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
----------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
----------------------------------------
The CCNet is a scholarly electronic network. To subscribe, please
contact the moderator Benny J Peiser at <b.j.peiser@livjm.ac.uk>.
Information circulated on this network is for scholarly and
educational
use only. The attached information may not be copied or
reproduced for
any other purposes without prior permission of the copyright
holders.
The electronic archive of the CCNet can be found at
http://abob.libs.uga.edu/bobk/cccmenu.html
*
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.