Sterling, E.P., All,
For the record, I like my peppered mammoth
with lemon butter...
Thick-cut, salt and pepper.
Jason, think about Tunguska. A 25 megaton airburst
that left no crater, no pits, not even the tiniest, no
material remains whatsoever, no isotopic traces in
reliable amounts, nothing with a side order of zilch.
(Ok, possible microscopic spherules in trees, not
2-3 mm particles, and disputed to boot).
Exactly; nothing was left; no evidence, no anything.
How, so, can you relate this to Tunguska, when the evidence that we
have for it is completely different?
Yet, had it occurred over Belgium, it would have
killed 90% of the population of the nation, or if over
metropolitan London simply removed the world's
then-largest city from the map. IF we did not have
the Russian newspapers, the native reports, Kulik's
photos of the trees (gone now), could anyone today
detect that it had ever occurred? And it hasn't even
been a lousy century! (The Centennial is next June!)
But you're lacking the isotopic evidence, etc. Not so with this layer
of...whatever it is.
Like a belief in the existence of the atom or any other
thing that we cannot and never will see with our own
eyes, vast numbers of craters have covered on Earth.
Mhm...
1) The flux of impactors at the Earth is identical to the
flux of impactors at the Moon, since the two bodies
occupy the same orbit and always have, the Moon like
a celestial tick on our neck.
Well they haven't always, but, irrelevant to this discussion.
2) The pristine state of the Moon allows for a very
accurate count of the number of impactors that have
struck the Moon (allowing for extrapolation for the
areas covered by flood basalts -- ~170,000 impactors
producing craters of one kilometer or more).
Fine, fine, information we all know.
3) It's mathematical child's play to scale up the lunar
impactor flux to the Earth's size and add in the increase
in "gravitational" cross section caused by the Earth's
stronger gravity (13.5 + 4.4 = ~18 times more impactors).
Not only that, but the stronger terrestrial gravity means
that ANY impactor will make a bigger crater on the Earth
than it would have if it had smacked the Moon instead.
(And for impactors that would make a crater 1 km or
more in diameter, the atmosphere is not a factor.)
Well, we also have to take into account that a fist-sized meteorite
will make a crater six or so meters across on the moon whereas on
earth such a thing would make nothing more than a pretty light show.
4) So we can easily determine the number of craters on
the Earth. No problem. The Earth has had approximately
three million (3,000,000) impactors, so we must have
three million (3,000,000) craters over one kilometer in
diameter!
Subtract the smaller craters and account for erosion...we're talking
about the past fifty thousand years, not 2+ billion. The number of
impactors over this timeframe was smaller than that of before, and
erosion has taken a lesser tole on such craters, as they're younger.
Before we all run outdoors to check out the vista of
craters, craters, craters everywhere -- sorry, they're gone.
After counting craters from the obvious to those hidden
to the eyes of all but gravitometers, 17,999 craters out of
every 18,000 craters have vanished utterly from the planet
without a trace!
See above...this makes sense given that most of the craters were
formed before the timeframe that is of any importance to this
discussion.
So, both these statements are true, in their fashion:
a) The Earth is the most cratered body in the solar system.
b) The Earth is the least cratered body in the solar system.*
(* except for the other really interesting place... Titan)
Well, maybe, maybe not...Mars should probably be more so.
>From 98,000 years BP to 14,000 BP, a northern polar
ice cap was in place, yes, with retreats and advances,
recensions and excursions, in this area or that area, or
all areas, changes whose precise timing is hard to pin
down, but for ALL of that 84,000 years, there was a
land based ice cap in most of the northern hemisphere,
varying in thickness from 1000 meters to 3000 meters.
Right-o.
Two miles of vertical ice. Now gone. What traces
of a crater in its upper surface do you expect would
survive? Just for fun, I went and modeled on the LPI
Impact Calculator a Ten Kilometer Comet a little less
dense than water making a 30-degree impact, releasing
8 million MegaTons TNT [or 8 TeraTons] energy
equivalent, and its crater wouldn't have reached through
an ice cap that thick; the crater was only 1100 meters
deep. Also, I don't know if anyone has seriously
analyzed a cratering event in deep ice! Ice, hard as
it seems, has properties midway between weak rock
and deep water (which produces much shallower
craters than rock).
But we have to account for a crater (well, impactor at least - or
maybe just call it a 'body') large enough to deposit such a layer of
dust, and I don't think that you're going to get that from such a
small impact.
Call the Earth the Eraser Planet. The Ice has to be one
of the best of the many erasers available. Three million
craters and only 170 of them still show... It's almost like
"they" were trying to trick us into an unreasonable
complacency, isn't it?
See above...your numbers are off due to a prejudice towards older
craters that were undoubtedly more common - and have suffered a great
deal more due to the effects of weathering.
We've had a lot of questions about the difference
between an asteroid impact and a comet impact.
The difference between an asteroid impact and
a comet impact of similar energy? The outcome
of each is different, though the crater's the same size:
http://www.news.uiuc.edu/scitips/02/1025craters.html
Right, and what it comes down to is size.
Surprised to find this, as I've never heard it mentioned
before: a 10-yr-old study, the last by Gene Shoemaker,
that demonstrated a He3 extraterrestrial dust layer at
36 million years ago that persisted for over two million
years and overlaps the times of the Popagai and the
Chesapeake Bay craters. He considered it the evidence of
a period of "comet showers." But other events are also
possible explanations.
http://mr.caltech.edu/media/lead/052198KF.html
How big are those craters again? If I recall, at least the Chesapeake
crater is fairly sizable...
One of the disadvantages of being a short-lived creature
with a recording civilization only a few thousand years old
in a universe 15 billion years old is the problem of detecting
threats that do NOT leave long persisting warnings behind.
Instead of 3,000,000 craters, there were a few, so we were
able to deduce the rest, but only in the last (less than) 50
years.
Prejudiced number...
We should not assume that we have now identified all
possible threats from the universe at large. A threat event
with few trace markers could be quite frequent and still be
very difficult to detect in the absence of such an event.
Well, mass extinctions should give us something of a clue even if we
can't find traces of an impact, but if I'm not mistaken, the mass
die-outs occurred several thousand years after the dust layer was laid
down, no?
Regards,
Jason
Sterling K. Webb
----------------------------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <[EMAIL PROTECTED]>
To: <meteorite-list@meteoritecentral.com>
Sent: Sunday, December 16, 2007 8:05 PM
Subject: Re: [meteorite-list] Mammoth Stew
Hi all -
1) From the descriptions, the spherules in the tusks
appear to be the result of the condensation of iron
plasma, the same as at Barringer crater.
2) When Nininger did his survey of spherules at
Barringer crater, I doubt if he looked several hundred
miles away from the crater - that's what I think of as
a ballistic re-entry. The internet site for this
impact has been greatly improved, and I'm sure that
some here must have been active in that.
I don't know about winds at the time of Barringer
impact, but I can't remember any statement as to angle
of impact. But then I can't remember many things
anymore.
3) I have no idea what the spherules' temperatures
were when they landed - but my guess is that they must
have been too high to use any type of barrel to
duplicate their hitting the bones. My guess is that
magnetic suspension and acceleration would be about
it.
4) As far as locating the 31,000 BCE crater goes, its
possible that the situation might be similar to the
K-T crater - that one took 10 years to find. Same
goes for impact point(s) for the 10,900 BCE event. If
you look at impact crater distribution maps, you'll
see that more have been found in the areas where
geologists live.
good hunting,
E.P. Grondine
Man and Impact in the Americas
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