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Re: Impact hypothesis

In a message dated 96-08-21 12:56:51 EDT, you write:

> One idea I've seen kicked around is that perhaps our solar system wandered
>  into a region of greater spatial density a few million years ahead of the
> T
>  line. The solar system of course moves in an orbit around the center of
>  Milky Way Galaxy, requiring about 250 million years to make one circuit. 
> This
>  might have reduced temperatures globally and perturbed a lot of asteroids
>  that they collided with the moon, Mars, Earth, and other planets. Perhaps 
> the
>  dense cloud the solar system might have passed through contained asteroids
>  and cometary nuclei which rained down on Earth for several million years
>  ahead of the really big whammy in the Yucatan. 

Actually, this has already been hypothesized. (by Rampino and Stothers I
The only difference is that they believed that the periodic bobbing of the
Solar System above and below the Galactic plane every 26-33 My,  in the
course of it's orbit somehow  increased the cometary flux which resultedf in
_apparent_ ~28 My mass extinction periodicity. Since they have not shown how,
let alone why, this could  or should happen  the hypothesis has been dropped.
Besides, we do not need to pass through a particular region of space to
increase the likelyhood of being hit. There are at least two belts of
cometary bodies, one is the Oort Cloud and I forget the other  (Kuiper Belt?)
that provide the fodder for comets and possibly asteroids. Being weekly held
in orbit by the sun, these bodies are very susceptible to interstellar and
solar system tidal forces. It is believed by some that  many "new" comets
were recently liberated from the Oort Cloud. _BUT_ not every mass extinction
horizon has been associated with an impact of any kind.   The main extinction
events under suspicion are the K-T, F-F, and possibly the E-O.
The K-T event has the advantage being the most studied(and debated.)
As for declining dinosaur diversity, this is just as hotly debated. IMO,
regardless of the answer, many other taxa were wiped out _along_ with the
dinosaurs and many of them were not in decline and the effects were global.
Marine and terrestrial ecosystems were effected and the impact theory is the
best explanation yet that explains this particular mass extinction.

Thomas R. Lipka
Paleontological/Geological Studies