees C).  There is that other related subject of
> whether chondritic meteorites are cool to touch when they land...but I'm not
> going there...
> 
> To reach 100 C, by "just being in an orbit near X", taking a carbonaceous
> chondrite as a model, I believe you would need to be a third of the way
> closer to Mercury's orbit from Venus' in today's Solar System.
> 
> You mention Spitzer data.  For comets on epic journeys through the Solar
> System, which have possibly been orbiting over 4.5 Billion years through all
> phases of development, there are many possible alternate sources of
> meaningful temporary heating during this history that could account for the
> gentle-moderate heating you mention, likely reasonable sized impacts and
> more so, shock heating from barreling through precursor Solar nebula
> components from their own soup they were formed out of in situ, not to
> mention other lower probabilities over time that chance favors.
> 
> Maybe you meant something else?  Even a lump of nickel iron is hard pressed
> to make 100 C in the Sunshine in Earth's neighborhood in outer space.  The
> high volatiles concentrations in carbonaceous chondrites are supportive of
> what I say, I think, though of course they are NOT conclusive.
> 
> Best wishes,
> Doug
> P.S. The Andromeda Galaxy, which dwarfs our own, may even collide with the
> Milky Way in 3 Billion years, two-thirds of the Sun's current age.
> 
>