PLEASE NOTE:
*
CCNet DIGEST, 3 November 1998
-----------------------------
(1) NEMESIS ATTACK: ASTEROID AS WEAPON
Bill Napier <wmn@star.arm.ac.uk>
(2) THE LEONIDS ARE COMING!
Science News
http://www.sciencenews.org/sn_arc98/10_31_98/bob1.htm
(3) CIVILISATION COLLAPSE, CLIMATIC DOWNTURNS & SOLAR
FLUCTUATION
Ken Hsu <ken@erdw.ethz.ch>
(4) THE DUST ENVIRONMENT OF COMET HALE-BOPP
M. Fulle et al., ASTRONOMICAL OBSERVATORY
TRIESTE
(5) HUBBLE SPACE TELESCOPE OBSERVATIONS OF COMETARY NUCLEUS &
COMA
P.L. Lamy et al., CNRS,MARSEILLE
(6) SHOCK-INDUCED EFFECTS OF METEORITE IMPACTS
I. Martinez & P. Agrinier, IPGP, PARIS
(7) MASS DEFICIENCY AT THE CHICXULUB IMPACT CRATER
J.O. Campos Enriquez et al.,UNIVERSITY
NACIONAL AUTONOMA MEXICO
==================
(1) NEMESIS ATTACK: ASTEROID AS WEAPON
From Bill Napier <wmn@star.arm.ac.uk>
Dear Benny,
UK readers of your CC bulletin may be interested to know that I
have
written a novel, NEMESIS, which has as its theme an asteroid
diverted
towards the USA as a weapon. It's being marketed by Headline
as a thriller.
Many of the issues familiar to your readers appear in the book
as matters for frantic and ferocious debate. (Is it a degassed
comet
or a stray from the main belt? What is the status of historical
catastrophism? What would happen to threat information if NASA
controlled MPC? Would deflection break the asteroid up? etc).
There are also a few giant egos running amok (I hasten to add
that the
characters are purely fictional!) And there is a twist in the
tail.
NEMESIS is about to appear in UK bookshops (hardback and trade
paperback). Contracts have either been signed or are being
negotiated
with publishers in Germany and Japan, with more likely to come.
Also, a
worldwide mass-market paperback version is scheduled for next
summer.
Concepts familiar to your readers will thus hopefully reach a
large
audience to whom the idea of an asteroid impact has hitherto been
seen
as no more than a Bruce Willis fantasy.
Best regards
Bill Napier
================
(2) THE LEONIDS ARE COMING!
From Science News
http://www.sciencenews.org/sn_arc98/10_31_98/bob1.htm
October 31, 1998
A memorable light show or just a bracing shower?
By RON COWEN
The night of Nov. 12, 1833, might as well have been July 4. The
skies
over the United States were ablaze with bursts of light
surpassing the
most extravagant display of fireworks.
It was, one eyewitness said, as if "a tempest of falling
stars broke
over the Earth . . . . The sky was scored in every direction with
shining trails and illuminated with majestic fireballs."
Even some
souls asleep in their beds were awakened by the silent light show
streaking through their windows -- an unusually intense display
of
shooting stars called the Leonid meteor storm.
Every year in mid-November, Earth encounters the Leonid storm, so
named because it was thought to come from the constellation
Leo. In
fact, this storm originates from a tenuous stream of dusty debris
expelled by Comet 55P/ Tempel-Tuttle during centuries of passes
near
the sun. These particles, known as meteoroids, spread out along
the
comet’s orbit.
As the meteoroids from this comet or others burn up in Earth’s
atmosphere, some 100 streaks of light an hour may grace the
skies.
Such events, known as meteor showers, are common; our planet
travels
through about 12 a year, each from a meteoroid stream spewed by a
different comet.
A more intense downpour, strong enough to qualify as a meteor
storm,
is rarer. A Leonid storm occurs roughly every 33 years,
when the
planet passes through a dense trail of debris that lies close to
the
comet (SN: 6/14/97, p. 371).
Chinese astronomers reported seeing the first storm in 902 A.D.
The
last one, in 1966, rivaled the spectacle of 1833 and was so
intense
that viewers likened the profusion of shooting stars to
snowflakes in
a snowstorm. This November 17, and perhaps the same time next
year,
the planet will once again plunge into a dense concentration of
the
Leonid stream.
Earth will pass three times farther from the comet than it did
during
the 1966 blockbuster. It’s not known whether this year’s
passage will
generate a mild storm, with thousands of shooting stars an hour,
or
just an intensified shower, with the streaks of light appearing
at
perhaps one-tenth to one-hundredth that rate.
Either way, East Asia is the prime location for witnessing the
peak of
the encounter, predicted to occur about 2:20 p.m. eastern
standard
time on Nov. 17 and to last at most a few hours. While it will be
a
moonless night in East Asia, daylight will render even the most
intense fireworks invisible in North America. Sky watchers in the
United States, however, might still be treated to a
better-than-average spectacle in the wee hours of the morning on
both
Nov. 17 and Nov. 18, says Brian G. Marsden of the
Harvard-Smithsonian
Center for Astrophysics in Cambridge, Mass. (see sidebar).
There’s a good chance the light show in 1999 will be at
least as
lavish as the one this year, says Peter Brown of the University
of
Western Ontario in London, Ontario. After that, thanks to Jupiter’s
tug on Tempel-Tuttle, the comet won’t get close to Earth for
another
century.
That’s one reason why Brown and his colleagues are setting
up shop
next month in Australia and in the Gobi desert in Mongolia. By
bouncing radar beams off the meteoroids and using optical TV
monitors
to track the streaks of light that the particles produce, the
researchers hope to gauge accurately the number of meteoroids
striking
Earth’s atmosphere and to test how well computer models have
predicted
the intensity of the event.
A close encounter between Earth and Tempel-Tuttle happens to
occur
just before or just after the icy comet passes closest to the sun’s
warming rays, forcing the comet to vent fresh debris. Because it
takes
time for the meteoroids to drift away from the comet, most of the
debris encountered by Earth is in fact composed of material
spewed by
the comet during previous passes.
The models developed by Brown and his colleagues, in which
they
simulate the release of debris from Tempel-Tuttle and track its
path
over several hundred years, suggest that the material Earth will
plow
through this year comes from dust expelled by the comet in 1899
and
1932. The team also finds that the 1999 event, best visible from
western Europe, might be about 50 percent stronger than the one
this
November.
In contrast, Donald K. Yeomans of NASA’s Jet Propulsion
Laboratory in
Pasadena, Calif., and his collaborators used a historical
approach to
predict that the Leonids will be equally intense this year and
next.
"We looked at all the showers from 902 A.D. through 1966 and
asked,
When were major storms witnessed?" says Yeomans. Factoring
in such
information as the proximity of the comet to Earth during a storm
and
the time lag between the comet’s closest approach to the sun
and the
occurrence of a downpour, the team predicts that the events of
1998 and
1999 may be the most spectacular showers since 1966.
The historical record suggests that the passages this year and
next
have characteristics in common with two previous sets of
encounters --
those in 1866 and 1867, in which observers saw a maximum of 5,000
shooting stars, or meteors, an hour, and those of 1931 and 1932,
in
which viewers counted at most about 200 meteors an hour. Yeomans
thus
estimates that observers this year will see somewhere between 200
and
5,000 meteors an hour.
That’s an admittedly wide range, he notes. "Nobody can
predict these
things well, so it’s a bit of a crapshoot," says
Yeomans. "There’s
nothing precise about predicting meteor storms."
Such uncertainties are proving particularly frustrating for
technology
companies, the U.S. military, NASA, and others who own or operate
any
of the 650 or so satellites in space. Researchers estimate that
each
craft’s chance of getting hit by a Leonid meteoroid is only
about 0.1
percent.
The debris is mostly tiny, and scientists don’t think any
particles
will punch holes in satellites. They are worried, however, about
the
electrical damage that may be wrought by these high-speed
meteoroids.
Traveling at 72 kilometers per second relative to Earth, more
than 200
times the speed of a 22-caliber bullet, Leonid particles have the
highest speeds of any group of meteoroids. That’s because
Earth and
the debris plow headlong into each other.
When a meteoroid no bigger than a grain of dust slams into a
satellite
at such speeds, the kinetic energy it delivers can generate a
cloud of
highly charged gas, or plasma. Depending on where the meteoroid
hits
and the construction of the satellite, the plasma may generate an
electromagnetic pulse that could short-circuit or destroy
delicate
electronic parts.
"It really comes down to our lack of understanding about
what happens
when something the width of a human hair hits a satellite 30,000
km
from Earth at 72 km/second," says Brown. "The simple
answer is that we
have only a vague idea. You can’t accelerate particles [this
large] on
Earth to that velocity."
The uncertainty breeds both apprehension and debate. The upcoming
Leonids "will represent the largest meteoroid threat to
spacecraft in
history," claims David K. Lynch of the Aerospace Corp. in
Los Angeles.
That assertion is "just nonsense," says Yeomans..
"That’s just Chicken
Little stuff."
Some scientists are quick to point out that during the 1966
Leonid
storm, not a single satellite was damaged. However, there are
more
than 10 times as many craft in space today as in 1966. In 1993, a
European Space Agency satellite spun out of control due to an
electrical disturbance generated by the impact of a particle from
the
Perseid meteor shower (SN: 10/2/93, p. 217).
This year "is the the first time . . . since we have had a
large number
of satellites that we’ve had a major, predicted meteor
storm, and this
is likely the largest one since 1966," says S. Pete Worden
of the U.S.
Air Force in Arlington, Va.
"If you’re the owner of a spacecraft that’s worth
$500 million up
there, even if the probability of being hit is less than 0.1
percent,
which it is, you may want to [take precautions]," Yeomans
notes. To
that end, NASA will not launch the space shuttle in mid-November,
and
the Hubble Space Telescope will point its mirror away from the
Leonid
meteoroids. Other satellites will be aligned edge on to the storm
so
that the smallest possible surface area will be exposed to the
debris.
In a few cases, says Worden, the military may shut power to
components
of satellites, such as antennae, that may be particularly
susceptible
to an electrical discharge. Some satellites might be switched off
entirely during the peak of the Leonid activity.
The craft that are at the greatest risk are those that reside in
extremely distant orbits, where Earth’s tug balances that of
the sun.
In such an orbit, about 1 million km closer to the sun than Earth
is,
a craft is far more likely to plow into a concentrated part of
the
Leonid trail. Ironically, these devices include the Solar and
Heliospheric Observatory, which just recovered from an unrelated
episode in which it lost power and spun out of control in June.
The only people in space, the cosmonauts in the Mir space
station, are
planning to ride out the height of the storm in Mir’s escape
vehicle,
the Soyuz capsule. In an emergency, the crew could fire up Soyuz
to
return to Earth.
Looking on the bright side, says Worden, "this is a really
unique
opportunity to test out all the satellites" a few years in
advance of
the solar maximum, a time when outbursts from the sun can hurl
clouds
of high-speed, charged particles toward Earth and generate global
electrical storms. Astronomers predict that a solar maximum will
occur
in 2000.
As the Leonid shower proceeds, observations by several teams,
including Brown’s, will guide government agencies and
businesses
concerned with the health of satellites. "The idea is to
give an early
indicator if it [turns out to be] a bad storm," says Brown.
"This is
the first real-time warning system" in place for a meteor
storm.
Radar observations by Brown and his collaborators may test an
intriguing, and somewhat disturbing, hypothesis that suggests the
coming Leonids could pose a greater threat to orbiting
satellites
than expected. In the October Astronomy and Geophysics, Duncan
Steel
of Spaceguard Australia in Adelaide proposes that the number of
Leonid
meteoroids in 1998 or 1999 could be 10 times greater than other
estimates.
Steel suggests that many of the freshest Leonid meteoroids are
composed entirely of organic, tar-like compounds -- key
constituents
of comets. In contrast to compounds rich in silicates or iron,
organic
compounds burn at relatively low temperatures high in Earth’s
atmosphere and are therefore not detected.
Some of the radar systems that Brown uses to detect meteoroids do
have
frequencies low enough to begin to detect Steel’s proposed
population,
Brown says. "I think, in broad terms, what Steel says is
probably
true." He notes, however, that Steel’s hypothesis holds
true only if
the composition of meteoroids is 100 percent organic. Even trace
amounts of silicates would cause a meteoroid to vaporize lower
down in
the atmosphere, where it could easily be detected by standard
equipment.
"My own feeling is that it’s not nearly as severe a
problem as he’s
presenting, but it underscores how much we don’t
understand," Brown
concludes. Come mid-November, sky watchers will have a chance to
find
out how stormy the Leonids will be.
Looking at the Leonids
Although astronomers aren’t at all sure that Earth’s
passage through
the Leonid meteor stream this November will prove a blockbuster,
it’s
a good bet that the annual light show will be more intense than
usual
-- even in North America, where viewers won’t see the peak
of activity
that sky watchers in East Asia will witness.
Instead of staying up late, it’s probably wiser to wake up
early on
Nov. 17 and 18. That’s because the radiant -- the point in
the sky
from which the Leonids appear to originate -- doesn’t rise
until about
12:30 a.m. local time and the show will be best at about 3:00
a.m.,
when the radiant is about 30 degrees above the horizon. The
radiant
lies within the constellation Leo’s western portion, a group
of stars
referred to as the sickle or backwards question mark.
The Leonid meteors are visible to the naked eye, and binoculars
would
only cut down the field of view, notes Brian G. Marsden of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Lie
down on a reclining chair, pointing your feet toward the east.
For the
best view, don’t gaze directly at the radiant; instead, look
30 or 40
degrees above or west of that point. The meteors should be
visible,
weather permitting, through sunrise. Dress warmly and remember,
if the
1998 event proves a dud, you’ll have a second chance in
1999.
-- R.C.
From Science News, Vol. 154, No. 18, October 31, 1998, p.
280.
Copyright 1998 by Science Service.
References:
Steel, D. 1998. The Leonid meteors: Compositions and
consequences.
Astronomy & Geophysics (October).
Yeomans, D.K., K.K. Yau, and P.R. Weissman. 1996. The impending
appearance of Comet Tempel-Tuttle and the Leonid meteors. Icarus
124:407.
Further Readings:
Cowen, R. 1997. Leonids: The coming storm. Science News 151 (June
14):371.
Sources:
Peter Brown
University of Western Ontario
Department of Astronomy
London, Ontario N6A 3K7
Canada
Brian G. Marsden
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
60 Garden Street
Cambridge, MA 02138
Duncan Steel
Spaceguard Australia P/L
P.O. Box 3303
Rundle Mall
Adelaide SA 5000
Australia
Donald K. Yeomans
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91125
==================
(3) CIVILISATION COLLAPSE, CLIMATIC DOWNTURNS & SOLAR
FLUCTUATION
From Ken Hsu <ken@erdw.ethz.ch>
Dear Benny:
Thank you for keeping me in the CCNet. I am very pleased that
your
book is out. I am now ordering a copy.
Your messages have been emphasizing the cosmic events. I do not
dispute the occurrences of cosmic events, or their short term
effect. But cosmic events, like volcanic eruptions, may
drastically
change the weather for a few years at most. Climatic changes are
long term changes. Cosmic input, like the K/T event, may have
feedback mechanisms to amplify the signals. Smaller events, such
as
those in the Holocene, most likely do not have lasting impact on
climate.
Being a geologist, I have been doing research on paleoclimatology
for decades. Basically, the driving force is solar radiation. The
Pleistocene ice ages are related to orbitary positions of the
earth,
or Milankovitch cycles. The Holocene changes are apparently best
correlated with solar activities.
From both scientific (lake sediments, ice cores, dust layers,
etc) and historical (archaeology, history) evidence, I have found
that quasiperiodical changes of about 1200/1300 years,which could
be
considered the 7th order fundamental harmonics of sunspot cycles.
The last 5000 years of history are divided into the following
epochs
1) Modern Age. Since 1840, global warming, economics not being
much
affected by climatic changes because of the Industrial
Revolution.
2) The Little Ice Age, 1280-1840. Age of colonization of
Europeans,
age of peasant rebellions in China, the age of southward
migrations
of the Anasazis.
3) The Age of Conquests, 600-1280. Immigration of pioneers into
no-man's land (Slavic people to Germany and Balkans, Alamanni to
Alpine foreland), locust-like movements of Arabs, Turks, Mongols,
and Vikings.
4) The Age of Migrations, 60 BCE-600 CE. Migration of Germanic
tribes. Southward migration of Chinese. Desperate people running
all
directions like lemmings. The Fall of the Roman Empire
5) The Greco-Roman World. 700 BCE - 60 BCE. The Cathagian and
Greek
emigrations, Roman conquests. The locust swarms of the Celts. The
Golden Age and Western Han of China.
6) The Centuries of Darkness, 1250-700 BCE. The migrations of
Indo-European tribes (Lausitzers, Urnfielders,Illyric, Italic,
Veneti, The Sea Peoples). The fall of the Bronze Age empires
(Hittites, Mycenae, Middle Kingdom of Egypt. Western Zhou Dynasty
in
China).
7) The blooms of Bronze Age, 1750-1250 BCE. The Egyptian and
Mesopotamian civilizations. The Aryan occupation of Indus Valley.
The Shang Dynasty of China.
8) The Collapse of Early Bronze Age cultures, 2400-1750 BCE. The
dispersal of the Indo-Europeans from their Nordic homeland.
The
collapse of Middle East Bronze-Age societies. The invasion of
Hyksos. The abandonment of Indus Valley. The final exodus from
the
Saharas.
9) The dawn of civilizations, 2400-3000 BCE. Sumer, Egyptians,
The
Age of Emperors and Kings in China. Indus Valley.
10) The first return of little ice age. 3000-3600 BCE. The Oetzi
man-in-ice. The first desiccation of the Saharas.
11) 3600 BCE- End of Younger Dryas, circ. 10000 BP. Expansion of
Neolithic Revolution.
12) post-glacial: Migrations of Paleolithic hunters.
I am completing a manuscript, entitled The Curse: A Theory of
History, which will be first published in German by Orelli
Füssli.
Those who are seriously interested can contact me for a copy of
manuscript of the relevant chapters.
I hope you spread the word in your CCNet.
Best regards,
Ken Hsu
================
(4) THE DUST ENVIRONMENT OF COMET HALE-BOPP
M. Fulle*), G. Cremonese, C. Bohm: The preperihelion dust
environment of C/1995 O1 Hale-Bopp from 13 to 4 AU. ASTRONOMICAL
JOURNAL, 1998, Vol.116, No.3, pp.1470-1477
*) ASTRONOMICAL OBSERVATORY TRIESTE,VIA TIEPOLO 11,I-34131
TRIESTE, ITALY
Two UK Schmidt plates of comet Hale-Bopp dust tail taken in
1996 May are analyzed by means of the inverse dust tail model.
The dust tail fits are the only available tools providing
estimates of the ejection velocity, the dust-loss rate, and the
size distribution of the dust grains ejected during years
preceding the comet discovery. These quantities describe the
comet dust environment driven by CO sublimation between 1993
and 1996, when the comet approached the Sun from 13 to 4 AU.
The outputs of the model are consistent with the available coma
photometry, quantified by the Af rho quantity. The dust mass
loss rate increases from 500 to 8000 kg s(-1), these values
being inversely proportional to the dust albedo, assumed here
to be 10%. Therefore, the mass ratio between icy grains and CO
results is at least 5. Higher values of the dust-to-gas ratio
are probable, because the model infers the dust-loss rate over
a limited size range, up to 1 mm sized grains, and because the
power-law index of the differential size distribution ranges
between -3.5 and -4.0, so that most of the dust mass was
ejected in the largest boulders that Hale-Bopp was able to
eject. The dust ejection velocity close to the observations,
between 7 and 4 AU, was close to 100 m s(-1) for grains 10 mu m
in size, much higher than that predicted by R. F. Probstein's
theory, thus confirming previous results of Neck-Line
photometry. This result is an indicator of CO superheating with
respect to a free sublimating CO ice, in agreement with the
high observed CO velocity. The fundamental result of the paper
is that such a high dust velocity remained constant between 13
and 4 AU, thus providing a strong constraint to all models of
the GO-driven activity of the comet during its approach to the
Sun: CO superheating must have been active since 13 AU from the
Sun. It might be provided by the abundant dust itself, or by
seasonal effects heating the subsurface layers, as was
suggested for comet 29P/Schwassmann-Wachmann 1. Another
similarity between the two comets is provided by the power-law
index of the time-averaged size distributions: -3.6 +/- 0.1 for
C/1995O1 and -3.3 +/- 0.3 for 29P/SW1. However, other
characteristics of the dust environments are very different, so
that, in general, it is impossible to distinguish a CO-driven
comet from a typical water-driven one. Copyright 1998, Institute
for
Scientific Information Inc.
=================
(5) HUBBLE SPACE TELESCOPE OBSERVATIONS OF COMETARY NUCLEUS &
COMA
P.L. Lamy*), I. Toth, H.A. Weaver: Hubble Space Telescope
observations of the nucleus and inner coma of comet 19P 1904 Y2
(Borrelly). ASTRONOMY AND ASTROPHYSICS, 1998, Vol.337, No.3,
pp.945-954
*) CNRS,ASTRON SPATIALE LAB,BP 8,F-13376 MARSEILLE 12,FRANCE
The nucleus of comet 19P/Borrelly was detected using the
Planetary Camera (WFPC2) of the Hubble Space Telescope (HST).
During the time of our observations, the comet was 0.62 AU from
the Earth, 1.40 AU from the Sun, and had a solar phase angle of
38 degrees. The high spatial resolution of the HST images
allowed us to discriminate clearly between the signal from the
nucleus and that from the coma. The lightcurve of the nucleus
indicates that it is a highly elongated body rotating with a
synodic period of 25.0 +/- 0.5 hr. Assuming that the nucleus
has a geometric albedo of 4% and is a prolate spheroid with a
rotational axis pointing in the direction determined by
Sekanina (1979), we derive that its semi-axes are 4.4 +/- 0.3
km and 1.8 +/- 0.15 km. The corresponding fractional active
area of similar to 8% suggests a moderately active comet. The
highly anisotropic coma is dominated by a strong sunward fan,
and the dust production rate exhibited signs of temporal
variability throughout our observations. Copyright 1998,
Institute
for Scientific Information Inc.
====================
(6) SHOCK-INDUCED EFFECTS OF METEORITE IMPACTS
I. Martinez & P. Agrinier: Meteorite impact craters on Earth:
major
shock-induced effects in rocks and minerals. COMPTES RENDUS DE L
ACADEMIE DES SCIENCES SERIE II FASCICULE A-SCIENCES DE LA TERRE
ET DES
PLANETES, 1998, Vol.327, No.2, pp.75-86
IPGP, 4 PL JUSSIEU, F-75252 PARIS 05, FRANCE
The basic principles of the physics of shock waves are
summarised,
showing how shock pressures, shock and post-shock temperatures,
and
shock durations can be estimated in the case of large meteorite
impacts on Earth. In a second part, the pertinence of laboratory
high-pressure dynamic experiments for simulating large meteroite
impact events and for calibrating their physical conditions is
discussed. It is concluded that most shock features are common to
natural and laboratory shocks, although the lifetime of
experimental
shocked states is shorter by several orders of magnitude. Then, a
review is made of the major shock effects observed in minerals
and
rocks. Quartz has been by far, the most extensively studied shock
mineral. particularly, planar deformation features (PDFs),
interpreted
as resulting from relaxations at the shock front, are unambiguous
shock indicators, for shock pressures approximately between 15
and 35
GPa. At higher pressures, the formation of high-pressure
polymorphs of
SiO2 in shocked quartz is also discussed. Shock effects in some
other
selected minerals, although less extensively studied, are also
reviewed, with special emphasis on the discovery of diamonds at
impact
sites and of all the high-pressure polymorphs of olivines an
pyroxenes, including silicate perovskite, in shocked meteorites.
Finally, the controversial links between large impacts and major
environmental effects are discussed in a fourth part. (C)
Academie des
sciences/Elsevier, Paris).
====================
(7) MASS DEFICIENCY AT THE CHICXULUB IMPACT CRATER
J.O. Campos Enriquez*), H.F. Morales Rodriguez, F. Dominguez
Mendez,
F.S. Birch: Gauss's theorem, mass deficiency at Chicxulub crater
(Yucatan, Mexico), and the extinction of the dinosaurs.
GEOPHYSICS,
1998, Vol.63, No.5, pp.1585-1594
*) UNIVERSITY NACIONAL AUTONOMA MEXICO, INST GEOFIS, DELEGAC
COYOACAN, MEXICO CITY 04510,DF,MEXICO
Using Gauss's theorem we estimated the mass deficiency of the
Chicxulub impact structure (Yucatan, Mexico) from its gravity
anomaly. The mass deficiency obtained from the residual gravity
anomaly map ranges between 1.06 x 10(16) and 1.67 x 10(16) kg.
Because the gravity anomaly has approximately radial symmetry,
we also estimated the mass deficiency from selected profiles.
In this way, we obtained slightly lower mass deficiencies (6.16
x 10(15) to 1.35 x 10(16) kg). The central gravity high, which
is supposed to be associated with the central structural high,
has a mean excess mass of 1.93 x 10(14) kg. By assuming a mean
density contrast of 100 kg/m(3) between the country rock and
the sedimentary and brecciated rocks, we estimated the
equivalent total mass (1.60 x 10(17) to 4.34 x 10(17) kg) and
volume (6.16 x 10(13) to 1.67 x 10(15) m(3)) of breccias and
sedimentary rocks responsible for the gravity anomaly. These
figures represent lower bounds on the mass and volume ejected
from the impact crater. They represent estimates made from
geophysical principles and data, and compare well with
independent estimates based on other principles such as scaling
relations. According to actual estimations of the sulfur
dioxide mass generated by the Cretaceous-Tertiary impact and
our results, only a small fraction (about 1%) of the anhydrite
in the target strata was vaporized. Copyright 1998, Institute for
Scientific Information Inc.
----------------------------------------
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