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Re: cometary shower going on NOW?
Victor D. Noto schrieb:
> Shoemaker confirmed that he thought ultimately the impact rate over
> the
> last 3.2 billion years was related to the travel of the solar system
> (Sun
> and/or Earths) travel around the galaxy. He made special note of the
> z
> motion of the Sun in relation to other G type suns and how the our sun
> only
> moves 75 parsecs through the galactic plane and other stars of similar
> type
> the z motion was 300 parsecs(z motion = up and down motion). Gene
> indicated that this shorter z motion put our solar system in the area
> of
> dense star clusters for a longer time and this may strip the Oort
> cloud of
> its outer shell of material causing increased cometary activity some
> period
> of time following the passage by other stars.
There are two excellent Sky & Telescope articles in the March 1990 issue
on this topic:
(a) Target Earth: It W i l l Happen (pages 261-265) by D. Morrison,
NASA-Ames Research Center and C.R. Chapman, Planetary Science Inst.
(b) Are Periodic Bombardments Real? (pages 266- 270) by P.R. Weissman,
JPL
Weissman also mentions the Sun's solar up and down motion: As the Sun
orbits the Milky Way's center every 250 million years or so, it also
crosses through the galactic plane about every 32 million years. This
so-called epicyclic motion resembles that of a merry-go-round horse and
has been linked by some scientists to alleged periodic comet showers and
mass extinctions (p. 267).
Where we stand: Accurate dating of the dyings is another problem. Raup
and Sepkoski used a widely accepted system known as the Harland Time
Scale, but if one uses some of the other chronologies proposed by
geologists and paleontologists, the periodicity changes or even goes
away. In addition, an absolute time-scale is obtained by tying the
fossil record to rocks dated by radioisotope techniques. However, Eugene
Shoemaker pointed out that no radioisotope "tie points" exist for events
between 140 and 240 million years ago. The ages of extinctions in that
interval are just informed guesses based on a linear interpolation
between known tie points (pp. 269-270).
Paul Weissman: My feeling is that periodic bombardment in the solar
system is not real. The evidence for regularity is weak, both in the
fossil data of Raup and Sepkoski and in Alvarez and Muller's small list
of well-dated craters. ...
If the periodicity in comet showers is real, then I suspect the cause is
one we already know about, most likely the Sun's 32-million-year
epicyclic motion above and below the galactic plane. But what turns such
motion into periodic impacts on Earth remains a total mystery, and until
that vital link can be found all we have is an unconfirmed hunch.
> ... and I wonder what caused these pigs demise perhaps desease,
> famine, flood, or climate change (EL NINO) etc etc....
Twelve voices were shouting in anger, and they were all alike. No
question, now, what had happened to the faces of the pigs. The creatures
outside looked from pig to man, and from man to pig, and from pig to man
again; but already it was impossible to say which was which (Animal
Farm, by George Orwell).
------- snip -------
Oink, squeal, grunt, ....
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