[meteorite-list] Comet: Talking Points, #1

[Sterling K. Webb]

Hi, List,

    Yeah, here we go again...

----- Original Message ----- 
From: "Marco Langbroek" <marco.langbroek at wanadoo.nl>
>
> Where? Where are the dust layers in peat deposits, 
> lake deposits, deep sea cores, ice cores from Greenland 
> and Antarctica for example? All there is, is a set of 
> narrow tree rings. No more.
>

It is necessary to examine the question quantitatively.

    Example: Earth encounters a very small globule of interstellar dust.
It takes 36 days to traverse the globule. The dust that impinges on the
Earth's atmosphere is accumulated in 36 days, during which solar flux
to the ground is diminished by an arbitrary percentage, say 5%. Such
varieties of dust are very fine, of small particle size. Most take years
to fall out of the atmosphere.
    The original "strong signal" of the dust, with a 36 day duration, 
has now been "smeared out" to 6000 to 8000 days. Because 
the particles are very fine, they tend to incorporate themselves 
in surface layers, and erode away at the rate of general local 
erosion. Those particles that land on rock, sand, ice, move to
soil or sea more quickly. The 6000 day smeared signal is smeared
out to 50,000 days or more before that portion that reaches the sea
or lakes is deposited there. 
    Because the particles are "fines," they settle to the ocean bottom
very slowly, 100,000 days or more. The mass "signal" is now 
down to about 10^-4 of its original strength. How much material 
is required to intercept 5% of the solar radiation and reflect 
or re-radiate it to space in the first place? The answer is a mere 
1/2 gram per square meter if ice, more if it's silicate dust, less 
if the dust is "fluffy." Deposited at the sea bottom in a only
100,000 days, that's 5 MICROGRAMS per square meter 
per day, during which time it is being mixed with the much 
greater amount of normal terrestrial sediment being deposited.
    How detectable is that? If the material is particles from a comet 
that are mostly ICE, they degrade to WATER. Just how do you 
"detect" water? If the material is silicate dust, most of it is 
indistinguishable from any terrestrial silicate except for isotopic 
imbalances from the terrestrial ratios. 
    When it's all settled we may be talking about "detectable"
isotopes as low as one millionth of one millionth of a gram, or
one femtogram! Maybe even 10 femtograms... Have you any 
concept of how difficult such a detection is? The German 
researcher who found the evidence of supernovae in sediments 
2.2-2.3 million years old, namely Fe60, went through pounds 
of muck. He spent many years doing it. Know how much Fe60 
he found? SIX ATOMS! Then, everybody went out for a beer!
    [For those who wonder why this is definitive, there is no other
place in the universe where Fe60 can be formed except in a
supernovae. They can't be contamination from the lab, etc.,
because there ain't any. Other supernovae-only isotopes are
Be10 , Iodine 129 and Sm146. Find'em and you can go
out for a beer, too! I'm buying.]
    You were expecting a Dust Bowl? Drifts accumulating
in the streets? Having to wear a dust mask? Not really... The 
Earth already picks up some dust. There's argument about how 
much. Tens of thousands of tons per year? Hundreds of thousands 
of tons per year? So, how much do these tiny quantities of dust 
I've been talking about amount to? Only 2,500,000,000 tons!!! 
Yes,  2-1/2 BILLION tons. But it's only 1/2 gram per square meter. 
Imagine the tiniest pinch of talcum powder thrown up into the air
and dispersed over more than a square yard! What would you see?
Nothing much, if anything... 
    So, you would never see the dust, only the suddenly and 
dramatically dimmed sunlight, the rapidly increasing cold, and
all the rest of the litany -- only the disastrous effects would be 
perceptible.
    The Earth's area is a half billion square kilometers, so we're
talking about 1/2 ton per square kilometer, the rate of mild
organic fertilizer usage, or less lime than my yard needs to grow
good grass again, or... you pick an example. It's not very
much, but it's 'way too much, both at the same time.
    Some of  the knowledgeable will jump in there and
point out that the eruption of Tambora that produced the 
1816 "year without summer" involved the injection of about 
200 billion tons into the atmosphere! Well, two things count
in causing climatic disaster: small particle size and high altitude,
just what "cosmic" events provide and volcanoes do not.
    Tambora injected a coarse mixture of particles into the 
lower atmosphere of the northern hemisphere. Only the finest, 
highest particles persisted very long, perhaps 10 to 100 million
tons, but it was they that dropped the temperature of the US briefly
by 20 to 25 degrees C., to freezing in July and August. After a 
few weeks, temperatures rebounded to the 90's (F.). They were 
low and fell out quickly.
    A 5% reduction in solar flux would be much worse, probably
worse than the 534 AD event hypothesized.
    So, that answers the "Where are the layers of dust?" question.
    Caveats and Weasels: The question of calculating the opacity
of dust is really complicated. The physical parameters involved are:
the particle cross sectional area; the mass opacity which is the area
per gram of absorber; the absorption coefficient, which is the ratio
of the area of absorber to its volume; if you're a chemist, the similar
extinction coefficient; the "optical depth" (there's a formula for that;
I got no symbols on this keyboard); and similarly the "transmittance."
I looked up the values I could find; I (ahem) extrapolated the ones
I couldn't. (This List is less fun than grad school, and it wasn't that
much fun.)
    So, I could be off by a magnitude either way. This would not 
change the bottom line: the amount of fine dust at the top of the 
atmosphere sufficient to cause disaster would, if it fell down on you 
all at once, would hardly be noticed or at most be dusted off 
without a thought!
    If you like experiments: on a nice sunny day, construct and then
stand in a 3 foot square black box 10 feet tall; put a three foot
square pane of glass on the open top of the box; then have 
somebody dust it lightly with talcum powder until it forms a layer 
1 millimeter thick; then look up. See much? (I'm one of those 
lowly physicists, not even an astro-, that Marco objects to;
I like experiments.)
    See much?! you exclaim; it's about like night in here! Well, 
that's ten grams of dust per square meter at the top of your 
atmosphere.

Sterling K. Webb