PLEASE NOTE:


*

CCNet DIGEST, 16 June 1998
--------------------------

(1) THE DRY STRATOSPHERE & MINI COMETS
    B. Hannegan et al., UNIVERSITY OF CALIFORNIA IRVINE

(2) RUBBLE PILE ASTEROIDS
    E. Asphaug et al., UNIVERSITY OF CALIFORNIA SANTA CRUZ

(3) CHARGED DUST DYNAMICS ABOVE THE SURFACE OF A COMET FAR FROM THE SUN
    A. Juhasz & K. Szego, RESEARCH INSTITUTE OF PARTICLE & NUCLEAR PHYS

(4) THE TURBULANT STATE OF COMETARY ION PICKUP
    I. Krauklis*) & A.D. Johnstone, UNIVERSITY COLLEGE LONDON

(5) EARLY RECOVERY OF COMET 55P/TEMPLE-TUTTLE
    O.R. Hainaut et al., UNIVERSITY OF HAWAII

(6) HISTORICAL INFORMATION ON METEORITE IMPACTS
    Duncan Steel <dis@a011.aone.net.au>

(7) NASA HISTORY OF COSMIC EARTH IMPACTS
    http://www.hq.nasa.gov/office/pao/History/impact.html

(8) MORE ON THE AD 540 EVENT
    Timothy Bratton <bratton@ACC.JC.EDU>

=================
(1) THE DRY STRATOSPHERE & MINI COMETS

B. Hannegan*), S. Olsen, M. Prather, X. Zhu, D. Rind, J. Lerner: The
dry stratosphere: A limit on cometary water influx. GEOPHYSICAL
RESEARCH LETTERS, 1998, Vol.25, No.10, pp.1649-1652

*) UNIVERSITY OF CALIFORNIA IRVINE, DEPARTMENT OF EARTH SYSTEM
   SCIENCES, IRVINE, CA, 92697

The stratosphere accumulates the bulk of meteoric and cometary material
that impacts the Earth and provides a measure of that flux. Recent
models and measurements of the effective age of stratospheric air
demonstrate our ability to simulate this buildup. Current observations
of H2O in the stratosphere and mesosphere are consistent with an
extraterrestrial source of H2O, but limit its influx to less than 2
Tg/yr. Such a flux is consistent with standard estimates but is 100
times less than the small-comet hypothesis. Copyright 1998, Institute
for Scientific Information Inc.

==================
(2) RUBBLE PILE ASTEROIDS

E. Asphaug*), S.J. Ostro, R.S. Hudson, D.J. Scheeres, W. Benz:
Disruption of kilometre-sized asteroids by energetic collisions.
NATURE, 1998, Vol.393, No.6684, pp.437-440

*) UNIVERSITY OF CALIFORNIA SANTA CRUZ, DEPARTMENT OF EARTH SCIENCES,
   SANTA CRUZ, CA, 95064

Recent numerical studies(1-5) suggest that 'rubble-pile' asteroids
(gravitationally bound aggregates of collisional debris) are common in
the Solar System, and that self-gravitation may equal or exceed
material cohesion for planetary bodies as small as several hundred
metres. Because analytical scaling relations for impact cratering and
disruption(6-8) do not extend to this size regime, where gravity and
material strength are both important, detailed simulations are needed
to predict how small asteroids evolve through impact, and also to
ascertain whether powerful explosions offer a viable defence against
bodies headed for a collision with Earth. Here we present simulations,
using a smooth-particle hydrodynamics code(9), of energetic impacts
into small planetary bodies with internal structure ranging from solid
rock to porous aggregate. We find that the outcome of a collision is
very sensitive to the configuration of pre-existing fractures and voids
in the target. A porous asteroid (or one with deep regolith) damps the
propagation of the shock wave from the impactor, sheltering the most
distant regions, while greatly enhancing the local deposition of
energy. Multiple-component asteroids (such as contact binaries) are
also protected, because the shock wave cannot traverse the
discontinuity between the components. We conclude that the first impact
to significantly fragment an asteroid may determine its subsequent
collisional evolution, and that internal structure will greatly
influence attempts to disrupt or deflect an asteroid or comet headed
towards Earth. Copyright 1998, Institute for Scientific Information
Inc.

==================
(3) CHARGED DUST DYNAMICS ABOVE THE SURFACE OF A COMET FAR FROM THE SUN

A. Juhasz & K. Szego: Charged dust dynamics above the surface of a
comet far from the Sun.  JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS,
1998, Vol.103, No.A6, pp.12015-12022

KFKI, RESEARCH INSTITUTE OF PARTICLE & NUCLEAR PHYSICS, POB 49, H-1525
BUDAPEST, HUNGARY

We investigate the dynamics of small dust particles ejected from the
sunlit side of the nucleus of a comet far from the Sun. The motion of
these particles is determined by gravity, electromagnetic forces, and
solar radiation pressure. The nucleus is not shielded from either the
solar wind or the solar UV radiation; therefore the surface of the
nucleus absorbs electrons and protons and emits photoelectrons. As a
result, the surface gets positively charged. Above the nuclear surface
a photoelectron plasma sheath forms in which the ejected dust grains
collect electrostatic charges and become responsive to the electric
field in the sheath. We show that most of the dust cloud created by an
impact (e.g., a lander) will settle after a few (similar to 4) hours
elsewhere and not, in the close vicinity of the impact site (i.e., on
the lander itself). Copyright 1998, Institute for Scientific
Information Inc.

=================
(4) THE TURBULANT STATE OF COMETARY ION PICKUP

I. Krauklis*) & A.D. Johnstone: The turbulent state of cometary ion
pickup generated fluctuations near comet P/Halley. JOURNAL OF
GEOPHYSICAL RESEARCH-SPACE PHYSICS, 1998, Vol.103, No.A6, pp.12005-12014

*) UNIVERSITY COLLEGE LONDON, MULLARD SPACE SCI LAB, HOLMBURY ST
   MARY, DORKING RH5 6NT,SURREY,ENGLAND

One of the principal effects of an outgassing comet is to introduce
additional mass into the solar wind flow. This mass-loading process
results in the generation of Alfven waves via ion-cyclotron resonance.
In the case of the Halley encounter this source of ''new'' fluctuations
was introduced into a low speed solar wind 0.89 AU from the sun.
Previous in situ observations in low speed flows at this distance have
described the fluctuations as turbulently evolved or ''aged.'' The
spectral characteristics and time dependent anisotropies of these new
cometary fluctuations have been effectively explained by a
conservation-of-energy description of the cometary ion pickup process.
An alternative description has been proposed whereby turbulent
cascading generates the observed spectrum, implying that the
fluctuations evolve significantly in the time between their generation
and their observation. Analysis of the rugged invariants and, in
particular, the Alfven ratio suggests that throughout the interaction
region upwind from the comet the cometary ion pickup generated
fluctuations became less evolved as the bow shock was approached. Hence
turbulent cascading of the fluctuation energy is unlikely to be an
effective description of the observations or to modify the spectral
characteristics of these fluctuations significantly in the upwind
interaction region. Copyright 1998, Institute for Scientific
Information Inc.

=================
(5) EARLY RECOVERY OF COMET 55P/TEMPLE-TUTTLE

O.R. Hainaut*), K.J. Meech, H. Boehnhardt, R.M. West: Early recovery of
comet 55P/Tempel-Tuttle. ASTRONOMY AND ASTROPHYSICS, 1998, Vol.333,
No.2, pp.746-752

*) UNIVERSITY OF HAWAII, INSTITUTE OF ASTRONOMY, 2680 WOODLAWN
   DR, HONOLULU,HI,96822

We present recovery observations of the parent of the Leonid meteor
stream - comet 55P/Tempel-Tuttle using the Keck II telescope. At
recovery, the comet was at r = 4.5 AU, with m(R) = 22.6, and there was
no indication of coma or activity. Pre- and post-recovery observations
from 6 observing runs (1994-1997 June) are presented. From these
measurements we estimate that the nucleus radius is R-N = 1.8+/-0.2 km
with a nucleus axis ratio greater than 1.5. As of 6/97 (r = 3.5 AU),
the comet activity had not yet started. Copyright 1998, Institute for
Scientific Information Inc.

====================
(6) HISTORICAL INFORMATION ON METEORITE IMPACTS

From Duncan  Steel <dis@a011.aone.net.au>

Forwarded from Roger Launius (rlaunius@hq.nasa.gov)
Subject: Historical Information on Meteorite Impacts Available On-Line

We have some new Web sites related to the history of cosmic Earth
impacts (meteors, comets, asteroids, etc.).  There is a narrative
summary  at http://www.hq.nasa.gov/office/pao/History/impact.html and
also a bibliography at
http://www.hq.nasa.gov/office/pao/History/ceibiblio.html and a page
with links to some cool related pages at
http://www.hq.nasa.gov/office/pao/History/ceilinks.html. Jason Pates,
NASA History Office summer intern, did the research and writing of
these and Nick Siler, another NASA History Office summer intern, did
the html formatting.  Hope you find this helpful before seeing "Deep
Impact" or "Armaggedon!"

=====================
(7) NASA HISTORY OF COSMIC EARTH IMPACTS

From http://www.hq.nasa.gov/office/pao/History/impact.html

National Aeronautics and Space Administration

Office of Policy and Plans

NASA History Office

COSMIC EARTH IMPACTS

INTRODUCTION

Every year, thousands of cosmic bodies bombard the Earth's atmosphere,
with a few hundred surviving the journey to impact land or sea.
Meteorites the size of a basketball strike Earth about once a month,
with about 75 percent landing in water. Larger asteroids with potential
for major global environment changes hit once every 100,000 years on
average, with similar-sized comets impacting approximately every
500,000 years.

TERMS: ASTEROIDS, METEORS AND COMETS

Asteroids are generally composed of rock, with some made of metals such
as iron and nickel. The vast majority of asteroids in our solar system
exist in the Asteroid Belt between Mars and Jupiter. They have
diameters that range from under one mile to nearly 500 miles across.
Upon entry into the atmosphere, these big rocks are made incandescent
by friction with the Earth's atmosphere, burning a luminous trail or
streak into the sky called a meteor. Exceptionally bright meteors are
also sometimes called bolides, or fireballs. The core of the object
falling towards Earth is considered a meteoroid. When a meteoroid
survives the descent and strikes Earth, it is renamed a meteorite.
Meteorites are no more likely to be radioactive than ordinary
terrestrial rocks, and no meteorite yet has been found to contain any
element not occurring naturally on Earth. Larger meteorites have left a
number of craters around the world both on land and on the ocean floor.

Comets are "dusty iceballs" of rock, frozen water and organic compounds
that consist of a head, or coma, and a gaseous tail potentially
thousands of miles long. Its head consists of a solid nucleus
surrounded by a nebulous coma up to 1.5 million miles in diameter. The
tail is an elongated curved vapor trail arising from the coma when
sufficiently close to the sun, and is thought to consist chiefly of
ammonia, methane, carbon dioxide, and water. Comets are observed only
in that part of their orbits that is relatively close to the sun.

IMPACTS

The Chicxulub crater on the north shore of the Yucatán Peninsula in
Mexico is a 65-million-year-old asteroid or comet impact scar the
source of which is thought to have brought about the end of the
dinosaurs, along with about 70 percent of life on Earth. The recently
discovered impact kicked up a global cloud of dust and sulfur gases
that blocked sunlight from penetrating through the atmosphere and
introduced Earth to a decade of near-freezing temperatures. Small
glassy bodies dating 65 million years called tektites, which are
believed to be of extraterrestrial origin, have been found all around
the Gulf. The crater is estimated to be 120 miles in diameter,
which has since filled with ocean sediment. Scientists believe events
the size of Chicxulub occur once every 50-100 million years.

The most famous and best-preserved exposed crater, the Barringer Meteor
Crater near Winslow, Arizona is almost a mile in diameter and has been
dated to approximately 50,000 years old. It was discovered in 1891 and
was created by a nickel-iron meteorite about 150 feet across, weighing
roughly 300,000 tons and traveling at a speed of 40,000 miles per hour.
The impact caused an explosion equivalent to 20 million tons of TNT
andhas left behind a chasm 570 feet deep.

The 1908 Tunguska Event in Siberia occurred when an asteroid or comet
estimated at 200 feet wide and weighing about 100,000 tons exploded
five miles above the Earth, releasing approximately 20-40 megatons of
energy and leaving tiny grains of cosmic matter embedded in the trees
knocked down by the blast. An area the size of Rhode Island was
devastated in the explosion, killing wildlife within 20 miles of the
impact, and creating fires that burned for weeks. Falls in the class of
the Tunguska and Barringer variety occur over land only about once
every millennium.

On March 23, 1989 an asteroid named 1989 FC with a diameter about 0.3
miles and a kinetic energy of over 1,000 one-megaton hydrogen bombs
passed within 430,000 miles of the Earth. This asteroid was not
discovered until it had passed its point of closest approach, and only
after calculating backwards its orbital path. Since then several other
celestial bodies of similar sizes have been measured as coming within
62,000 miles of Earth.

CONCLUSION

Although scientists have now identified over 150 impact sites, the
history of Earth impacts will continue to grow as new discoveries of
hidden craters are made. Nearly all known craters have been recognized
as such since 1950 and a number of new structures are found each year.
Evidence suggests that there have been many thousands of other impacts
over the course of the Earth's history, some of which have yet to be
uncovered on the ocean floor or in hidden surface locations.

Updated June 10, 1998

NASA Chief Historian: Roger D. Launius
NASA History Web Curator: Steve Garber
For further information contact
Jason Pates or history@hq.nasa.gov

==================
(8) MORE ON THE AD 540 EVENT

From Timothy Bratton <bratton@ACC.JC.EDU>

Dear (History of Astronomy) List Members:

This is the strongest argument I've seen to date against Vesuvius as
the cause of the climatic conditions of A.D. 536, and it strengthens
the asteroid theory considerably.  Reactions?
 
From c.leroy@rocketmail.com Fri Jun 12 20:42:08 1998
Date: Fri, 12 Jun 1998 13:49:54 -0700 (PDT)
Subject: AD 540 Event & Vesuvius
 
I read your defense on Thursday's CC-Net Digest of Vesuvius as the
agent responsible for producing the climate downturn at AD 540 with
great interest. However, I do not believe Vesuvius can properly be
given the credit because the eruption at AD 536 left no acidity signal
in Greenland's GISP2 ice core; see G.A. Zielinski et al., SCIENCE 264,
13 May 1994, 948-952.

Their Table 1 shows that the acidity signals at AD 77 and 472 are
attributable to Vesuvius; but no significant acidity signals exist
between AD 508 and 639. Mike Baillie has been clear from my reading
that no appropriate acidity signal exists in the Greenland ice to
justify a significant volcanic vector for the AD 540s _Klimasturz_.
Zielinski et al. bear this out.

Surely, if a volcanic vector were a major player in the AD 540 event,
its activity would have registered in Greenland, as the AD 79 eruption
did if the signal at AD 77 (off due to counting error) is the result of
Vesuvius. To the best of my knowledge, every known major eruption above
10 degrees S. latitude in the past 2000 years has registered in the
Greenland ice. It is hard to imagine a volcanic vector for the AD 540
event <not> to have registered there, too.

Furthermore, Baillie's suggestion of a cometary/cosmic vector does not
require any crater to have been produced since, in the context of Clube
and Napier's Taurid Complex model, the major agents would have been
periodic heavy firefall storms punctuated by recurring Tunguska-class
events, which is suggested by the Chinese and Anglo-Saxon literary
sources. The chemical signatures of such activity may well be more
subtle than usually envisioned, but should be retrievable,
nevertheless, when proper protocols and resources are deployed in their
search, as I indicated in my letter in The Sciences in 1991 following
Kevin Pang's "Volcano Weather" in the Jan/Feb issue, which was posted
to the CC-List in 1997. After all, the chemical signatures for
Tunguska, even at ground zero required a concentrated effort to find,
not to mention the dearth of signatures in Greenland's ice.
 
                                              Sincerely,
                                              Leroy Ellenberger,
                                              St. Louis, MO
-----

From Timothy Bratton <bratton@ACC.JC.EDU>

Dear List Members:

Here's an addendum by Leroy Ellenberger to his last remarks.
 
Date: Fri, 12 Jun 1998 15:01:22 -0700 (PDT)
From: Leroy Ellenberger <c.leroy@rocketmail.com>
Subject: Re: AD 540 Event & Vesuvius
 
When I wrote before, I did not take into consideration Mike Baillie's
recent report to the CC-list that the GISP2 core was damaged between
the years AD 545 and 613, as Bob Kobres reminded me immediately.
However, if that ice core date of 545 is accurate (and there is every
reason to think that it is), then an eruption of Vesuvius in 536 would
be expected to show up in that core since it takes only one or two
years for the acid to travel from Italy to Greenland. On the other
hand, if the 536 is off too much, then its presence in the GISP2 might
well be compromised.

Given this defect in the GISP2 record, which is not reported by
Zielinski et al. in their SCIENCE paper cited previously, then every
effort should be made to obtain the acidity record from the Danish team
from their neighboring and contemporary GRIP core. 

                                               Leroy Ellenberger,
                                               St. Louis, MO

Dr. Timothy L. Bratton                  bratton@acc.jc.edu
Department of History/Pol. Science      work: 1-701-252-3467, ext. 2022
6006 Jamestown College                  home: 1-701-252-8895
Jamestown, ND 58405                     home phone/fax: 1-701-252-7507

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*

RE: MORE NUANCED AND CAREFUL EVALUATION OF EXTINCTION EVIDENCE IS
    DESIRABLE


CLARK'S RESPONSE

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

Dear Benny,

I thank Timo Assmuth for his cogent remarks about extinctions. However,
I meant exactly what I said and wonder if he would disagree. What I
said was (June 2nd CC Digest):  "...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.  Assmuth has added a word to my quote "for clarification", and
he then has criticized a different sentence, namely "...virtually every
animal and plant SPECIES on the planet die[d]." The remarks he makes
about the changed sentence are sensible and conform to what I
understand about the K/T extinctions -- roughly 70% disappeared, but
there are wide variations in the manner of the extinctions.

I was trying to indicate another misconception people have, but
evidently in too abbreviated wording for Assmuth and perhaps others to
have appreciated:  Just because a species survives through an
extinction in the fossil record, hardly means that the INDIVIDUALS in
the species thrived. My picture of the K/T is of a holocaust in which
the overwhelmingly vast majority of individuals died even of the
species that made it through. Perhaps there were environments in which
even most of the individuals survived the changes, but I suspect that
Assmuth would agree that, in many cases, species survived in part
because isolated microenvironments (refugia) existed that were less
deadly than most of their previous habitats.

I intended to counteract the vague impressions that people have that
while 70% of species were being wiped out, 30% of life was doing just
fine. It wasn't! The fossil record bespeaks of a mass killing that was
unfathomly devastating.

Clark Chapman

==================

TIMO'S REPLY

From Timo Assmuth <Timo.Assmuth@vyh.fi>

Dear Clark, Benny, and others (I really mean this - I think the NEO
community, in all its variety of opinion and at times heated debates,
also about 1997 XF11, is a very dear and fruitful one for the quest of
knowledge also to aid the preservation of life and its home on Earth).

I thank Clark for his additional remarks and clarifications. They
promote some more thoughts which may be relevant in the discussion.

As Clark noted, I did not cite his sentence verbatim, in an attempt to
condense it, but added "species" in the sentence dealing with the death
of virtually all animals and plants (individuals). In doing so, I may
have muffled his meaning instead of clarifying (in which case I
apologise). I didn't mean to question his emphasis of the plight of
survivors - it's certainly important for evaluations of the significance
and policy implications of the NEO threat.

However, I believe that also my own main points are still valid:

1. We may not know for certain that 'virtually all' (meaning precisely
what?) even of individual plants and animals (including diverse and
individual-rich groups such as insects), or 'most' species, died 'at
the K/T boundary' (meaning when and how long after impact ? - N.B. the
temporal resolution varies among samples and species assemblages).
For instance, if one were to base estimates of the share of species
which went extinct to a higher degree on some other (diverse but perhaps
less observable) groups of organisms, these estimates might be lower.

2. In particular, we do not seem to know for sure, although it is
implied commonly and, I felt, also in Clark's letter, to what degree
these deaths, sufferings and changes of life - at the levels of
individuals, populations, species (also in terms of definite survival)
and communities - were caused by the Chicxulub impact or ANY NEO impact.

Therefore, I think we may discern in our exchange of ideas
a) arguments for precaution in the sense of NEO risk aversion: e.g.,
also the survivors were and would again be severely hit by such a blow,
apart from the common concern for the non-surviving (species),
b) other arguments for caution in the sense of questioning the degrees,
characteristics and causes of NEO effects (just as we question the
geocentric narrowly uniformitarian assumption of no NEO effects).

I think all kinds of uncertainties and arguments are important to
remember, take into consideration and in many cases even express.
A bit too often does one read of the Yukatan-crosser having 'wiped out
the dinosaurs' (and, what is still more debatable, nearly all other
species), as an indisputable fact, and with the connotation that other
factors cannot have been significantly contributing.

For one thing, there may have been other NEOs around that time affecting
life - by direct hits OR by other agents, including fluxes of fragments
and associated atmospheric/other changes (=possibility which emphasizes
but perhaps also redirects 'NEO concern').

However, ALSO OTHER extraterrestrial as well as terrestrial, even
endogeneous (biological) factors may have contributed significantly
(=possibility which may challenge some of the more far-stretched NEO
concerns, and which is to be taken seriously for scientific, practical
and also 'tactical' reasons).

Wishing you well, yours sincerely,
Timo Assmuth/FEI 15.6.



CCCMENU CCC for 1998

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