Re: [Fwd: Earth Battered Through History By Comets]
Wed, 25 Aug 1999 19:24:55 +0000

At 09:20 AM 08/24/1999 EDT, wrote:
>Sometime during the mid- to late Seventies I read an article about
>carbonaceous chondrites (CC). In it the authors suggested that, because
>tended to be light and delicate compaired to stoney or metal meteorites,
>also tended to explode in the atmosphere rather than impact the ground.
>suggested that a CC was the explanation for Tunguska.

Before my computer was stolen, I had a computer program that I had written
which modeled the interactions of a meteor falling through the atmosphere.
It contained the strength of various materials, like ice, iron, chondrites
or stony meteors. The interesting thing that this program showed was that
if a meteor was above a certain diameter and moving above a certain speed,
the pressure wave caused by the impact of the atmosphere on the outer
surface would not reach the center of the object before it hit the earth.
If the pressure were greater than the tensile/compressive strengths, the
meteor would explode. The point of all this is that it is a whole lot
easier to get a chunk of ice (comet) to explode in mid air than anything
else. My guess on Tunguska is that it was a small comet rather than a
carbonaceous chondrite. Here is some basic data on Tunguska.

"The 1908 Tunguska airburst provides a calibration, with a shock wave
sufficient to fell trees over an area of >= 1,000 km 2 and a fireball that
ignited fires over a smaller area near ground zero. The yield of the
Tunguska blast has been estimated at 10-20 from microbarograph
measurements in Europe and other methods. If we assume that the radius of
forest devastation would apply also to destruction of nearly all buildings
(chiefly poorly constructed residences), which would thus kill most people,
the area of lethal damage is given by a=100y2/3, where Y is the yield in MT
and A is in Km2. This corresponds to an overpressure of about4 p.s.i.
(-2.8 x 105 dyne cm-3)." ~ Clark R. Chapman and David Morrison, "Impacts
on the Earth by Asteroids and Comets: Assessing the Hazzard," Nature, 367,
Jan. 6,1994, p. 34.

>Whether or not this is true, the Tunguska event represents a possible answer
>to Glenn's question. No crater was ever found in Siberia, yet the
>atmospheric explosion still managed to throw enough dust and soot into the
>air to affect the weather for some years after. An alternative answer would
>be impacts in the oceans, which would leave no obvious craters but could
>still throw up enough material into the atmosphere to change the weather for
>a few decades.

Kevin is correct that a Tunguska style event is a possible explanation. But
I don't think it will work for the following reasons. First there was not
much of a dust cloud. It is true that there was a lot of material in the
air, but it may not have been dust but may have been water in the form of
noctilucent clouds. The sky at night was orange giving enough light for
people to read newspapers by night. However, it only lasted a few
days--dust lasts much longer.

Secondly, the dust veil index does not have a blip in 1908 which would
correspond to that impact. Here is the data for the DVI
year lat. long. event (volcano) DVIG DVIN DVIS
1906 54.00-168.0BOGOSLOF 20 20
1907 52.00 157.5SHTYUBELYA 150 150
1907 19.50-156.0MAUNA LOA 10 10
1910-53.00 73.5HEARD ISLAND

1911-56.50 -28.5LESKOV ISLAND

1911 14.00 121.0TAAL 30 15 15
1912-77.50 167.0EREBUS

1912 58.00-155.0KATMAI 150 150

Where DVIG is the global Dust veil index, DVIN is for the northern
hemisphere, DVIS is for the southern. You can see that there was nothing in
1908 of significance globally.


Actually, I don't believe that Tunguska left a big dust plume.

>My main objection to this theory is that it sounds too much like "jumping on
>the bandwagon"; impact disasters are now trendy and people are starting to
>claim them as causes for events simply for that reason. I would think a
>better explanation for possible historical climate changes would be volcanic
>activity. Even an eruption as small as Pinatubo affected the weather for a
>number of years; an eruption only 10 to 50 times larger might be enough to
>create the disaster needed to bring down a civilization.

Kevin is correct here. Impacts are trendy, and volcanic activity is a much
more reasonable explanation, if one is required.

Foundation, Fall and Flood
Adam, Apes and Anthropology

Lots of information on creation/evolution