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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
events.
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.