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Re: astroblemes

[Nemesis hypothesis reference deleted]
Just because the Nemesis model has been properly discredited does not 
mean that periodicity is not to be found in impacts. The Nemesis model
was a good first guess, but it has been shown that the orbit required 
for Nemesis is unstable, and thus any Nemesis would be perturbed out of 
that orbit by passing stars.

The periodic nature of impacts is now being viewed by some as related to the 
passage of our solar system through the galactic plane, something which
occurs approximately every 32 million years. The galactic plane is matter
enriched and might cause gravitational perturbations in the Oort cloud
and thus trigger comet showers on a periodic basis. There may be other
astronomical triggers possible as well. (see below)

>Also, there is no real evidence for high rates of impacts
>at or around the K-T boundary.  So, if that case has only one or
>two major impacts, it is unlikely that the other, less studied
>cases, have more.

How about this evidence:

In the last 30 million years, where the evidence is the freshest and the 
data is the best, we know of only 3 craters at least 20 km diameter:
in diameter:

Crater       diameter      time of impact
Haughton        20            21 mya 
Reis            24            15 mya
El'Gygytgyn     23             4 mya

Now let's examine the 30 million year interval spanning the K/T:

Crater       diameter      time of impact
Manson          35km            74 mya
Chicxulub     >200km            65 mya
Kamensk         25km            65 mya
Ust-kara        25km            57+-9 mya
Kara            60km            57+-9 mya
Logancha        20km            50+-20 mya

We know of at least 5 impacts in this 30 million year interval that
are greater than *any* in the last 30 million years. I cite this not
as evidence of periodicity, but rather to rebut the statistical 
generalization(i.e. no evidence for high rates around the K/T boundary)
mentioned above.

Disclaimer: The following is pure speculation that may be of interest 
to some readers.

My personal theory is that the largest impacts do have a periodic nature, 
which is related to the geometry of our galaxy itself.

To illustrate this theory, lets look at the 3 biggest(by far) extinction

Period                     time            crater        diameter
Cretaceous/Tertiary        65 mya         Chicxulub     >200km
Permain/Triassic          250 mya         S. Atlantic   >300km*
Ordovician/Silurian       439 mya         no known crater(yet!)

Notice the number of years separating these events.(~185 million years)
Now lets run back the clock 185 million more years:

Precambrian               593+-32 mya     Lake Acraman   150km

The 185 million year period is approximately the time required for our
solar system to make one lap around the galactic nucleus. What are the
odds of these really huge strikes(>150 km craters) being spaced at almost
precise 185 million year intervals? Think about an analogy of our own 
solar system and why we have meteor showers. Every time we make a lap
around the sun we pass through the trails of comets, which result in
meteor showers. On the galactic scale, we may pass through dust clouds
or some other matter-rich regions which may perturb comets and send a
few of them on a one-way ticket to the inner solar system.