[meteorite-list] Mammoth Stew

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 17 Dec 2007 14:06:11 -0600
Message-ID: <023e01c840e8$45764bb0$b842e146_at_ATARIENGINE>

Hi, Jason, List,

1. The point of comparison to Tunguska is that
an airburst is radically different from a ground burst
and leaves, proportionately to its intensity, remarkably
few discernable markers.

2. The statement that the Earth and Moon haven't
"always" shared the same orbit is quibble for quibble's
sake. The collision between the two occurred in the
first 100 million years or possibly much less of the
solar system. The Australian zircons say more than
4.46 billion years. The mutual collision disassembled
the Moon and melted the Earth into the mantle, so
the surface record starts or is re-set at that point. (I
think that the proto-Moon formed at one of the Earth's
Trojan points. A Trojan is only stable if the mass of
the Trojan is insignificant compared to the primary
orbital mass; once it grows to a measurable fraction,
it is perturbed "along" the orbit until it is "captured"
by the primary. That's how the Earth and Moon got
hitched, sez the witch doctor, poking the fire...)

3. I am only counting impactors that result in a 1000
meter crater or larger. Atmosphere not a factor in that
size range. The fact that the Earth has "new" craters of
only 10 meters diameter demonstrates our atmosphere
only "protects" from mere nuisances and nothing more.
Moreover, you imply that it is "easier" to make craters
in the Moon (your incorrect fist-sized impactor, six meter
crater), and that is not true. The ratio of impactor energy
to crater size is the same for both bodies, both of which
have the same crustal density. The Moon is surfaced
entirely with "tougher" rocks (basalts largely) and not
overlayed with "soft target" to the extent of the Earth
(all that water? half a kilometer of clay?). Further, the
Earth's gravity accelerates an approaching close object
by an additional 11,200 m/sec in velocity over its distant
relative velocity, while the Moon only adds a measly
2380 m/sec. It's HARDER to crater the Moon than
the Earth, on a crater by crater basis.

4. You used the puzzling phrase "such a small impact"
in response to my having modeled an impact that was
8% of a Chicxulub and still wouldn't have punched
through the Pleistocene ice cap! What does it take for
you to call an impact large? As for traces, we have lots
according to the Firestone gang: nanodiamonds, carbon
layer, bucky balls, metallic residues, blah, blah, all many
thousands of miles away from the impact point. Actually,
I have to thank you for (unintentionally) starting me down
the path toward modeling "impacts in ice caps." Such
thick emplacements cover 10% to 20% of the Earth's
surface during glaciations, yet leave few traces of even
a major impact: no rock melt, no impactites, no crater,
no tidal waves. We've ignored this possibility completely
up till now. Note that the traces the Firestone gang have
found seem to be able to be formed entirely from the
impactor? That is what would happen in an "ice cap hit"!
There would be those bothersome shock waves, the
injection of a huge amount of water vapor into the Earth's
atmosphere, after the wildfires from the thermal flash, of
course. I don't think this obvious possibility has ever
been "thought through."

5. You say, "most of the craters were formed before
the [recent?] timeframe." Well, that's exactly what the
argument's about, isn't it? This is the comfortable, "that's
all in the past" argument. Let's review the cratering history
of the solar system. After initial accretion, a tapering off.
Then, at 3.8 to 3.9 billion years ago, an intense episode,
the "Late Bombardment," followed by an exponential
decline for more than 3 billion years. Then, at 0.6 billion
years ago, cratering rates begin to rise dramatically, until
0.4 billion years ago, when they have increased fourfold
in 0.2 billion years. They again decline. until 125-100 million
years ago, when they increase, roughly doubling. If nothing
else, this demonstrates that the system impactor flux varies
dramatically through solar system history, for whatever
reason. The role of comets, stellar encounters, Oort Cloud
shenanigans, asteroidal family dustups is all unclear and
yet to be pinned down. Good old ignorance. But the view
that solar systems start in an impact squabble and then
settle down happy forever is dead wrong. Perhaps, after 4 or
5 billion years, long-tern inherent dynamic instabilities catch
up with a solar system and such systems begin to pummel
themselves to death. We don't know. Find me a 10 or 20
billion year old stable solar system... Maybe such contrivances
as solar systems just don't last? We are always (see the press
releases) finding very young solar systems; have you ever
noticed our finding a very old one? Me, neither.

6. You call the 3,000,000 crater figure a "prejudiced number."
You are simply wrong about that. That's the number of craters
made on Earth; I didn't say they survived. Oh, it could be off
by 300,000 craters either way, but it's "order of magnitude"
correct. What the current rate is -- that IS the dispute, to be
determined, not dismissed. Cratering, where we are able to
closely date it, appears to demonstrate "clustering." If impacts
come in intense episodes of 50,000 to 100,000 years with
deep calm between... Well, that means a "local" rate can be
radically different from a long-term time-averaged rate. What's
the impact weather like this era? That's the question. It depends,
in scientific terms, on whether cratering is entirely "stochastic"
or not. If there are "mechanisms" of cratering, it's not. Since
we know of several obvious mechanisms (the breakup of
major asteroids, comet infall from outside perturbations), it
does not seem likely that the process is stochastic or perfectly
random. If it is not, the comfortable notion that "such things
don't happen anymore" is unsupportable.

7. The insistence on perfect matching of extinction and impact
timing is a red herring. In massive impacts (K-T, the Permian
Wollop), yes, it should be close. But extinction can be "smeared"
out over thousands of years by environmental change. The fact
IS that 10,000 years ago, there were flourishing some 300-odd
major (big) mammal species that I've never seen and never will.
They had all been through de-glaciations before, so it wasn't
that. Either "something" happened or Man the Mass Murderer is
responsible. (A ridiculous theory; when game is scarce, you move
on and game recovers. Neolithic man never hunted game down to
extinction. It takes so long, you'd starve first. They weren't stupid,
you know. They were opportunists, and they lived off the fat of
the land, not the lean.)



Sterling K. Webb
--------------------------------------------------------------------------
----- Original Message -----
From: "Jason Utas" <meteoritekid at gmail.com>
To: "Meteorite-list" <meteorite-list at meteoritecentral.com>
Sent: Monday, December 17, 2007 12:58 AM
Subject: Re: [meteorite-list] Mammoth Stew


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" <epgrondine at yahoo.com>
> To: <meteorite-list at 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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Received on Mon 17 Dec 2007 03:06:11 PM PST