Tunguska event (followup to Dynamics behind....)

As promised (for those who may be interested), my e-mail
(part of which I'm posting here) from Kevin Zahnle of
Ames Research Center, is probably as complete and "official"
as is available at present. Apparently, no one still has been
able to fully explain the explosion (note the next-to-last sentence
in the first paragraph).
---------

here is some text I published in Nature ca. 1996. I don't think
I've written anything about Tunguska since:


Perhaps the earliest widely-held theory for the Tunguska explosion was
that the world was about to end. As the minutes passed this theory
was dropped in favor of other, less final theories, until today one is
hard-pressed to find anyone who truly believes that the world ended on
the morning of June 30, 1908. It is now generally accepted that the
explosion marked the end of another world, a small (50-100 m) comet or
asteroid destroyed by aerodynamic forces some 10 km above the trees.
Yet, because no undoubted remnants of the impactor have been
recovered, the Tunguska event retains an air of mystery. The paper by
V.V. Svetsov on page xxx of this issue (ref 1) may provide a part of
the explanation.

The Tunguska impactor exploded above a sparsely inhabited region in
central Siberia with the force of a 15 MT bomb (roughly equivalent to
man's biggest). The blast wave flattened trees over 2000 km2 and
excited a magnitude 5 earthquake. Thermal radiation scorched trees
and set fires over much of this range, and even 70 km away an observer
removed his shirt for fear it would ignite (ref 2).

The earliest scientific expeditions to the impact site, launched
almost two decades after the event, focused on the search for
meteorites (ref 2). But no meteorite was found. Instead the
explorers found an extensive region of trees felled in a striking
radial pattern, at the center of which they found standing trees
stripped of branches. Apparently the meteor exploded in the air.
Although even more interesting theories have been suggested, recent
attention has focused on whether the Tunguska body was a comet or an
asteroid; i.e., on whether the impactor was a rock.

Partisans of a comet argue that an asteroidal Tunguska would
inevitably have left large meteorites scattered around the impact
site. Comets, it is presumed, would not. At present, there are two
extant reports of possible Tunguska meteoritic debris. One is of a
modest iridium excess in local peat (ref 3). The other is of a
relatively high abundance of microscopic dust particles in tree resins
exposed between 1902 and 1920 in local conifers that survived the
explosion (ref 4). These may prove to be exogenic, although neither
is diagnostic of the type of impactor. Other arguments raised in
favor of a comet include a possible genetic link to the Taurid complex
of meteors and to comet P/Encke (ref 5), and the suggestion that white
nights over Europe were caused by Earth sweeping up the comet's tail.

Partisans of an asteroid argue that the Tunguska meteor penetrated the
atmosphere so deeply that, when it finally did explode, it did so
while subjected to much higher aerodynamic stresses than those
experienced by smaller meteors (ref 1,6,7). The higher stresses broke
it into smaller pieces. For reasonably strong rock, Svetsov estimates
that typical fragments would have been 1-3 cm. But very few 1-3 cm
meteorites reach the Earth. Rather, they are just small enough to be
ablated away by thermal radiation in the fireball (ref 1). An
important assumption is that the ablated products are mostly melt
droplets, rather than vapour, making ablation relatively efficient.
What remains is a spray of melt droplets borne on blast waves.

A difference between this work and earlier work (e.g., ref 6) is its
high luminous efficiency. As Svetsov considers only the opacity of
hot air, it is unsurprising that he deduces a luminous efficiency
(~30%) like that of a nuclear explosion of comparable yield. This is
much higher than for typical meteors (1%), for which the optical
depth of shocked air is small. On the other hand, Svetsov neglects
the opacity of dust and vapours from the impactor in calculating the
thermal radiation field. Air must be heated to 6000 K before it
becomes opaque to visible light, but ablated vapours can be opaque at
lower temperatures. Thus the effective radiating temperature of an
impact fireball is probably less than for a nuclear fireball, and the
luminous efficiency (and ablation rates) somewhat less than what
Svetsov computes.

It is harder to explain away the iridium. Exogenic iridium was the
telltale clue (the "smoke", as it were) that an impact terminated the
dinosaurs. The Greenland and Antarctic ice sheets record the
background iridium accretion rate, but no additional signal is seen on
or around 1908 (ref 7). If a chondritic or cometary Tunguska were
uniformly spread over the northern hemisphere, one expects an annual
Ir signal 10-100 above background (ref 7).

The surest way to hide iridium is not to supply it, which is possible
if the impactor were an achondrite, a relatively rare kind of stone
that has little iridium because it was once part of a larger
differentiated body. Otherwise the ejecta must not reach Greenland.
The ejecta could have been launched into space, or alternatively,
broadcast over a limited (~1000 km) region. A third possibility
recalls the white nights over northern Europe. These began the night
of the Tunguska event, and were presumably caused by sunlight
illuminating mesospheric particles of some kind (ref 2). Unlike
volcanic aerosols, the noctilucent cloud did not remain noteworthy for
more than two or three days. The particles either evaporated (if ice)
or fell (if dust). If the latter, the stratospheric residence time
was too brief for the particles to spread out, and so precipitation
may have been patchy.

Comet Shoemaker-Levy 9 (SL9) also exploded in an atmosphere
(Jupiter's), and one might hope to draw some analogies to Tunguska.
The SL9 ejecta blankets (ref 8) extended many thousands of kilometers
from the impact site, preferentially along the wake. Ejecta
velocities were 10-15 km/s. Scaling considerations (ref 9) suggest
that Tunguska ballistic ejecta velocities were probably 3-10 times
slower, i.e. 1-5 km/s, which implies that the ejecta would have fallen
within 2500 km of the impact site. At these velocities there is no
escape, and white nights may have depended in part on mesospheric
winds. The analogy also suggests that the ejecta were mostly launched
towards Manchuria, as the Tunguska impactor came from the southeast.
Finally, the visible SL9 particles were mostly very small and slow to
fall (ref 8). Analogous cometary dust from Tunguska would need have
been Ir depleted for it not to fall on Greenland.

Small impact craters on Earth are almost always associated with
relatively rare iron impactors (ref 11). The 1.2 km Meteor Crater in
Arizona, for example, was produced by an iron body of essentially the
same energy as the Tunguska explosion. The smallest crater known to
be made by a chondrite is the 3.4 km New Quebec crater. This raises a
problem with comets: if comets with energy of 15 MT can reach the
troposphere, then the much more numerous stony asteroids, which all
models agree will penetrate deeper, should be cratering the land every
1000 years. If Tunguska were a comet, where are all the Meteor
Craters made by rocks?


1. Svetssov, V. this issue (1996).
2. Krinov, E. L., Giant Meteorites (Pergamon, Oxford, 1966).
3. Korina, M. I. et al., LPSC 18, 501-502 (1987).
4. Longo, G., Serra, R., Cecchini, S. & Galli, M., Planet. Space Sci.
42, 163-177 (1994).
5. Kresak, L., Bull. Astron. Inst. Czechoslovakia 29, 129-134 (1978).
6. Chyba, C. F., Thomas, P. & Zahnle, K., Nature 361, 40-44 (1993).
7. Hills, J. G. & Goda, M. P., Astron. J. 105, 1114-1144 (1993).
8. Rasmussen, K. L., Clausen, H. B., & Kallemeyn, G. W., Meteoritics
30, 634-638 (1995).
9. West, R. A. in The Collision of Comet Shoemaker-Levy 9 and Jupiter
(eds. Noll, K., Weaver, H. & Feldman, P.) 269-292 (Cambridge, 1996).
10. Zahnle, K. in The Collision of Comet Shoemaker-Levy 9 and Jupiter
(eds. Noll, K., Weaver, H. & Feldman, P.) 183-212 (Cambridge, 1996).
12. Grieve, R. A. F., Meteoritics 26, 175-194 (1991).

------
Oracle

"O" == OracleofBugtussle writes:

O As promised (...), my e-mail (...) from Kevin Zahnle of Ames
O Research Center, is probably as complete and "official" as is
O available at present. Apparently, no one still has been able to
O fully explain the explosion (note the next-to-last sentence in the
O first paragraph).

here is some text I published in Nature ca. 1996. I don't think
I've written anything about Tunguska since:

Perhaps the earliest widely-held theory for the Tunguska explosion
was that the world was about to end. As the minutes passed this
theory was dropped in favor of other, less final theories, until
today one is hard-pressed to find anyone who truly believes that
the world ended on the morning of June 30, 1908. It is now
generally accepted that the explosion marked the end of another
world, a small (50-100 m) comet or asteroid destroyed by
aerodynamic forces some 10 km above the trees. Yet, because no
undoubted remnants of the impactor have been recovered, the
Tunguska event retains an air of mystery. The paper by
V.V. Svetsov on page xxx of this issue (ref 1) may provide a part
of the explanation.

This is not my take on it. It is true that there is not a consensus
regarding what exploded. How to make something entering the
atmosphere at more than 10 km/s explode is not in doubt. You focussed
on the penultimate sentence in this paragraph. Note the preceding
sentence.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

Thanks!

Joseph Lazio wrote:

This is not my take on it. It is true that there is not a consensus
regarding what exploded. How to make something entering the
atmosphere at more than 10 km/s explode is not in doubt.

So, I'll ask you ONCE more to explain it to me. In fact, you could
re-read my *original* post on this, but I doubt you'd get it yet,
based on your performance so far.

I have the feeling that the "general consensus" that Kevin referred
to is simply a matter of "no other reasonable choice," considering
that we know that there indeed WAS an explosion over Siberia on 6-30-08.

When you seemed to be dancing
in one of your e-mails to me, and I responded, once again, to press you
on HOW (remember this from my first post?) the object exploded with many
of the characteristics of an H-bomb, leaving no remnants to fall to the ground,
you, incredibly, replied by asking my how an H-bomb explodes.

Are you telling me that you don't know the difference between a
multi-million dollar thermonuclear weapon and a hunk of rock, or
comet, flying through the air?

You are doing a pretty good job of portraying a kook.
Or is it just a portrayal?

--
Oracle

Of course it was not a Meteorite and Back Engineering techniques as used by
the True Geology demonstrate this readily !
Likewise the same technique demonstrate the Tunguska event exploded with
many
of the characteristics of an H-bomb, leaving no remnants to fall to the
ground ( quoting ) ... aparenting it indeed to such nuclear type of
explosion.

Unfortunately Geologists at large are completely impervious or further
ignorant of the real nature of our Earth Environment ( Based of course on
the UPL )

Well seen and congratulations therein.

ncp

"OracleofBugtussle" a écrit dans le message de
om...
Joseph Lazio wrote:

This is not my take on it. It is true that there is not a consensus
regarding what exploded. How to make something entering the
atmosphere at more than 10 km/s explode is not in doubt.

So, I'll ask you ONCE more to explain it to me. In fact, you could
re-read my *original* post on this, but I doubt you'd get it yet,
based on your performance so far.

I have the feeling that the "general consensus" that Kevin referred
to is simply a matter of "no other reasonable choice," considering
that we know that there indeed WAS an explosion over Siberia on 6-30-08.

When you seemed to be dancing
in one of your e-mails to me, and I responded, once again, to press you
on HOW (remember this from my first post?) the object exploded with many
of the characteristics of an H-bomb, leaving no remnants to fall to the
ground,
you, incredibly, replied by asking my how an H-bomb explodes.

Are you telling me that you don't know the difference between a
multi-million dollar thermonuclear weapon and a hunk of rock, or
comet, flying through the air?

You are doing a pretty good job of portraying a kook.
Or is it just a portrayal?

--
Oracle

"ncp" wrote in message
...
Of course it was not a Meteorite and Back Engineering techniques as used
by
the True Geology demonstrate this readily !

Turdhard, get off this frequency!!! The second part of your fake e-mail
address describes you perfectly (mole).

On Wed, 17 Dec 2003 11:54:31 +0100, "ncp"
wrote:

Of course it was not a Meteorite and Back Engineering techniques as used by
the True Geology demonstrate this readily !
Likewise the same technique demonstrate the Tunguska event exploded with
many
of the characteristics of an H-bomb, leaving no remnants to fall to the
ground ( quoting ) ... aparenting it indeed to such nuclear type of
explosion.

Unfortunately Geologists at large are completely impervious or further
ignorant of the real nature of our Earth Environment ( Based of course on
the UPL )

Well seen and congratulations therein.

Ready to lose *another* account JP?

--

Find out about Australia's most dangerous Doomsday Cult:
http://users.bigpond.net.au/wanglese/pebble.htm

"You can't fool me, it's turtles all the way down."

In article ,
OracleofBugtussle wrote:

Joseph Lazio wrote:

This is not my take on it. It is true that there is not a consensus
regarding what exploded. How to make something entering the
atmosphere at more than 10 km/s explode is not in doubt.

So, I'll ask you ONCE more to explain it to me. In fact, you could
re-read my *original* post on this, but I doubt you'd get it yet,
based on your performance so far.

I have the feeling that the "general consensus" that Kevin referred
to is simply a matter of "no other reasonable choice," considering
that we know that there indeed WAS an explosion over Siberia on 6-30-08.

When you seemed to be dancing
in one of your e-mails to me, and I responded, once again, to press you
on HOW (remember this from my first post?) the object exploded with many
of the characteristics of an H-bomb, leaving no remnants to fall to the
ground,
you, incredibly, replied by asking my how an H-bomb explodes.

Are you telling me that you don't know the difference between a
multi-million dollar thermonuclear weapon and a hunk of rock, or
comet, flying through the air?

You are doing a pretty good job of portraying a kook.
Or is it just a portrayal?

--
Oracle

Of course! Why bother with science when you can argue _ad hominem_ &
have fun name-calling?

Anagram for OracleofBugtussle: A SUBERECT FOOL SLUG


One of thousands, & we're just getting started.

----
Hung Wu

[P.S.] SO: U A TROLL? BUG FECES!

[P.P.S.] Or: TROLL ABUSE CUES FOG...

"Tyrannus von Krummholz" wrote in message
...
In article ,
OracleofBugtussle wrote:

Joseph Lazio wrote:

This is not my take on it. It is true that there is not a consensus
regarding what exploded. How to make something entering the
atmosphere at more than 10 km/s explode is not in doubt.

So, I'll ask you ONCE more to explain it to me. In fact, you could
re-read my *original* post on this, but I doubt you'd get it yet,
based on your performance so far.

I have the feeling that the "general consensus" that Kevin referred
to is simply a matter of "no other reasonable choice," considering
that we know that there indeed WAS an explosion over Siberia on 6-30-08.

When you seemed to be dancing
in one of your e-mails to me, and I responded, once again, to press you
on HOW (remember this from my first post?) the object exploded with many
of the characteristics of an H-bomb, leaving no remnants to fall to the
ground,
you, incredibly, replied by asking my how an H-bomb explodes.

Are you telling me that you don't know the difference between a
multi-million dollar thermonuclear weapon and a hunk of rock, or
comet, flying through the air?

You are doing a pretty good job of portraying a kook.
Or is it just a portrayal?

--
Oracle

Of course! Why bother with science when you can argue _ad hominem_ &
have fun name-calling?

Anagram for OracleofBugtussle: A SUBERECT FOOL SLUG


One of thousands, & we're just getting started.

----
Hung Wu


Complains of name-calling with a response doing the same does not help your
credibility.

In article , George
wrote:

"Tyrannus von Krummholz" wrote in message
...
In article ,
OracleofBugtussle wrote:

Joseph Lazio wrote:

This is not my take on it. It is true that there is not a consensus
regarding what exploded. How to make something entering the
atmosphere at more than 10 km/s explode is not in doubt.

So, I'll ask you ONCE more to explain it to me. In fact, you could
re-read my *original* post on this, but I doubt you'd get it yet,
based on your performance so far.

I have the feeling that the "general consensus" that Kevin referred
to is simply a matter of "no other reasonable choice," considering
that we know that there indeed WAS an explosion over Siberia on 6-30-08.

When you seemed to be dancing
in one of your e-mails to me, and I responded, once again, to press you
on HOW (remember this from my first post?) the object exploded with many
of the characteristics of an H-bomb, leaving no remnants to fall to the
ground,
you, incredibly, replied by asking my how an H-bomb explodes.

Are you telling me that you don't know the difference between a
multi-million dollar thermonuclear weapon and a hunk of rock, or
comet, flying through the air?

You are doing a pretty good job of portraying a kook.
Or is it just a portrayal?

--
Oracle

Of course! Why bother with science when you can argue _ad hominem_ &
have fun name-calling?

Anagram for OracleofBugtussle: A SUBERECT FOOL SLUG


One of thousands, & we're just getting started.

----
Hung Wu


Complains of name-calling with a response doing the same does not help your
credibility.


George:

Regrets for the perceived offense.

At no point did I *call* Oracle something, though I did make fun of his
handle. (He actually shows a measure of courage in his choice of it.)
It's not even his views I wish to disparage, but his style (as in "Your
persistent questioning suggests you are a kook"). We've probably all
seen worse here...

----
Hung Wu