PLEASE NOTE:


*

CCNet DIGEST, 2 June 1998
-------------------------

(1) DIFFERENCES BETWEEN CIVILIZATION-THREATENING IMPACTS AND
    EXTINCTION-LEVEL EVENTS
    Clark Chapman <cchapman@boulder.swri.edu>

(2) TUNGUSKA YIELD
    Mark Boslough <mbboslo@valinor.sandia.gov>

(3) PERIODIC ACCRETION OF INTERPLANETARY DUST
    S.J. Kortenkamp & S.F. Dermott, CARNEGIE INSTITUTE WASHINGTON

(4) QUESTION OVER 'PLANET' DISCOVERY
    The BBC
    http://news.bbc.co.uk/hi/english/sci/tech/newsid_102000/102797.stm

(5) ANATOMY OF AN ANOMALY: GEOLOGICAL EVIDENCE OF DEVONIAN IMPACT
    CATASTROPHE IN SOUTHERN NEVADA
    J.E. Warme & H.C. Kuehner, COLORADO SCHOOL OF MINES

(6) MYSTERIOUS IRON-NICKEL-ZINC ARCTIC SPHERULES
    D.A. Darby, OLD DOMINION UNIVERSITY

================
(1) DIFFERENCES BETWEEN CIVILIZATION-THREATENING IMPACTS AND
    EXTINCTION-LEVEL EVENTS

From Clark Chapman <cchapman@boulder.swri.edu>

Dear Benny,

In the spirit of Duncan Steel's critique of a news story in the latest
CCNet Digest, let me comment on the piece from The New Scientist quoted
in the same Digest. While this article is generally excellent in
accuracy and tone, it perpetuates something that I regard as one of the
biggest misconceptions about the impact hazard: insensitivity to the
enormous differences between what I have called a
"civilization-threatening event" and the impacts capable of causing
mass extinctions ("Extinction Level Events" in the terminology of "Deep
Impact"). The magnitudes of these catastrophes are so beyond our
personal experiences (fortunately!) that it is easy to make enormous
quantitative mistakes and overlook them.

The New Scientist article says that "Extinction Level Events" are due
to half-kilometre sized objects. Not true. By the way, I think they
mean one-km diameter objects, since that's the size for which it has
been estimated that about 2,000 larger than that exist in
Earth-approaching orbits. Actually, it is the 1.5 km wide (1 mile wide)
objects that David Morrison and I have considered to be the best
estimate of the size for the onset of a civilization-threatening
scenario.

But the K/T boundary holocaust was as though *hundreds* of mile-wide
asteroids struck at once! The wreckage of the ecosphere was so extreme
that not only did virtually every animal and plant on the planet die,
but there weren't even a few individuals of most species left, capable
of reproducing and carrying on their species.

A *far less* horrible impact by a single mile-wide impact would,
nevertheless, do more than "ruin our day." It would make the Great
Plague and World War II look inconsequential in comparison. But it
would leave no trace of its sub-extinction-level destruction in the
fossil record.

I think it is important for educators interested in the impact hazard
to emphasize that all globally destructive impacts are *not* equally
horrible.

Clark Chapman

======================
(2) TUNGUSKA YIELD

From Mark Boslough <mbboslo@valinor.sandia.gov>

Duncan,

We argued in our NEO proceedings paper that Tunguska could have had a
yield as low as 3.5 Mt. I think that 12.5 Mt is also an overestimate.
The higher number comes from a combination of treefall and seismic
data, but the treefall estimate assumes healthy forest with >90% of
trees blown down (neither true) and the seismic argument ignores
reaction force of a ballistic plume.  I do not think Tunguska was
larger than 5 Mt.

Regards,

Mark Boslough

====================
(3) PERIODIC ACCRETION OF INTERPLANETARY DUST

S.J. Kortenkamp*) & S.F. Dermott: A 100,000-year periodicity in the
accretion rate of interplanetary dust, SCIENCE, 1998, Vol.280, No.5365,
pp.874-876

*) CARNEGIE INSTITUTE WASHINGTON, DEPARTMENT OF TERRESTRIAL MAGNETISM,
   5241 BROAD BRANCH RD NW, WASHINGTON, DC, 20015

Numerical modeling of the orbital evolution of interplanetary dust
particles revealed that, over the past 1.2 million years, the rate of
accretion of dust by Earth has varied by a factor of 2 to 3. These
variations display a 100,000-year periodicity and are anticorrelated
with Earth's changing orbital eccentricity. Extraterrestrial helium-3
concentrations in a deepsea sediment core display a similar periodicity
but are 50,000 years out of phase with the predicted variations. Also,
because collisions between large bodies in the asteroid belt are
inevitable, it is expected that large-amplitude stochastic variations
on 10(7)- to 10(8)-year time scales would be superimposed on the
10(5)-year periodic variations. Copyright 1998, Institute for
Scientific Information Inc.

======================
(4) QUESTION OVER 'PLANET' DISCOVERY

From the BBC
http://news.bbc.co.uk/hi/english/sci/tech/newsid_102000/102797.stm

A scientist from the world famous Royal Greenwich Observatory says Nasa
scientists claiming to have found the first planet seen outside our
solar system are jumping the gun.

Dr Robin Catchpole believes a lot more work needs to be done before
such conclusions are drawn. 

The new object, in the constellation of Taurus, was found by the Hubble
space telescope.  Nasa believes it may be a planet two or three times
the mass of Jupiter, the largest planet in our solar system.
Alternatively it could be a brown dwarf star, one that was too small to
sustain the nuclear reactions in its core that normal stars need to
shine.

But although Dr Catchpole admits that the brown dwarf explanation is
feasible, he says more work is needed before further conclusions can be
drawn. "I really don't think there's any evidence to say that what
we are seeing here is a planet", he told BBC World Service, adding that
scientists needed to use another instrument to determine what sort of
light the object was giving out.

A spectrum of the object would tell researchers whether it was hot and
luminous like a star or cool and reflective like a planet.  "This one
looks far too bright to be reflecting the light of the stars nearby,"
he said.


Although he accepted the discovery had potential, he said: "A lot more
work has to be done before it's right to go out and tell everybody
you've found a planet." This could be done with the biggest telescope
in the world which this week produced its first pictures of space. The
new instrument built by eight European nations in northern Chile would
be able to gather the light, analyse it and tell us exactly what it is
made of.

Dr Catchpole believes Nasa's release of the story was symptomatic of an
increasing tendency for scientists "to grab the headlines as soon as
possible." He said this was driven by the need to get funding for
research and searching for life in other worlds was an area where there
was a lot of competition.

"The big question that everybody wants to know about is, is there life
out there? Are there planets out there? So the first person who really
gets one is going to make a major discovery."

(C) 1998 BBC

=====================
(5) ANATOMY OF AN ANOMALY: GEOLOGICAL EVIDENCE OF DEVONIAN IMPACT
    CATASTROPHE IN SOUTHERN NEVADA

J.E. Warme*) & H.C. Kuehner: Anatomy of an anomaly: The Devonian
catastrophic Alamo impact Breccia of southern Nevada. INTERNATIONAL
GEOLOGY REVIEW, 1998, Vol.40, No.3, pp.189-216

*) COLORADO SCHOOL OF MINES, DEPARTMENT OF GEOLOGY & GEOLOGICAL
   ENGENEERING, GOLDEN, CO, 80401

The Alamo Breccia is a carbonate rock breccia of Late Devonian age in
southern Nevada. It is an anomalous sedimentary unit because it has the
properties of a massive debris-flow and turbidity-current deposit that
would be expected to occur in deep water, but is intercalated over much
of its area with typical shallow-water carbonate-platform beds. The
Breccia was created by the catastrophic detachment and flow, over a
nearly horizontal surface, of previously deposited platform carbonates.
It crops out in 14 or more mountain ranges that cover an area of
similar to 10,000 km, conservatively averages similar to 50 m in
thickness, and contains a volume of 500+ km. Along the base it contains
trains of individual detached blocks as much as 500 m long and 90 m
high. Clasts generally grade upward to gravel-, sand-, or mud-sized
particles at the top. The Breccia was generated by forces unleashed
during the impact of an extraterrestrial object with Earth. The impact
produced shocked quartz grains, unique ejecta spherules, and an iridium
anomaly-which are present within the Breccia but absent from confining
beds. Internally the Breccia is segmented vertically into as many as
five sequentially thinner graded units created by successive tsunamis.
In one range, peculiar deformed dolostone, shocked quartz sandstone,
and sedimentary dikes and sills occur under the Breccia and deep-water
limestones rest over it, indicating a near-crater location. Surrounding
detached megablocks and tsunamites suggest an annular crater trough.
The Breccia formed within the span of a few hours or days, and falls
entirely within a single early Frasnian conodont zone, at similar to
367 Ma. The well-documented middle Late Devonian (Frasnian/Famennian)
extinctions are similar to 3 Ma later. An impact scenario explains the
known features of the Alamo Breccia: impact occurred on the Late
Devonian outer platform or slope; seismic shock delaminated the upper
similar to 50 to 100 m of the platform, loosening carbonate-platform
bedrock and creating trains of large blocks that may rest in an annular
trough; successive tsunamis reworked the loosened material, which was
augmented by unknown proportions of ejecta containing shocked quartz
from the crater, carbonate spherules from the vapor cloud, and iridium
from the projectile. Mass flows west of the platform likely represent
tsunami backwash, shock-induced failure along the platform margin, and
slumps from offshore topographic highs. Copyright 1998, Institute for
Scientific Information Inc.

======================
(6) MYSTERIOUS IRON-NICKEL-ZINC ARCTIC SPHERULES

D.A. Darby: Mysterious iron-nickel-zinc arctic spherules, CANADIAN
JOURNAL OF EARTH SCIENCES, 1998, Vol.35, No.1, pp.23-29

OLD DOMINION UNIVERSITY, DEPARTMENT OF OCEANIC, EARTH & ATMOSPHERIC
SCIENCES, NORFOLK, VA, 23529

An investigation into the sources of ice-rafted detritus in the central
Arctic Ocean using microprobe analyses of detrital Fe oxide minerals
discovered unique magnetic spherules in 20 of 144 potential source
sample sites from the shelves and coastal areas around this ocean. The
spherules occur only in samples from the Queen Elizabeth Islands,
Canada. These grains are characterized by a 45-60 mu m diameter,
pitting throughout, and are occasionally found as multiple joint
spherules. They have the optical properties of magnetite and, most
remarkably, contain both ZnO and NiO in subequal amounts of up to 25%.
The Ni suggests either an anthropogenic or meteoritic source. These
spherules were probably ice-rafted into the central Arctic Ocean, where
they are found in sediments of Holocene age and back to at least 780
ka, eliminating an anthropogenic source. Because Zn is too volatile to
survive entry of a meteor through the earth's atmosphere, these
spherules were probably formed during impact of an Fe-Ni meteor in an
area of abundant Zn, perhaps the Zn rich Paleozoic carbonates of the
Queen Elizabeth Islands. Examination of the ejecta and sediments
filling the 22 Ma Haughton Astrobleme impact site on Devon Island, a
carbonate terrain, revealed few magnetite spherules. None of these were
pitted or contained Ni, but a few percent of ZnO were found in three
spherules. Thus, the origin of these magnetite spherules remains
unknown. The unique appearance and geochemistry of these spherules are
useful in tracing Arctic Ocean ice-rafted detritus to its source.
Copyright 1998, Institute for Scientific Information Inc.

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