[meteorite-list] Mammoth Stew

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Sun, 16 Dec 2007 22:58:39 -0800
Message-ID: <93aaac890712162258v3b2d3506p7e6f47e6f8433fd8_at_mail.gmail.com>

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 01:58:39 AM PST