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*

CCNet 48/2001 - 27 March 2001
-----------------------------


"A 100 metre-wide space rock known as 2001 EC16 paid a passing visit
to Earth's vicinity earlier today. As it swept by at a little over 1.7
million km from Earth - approximately four and a half lunar distances -
the only people to pay it much attention were a dedicated band of
astronomers. However, this will not always be the case. Although there was
no danger of a collision between the Earth and 2001 EC16, the day will
surely come when luck runs out for our world (and humanity). Our only
chance of survival is to detect the space invader long before a
head-on collision occurs. This is where two of ESA's future missions -
GAIA and BepiColombo - may be able to play a vital role in forewarning us of
impending impacts."
--European Space Agency, 23 March 2001


"Russian controllers of the Mir space station, which was
successfully burned up in the atmosphere last week and its remains sunk in
the Pacific Ocean, said on Monday they had not revealed the true risks
involved in the operation. [...] Russian specialists had previously
calculated the probabilities of the craft's debris falling on populated
areas but did not publish the information to avoid panic, Semyonov
told the Itar-Tass news agency."
--Business Day, 26 March 2001



(1) FUTURE ESA MISSIONS MAY HAVE A MAJOR IMPACT - BUT NOT ON SEARCH FOR
SMALL ASTEROIDS
    Ron Baalke <baalke@jpl.nasa.gov>

(2) RUSSIANS REVEAL TRUE RISK INVOLVED IN DITCHING MIR
    Business Day, 26 March 2001

(3) ONE IN A TRILLION: THE WORLD'S ONLY SPACE JUNK SURVIVOR
    ABC News, 22 March 2001

(4) TODAY'S ECO SCARE: "PARTS OF THE WORLD MAY BE HEADED TOWARD DOOM,
SCHOLARS SUGGEST"
    Philadelphia Inquirer, 25 March 2001

(5) "IS THERE A DOOSMDAY ASTEROID?"
    Duncan Steel <D.I.Steel@salford.ac.uk

(6) EMP AND ASTEROID ENTRY
    Phil Bland <P.A.Bland@open.ac.uk>

(7) PERIODICAL MASS EXTINCTIONS AND RECOVERY
    Prokoph A, Fowler AD, Patterson RT

(8) A SHORT DURATION OF THE K/T BOUNDARY EVENT
    Mukhopadhyay S, Farley KA, Montanari A

(9) AMINO ACIDS IN THE K/T BOUNDARY SEDIMENTS
    Brisman K, Engel MH, Macko SA

(10) MORE ON THE CHICXULUB IMPACT CRATER
     Pope KO, Rejmankova E, Paris JF

(11) DETECTION OF SMALL COMETS WITH A GROUND-BASED TELESCOPE
     Frank LA, Sigwarth JB

(12) ANALYSIS OF COMA DUST PROPERTIES IN COMET HALE-BOPP
     D.X. Kerola & S.M. Larson

===============
(1) FUTURE ESA MISSIONS MAY HAVE A MAJOR IMPACT - BUT NOT ON SEARCH FOR
SMALL ASTEROIDS

From Ron Baalke <baalke@jpl.nasa.gov>

Future missions may have a major impact
ESA Science News
March 23, 2001
http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=26578

A 100 metre-wide space rock known as 2001 EC16 paid a passing visit to
Earth's vicinity earlier today. As it swept by at a little over 1.7 million
km from Earth - approximately four and a half lunar distances - the only
people to pay it much attention were a dedicated band of astronomers.

However, this will not always be the case. Although there was no danger of a
collision between the Earth and 2001 EC16, the day will surely come when
luck runs out for our world (and humanity). Our only chance of survival is
to detect the space invader long before a head-on collision occurs. This is
where two of ESA's future missions - GAIA and BepiColombo - may be able to
play a vital role in forewarning us of impending impacts.

GAIA will be ESA's successor to the extremely successful Hipparcos
astrometry mission. As well as measuring the positions and brightness of
stars and galaxies with unprecedented precision, GAIA will be able to detect
all kinds of transient objects in its field of view - supernovae, flaring
stars and ... asteroids.

"GAIA will detect objects brighter than magnitude 20, so we should observe
about one billion objects over the whole sky," said Project Scientist
Michael Perryman. "This means that if there is something there, we will see
it. In the case of near-Earth asteroids (NEOs), we should find objects as
small as 500 metres in diameter."

"The precision optics on GAIA will also give a colossal improvement in
orbital measurements, allowing astronomers to make very precise, long-term
orbit determinations," he added. "This will allow them to work out whether
asteroids measured by GAIA will eventually collide with Earth."

"Although most of the large near-Earth objects will probably have been found
by the time of GAIA's launch around 2010," he explained, "the NEO catalogue,
compiled from ground observations, will not contain all of the class of NEOs
called Atens - asteroids that spend much of their time inside Earth's orbit.
GAIA will be able to observe fairly close to the Sun, so it will carry out a
reasonably comprehensive survey of these objects."

Even GAIA will not be able to detect the entire population of asteroids that
cross Earth's orbit and disappear in the glare of the Sun. However, another
ESA mission should make a significant contribution to the ongoing census of
potentially hazardous asteroids.

Although BepiColombo's prime objective is to explore Mercury, it will also
be able to search for unknown asteroids in the uncharted region of space
between the planet nearest to the Sun and our Earth. These NEOs are
particularly dangerous, since they can approach the Earth unseen against the
brilliance of the Sun.

"The BepiColombo mission will involve two orbiters and a lander," explained
Mission Scientist Rejean Grard. "One of the orbiters will be used for
planet-wide remote sensing and radio science on a polar orbit with periapsis
and apoapsis altitudes of 400 and 1500 km. However we are also planning to
have a telescope on this orbiter."

"This telescope would be able to monitor a strip of sky of 6 degrees by 360
degrees as it looks along the orbiter's direction of motion," he added. "By
detecting asteroids that cross this field of view and comparing measurements
made at different times, we could determine their orbits. Preliminary
evaluations indicate that there could be up to 100 objects in the selected
strip of sky at any one time."

The Cosmic Shooting Gallery.

2001 EC16 belongs to a growing family of space rocks larger than 100 meters
across that can come closer to Earth than 0.05 AU (7.5 million km).
Fortunately, none of the known NEOs are presently on a collision course with
our planet, although astronomers are finding new ones all the time. At the
present time, 291 known potentially hazardous asteroids have been detected.

How many NEOs are out there? No-one knows. Astronomers estimate that there
are between 750 and 1100 near-Earth asteroids bigger than 1 kilometre in
diameter. There are probably millions of smaller objects in orbits that
carry them close to Earth.

Why does it matter? The amount of damage caused by an asteroid impact
depends on its size. Asteroids bigger than 1 kilometre would release energy
equivalent to 100 000 megatonnes of TNT - equivalent to 10 million times the
power of the atomic bomb that flattened Hiroshima. The result would be
devastation on a global scale.

The good news is that such events happen on average only once every 300 000
years. The bad news is that collisions with medium-sized objects are much
more frequent - once in a few thousand years on average.

Even objects the size of 2001 EC16 pack a significant punch. In 1908, an
asteroid or comet just 50 metres across blew up in the atmosphere above
Siberia. If this explosion had occurred over central London, the entire city
would have been flattened.

MODERATOR'S NOTE: Interesting: for the first time, the European Space Agency
has utilised the close approach of an asteroid (2001 EC16) as a peg to send
out a press release. ESA's message seems clear and unambiguous: In the near
future, Europe will make sure that objects such as 2001 EC16 will be
detected long before potential impact: "Although there was no danger of a
collision between the Earth and 2001 EC16, the day will surely come when
luck runs out for our world (and humanity). Our only chance of survival is
to detect the space invader long before a head-on collision occurs. This is
where two of ESA's future missions - GAIA and BepiColombo - may be able to
play a vital role in forewarning us of impending impacts." The only problem
is, neither GAIA nor BepiColombo will be searching for near Earth objects
such as 2001 EC16; in other words, it is highly unlikely that 2001 EC16
would have been detected by the two ESA projects in question. That's why the
British NEO Task Force Report has highlighted the need for a *dedicated*
search programme that focuses on small to medium-sized near Earth objects
which have the potential of destroying whole regions or even small
countries. ESA should be commended for including GAIA and BepiColombo in
their future NEO search activities. Nevertheless, Europe will have to do
much better if it wishes to help find those objects - most likely to hit us
hard in the foreseable future - before they find us. BJP.

==========
(2) RUSSIANS REVEAL TRUE RISK INVOLVED IN DITCHING MIR

From Business Day, 26 March 2001
http://www.bday.co.za/bday/content/direct/1,3523,818610-6080-0,00.html

Russians reveal true risks involved in ditching Mir

RUSSIAN controllers of the Mir space station, which was successfully burned
up in the atmosphere last week and its remains sunk in the Pacific Ocean,
said on Monday they had not revealed the true risks involved in the
operation.

If the final descent of Mir had gone wrong, debris could have fallen in
Europe, Asia, Africa and America, says Yury Semyonov, director of the
Energia space rocket corporation which ran Mir for the Russian government.

Russian specialists had previously calculated the probabilities of the
craft's debris falling on populated areas but did not publish the
information to avoid panic, Semyonov told the Itar-Tass news agency.

Now all had gone smoothly, with remnants of the station hitting the south
Pacific clear of humans in a textbook operation on Friday, Semyonov said
debris would have posed most danger for Latin America, the probability of it
falling there set at 8,6 percent.

The risk of fragments raining on Africa was 8,2 percent and on parts of Asia
was 6,9 per cent.

Probabilities were 1,7 percent for the US, 1,5 percent for Australia, and
0,5 for West Europe.

Russia's Aviation and Space Agency took out a $200-million insurance policy
against any damage or injury caused when the Mir was toppled from its orbit
after 15 years of service.

Russian engineers said before the operation that the chances of successfully
sinking several tons of Mir's remains were 97 to 98 percent. - Sapa-DPA.

© BDFM Publishers 2000

MODERATOR'S NOTE: Before you get overexcited about this apparent case of
Russian cover-up, one should keep in mind that there isn't that much of a
difference between a 91% or a 97% success rate estimate. BJP

==========
(3) ONE IN A TRILLION: THE WORLD'S ONLY SPACE JUNK SURVIVOR

From ABC News, 22 March 2001
http://www.abcnews.go.com/sections/scitech/DailyNews/spacejunk_survivor_010322.html

March 22 - Scientists reckon the odds of being hit by a piece of falling
space junk are around one in a trillion. Tulsa resident Lottie Williams was
that unlucky one.

Williams, 48, was exercising in a Tulsa park one morning four years ago when
she was hit on the shoulder by a six-inch piece of blackened metallic
material.

A used Delta II rocket had crashed into the Earth's atmosphere half an hour
earlier, and scientists at NASA believe that Williams was hit by a part of
it - making her the only person in the world known to have been hit by
man-made space debris.

FULL STORY at
http://www.abcnews.go.com/sections/scitech/DailyNews/spacejunk_survivor_010322.html

===========
(4) TODAY'S ECO SCARE: "PARTS OF THE WORLD MAY BE HEADED TOWARD DOOM,
SCHOLARS SUGGEST"
 
From Philadelphia Inquirer, 25 March 2001
http://inq.philly.com/content/inquirer/2001/03/25/national/COLLAPSE25.htm
 
The argument: Today's world has much in common with failed societies such as
Rome and Tikal.

By Robert S. Boyd
INQUIRER WASHINGTON BUREAU

SAN FRANCISCO - Historians and archaeologists who study the downfall of
ancient civilizations are warning that parts of the modern world may be
heading the way of history's fallen empires.

Researchers say the overcrowded cities, water shortages and electricity
brownouts in 21st-century California, India and Brazil are ominous reminders
of the fate of ancient Rome, Babylon and the Mayan empire.

Previous prophets of doom, such as the English political economist T.R.
Malthus and the "Club of Rome," which in 1972 predicted that the world's
population would overwhelm its resources, have been proved wrong so far by
the rapid progress of technology. This time, however, some researchers say
the complexity caused by high technology could be mankind's undoing.

The Mayas, who dominated Central America in the ninth century, built
elaborate irrigation systems to support their booming population. But they
"suffered from problems that are startlingly similar to those today," said
Vernon Scarborough, an archaeologist at the University of Cincinnati.

"Overpopulation was a major factor in making the Maya vulnerable to
failure," Scarborough said at a conference on the "Collapse of Complex
Societies" in San Francisco last month. "The trigger event of the collapse
appears to have been a long drought beginning about 840 [A.D.]."

Although many factors, such as war and disease, contributed to the
calamities of antiquity, speakers at the conference singled out two causes:
too many people and too little fresh water. That one-two punch can become
lethal, they said, when environmental problems such as a prolonged drought
or a change in climate put too much stress on a society.

The movement of peoples into big cities such as Rome and Tikal, the Mayan
capital, created great wealth, rich cultures and complex bureaucracies that
ultimately proved to be unsustainable.

"Complex societies have been collapsing for 12,000 years - as long as they
have existed," said Joseph Tainter, an expert on prehistoric American
Indians at the Rocky Mountain Research Station in Albuquerque, N.M.

"High population levels gave early societies a fragility that made them
especially vulnerable to environmental changes," said Christopher Scarre, an
archaeologist from Cambridge University in England.

"The societies themselves appear to have contributed to their own demise,"
Scarre continued. "They encouraged the growth of population to levels which
carried the seeds of their own decline through overexploitation of the land.
. . . It became only a matter of time before disaster struck."

According to Scarre, those failures were the opening chapters in "the long
history of human interaction with the environment, which continues to be
debated in contemporary concerns over CFCs [ozone-destroying
chlorofluorocarbons], greenhouse gases and genetically engineered crops."

In his talk, Tainter pointed to California's electricity crisis and
never-ending quest for enough water as today's version of the pressures that
wrecked early societies.

The Akkadian empire in Mesopotamia, the Old Kingdom of Egypt, the Indus
Valley civilization in India, and early societies in Palestine, Greece and
Crete all collapsed in a catastrophic drought and cooling of the atmosphere
between 2300 and 2200 B.C.

Intensive agriculture and excessive irrigation led to the salinization of
the Mesopotamian plain between the Tigris and Euphrates Rivers in what is
now Iraq. Scarre said this "ultimately damaged the very landscape these
societies were striving to improve."

In the American Southwest, the ancient Anasazi civilization "could not
sustain three decades of exceptional drought and reduced temperatures in the
13th century A.D.," according to a paper in Science magazine by Harvey
Weiss, an anthropologist at Yale University in New Haven, Conn., and Raymond
Bradley, a geologist at the University of Massachusetts, Amherst.

Now the world is facing an increasingly serious shortage of fresh water.
Although water covers three-quarters of our planet, 95 percent of it is
salty and 70 percent of the rest is locked up in ice.

Aquifers in Texas are being drained faster than they are refilled. The Aral
Sea in Central Asia is drying up as the rivers that feed it are diverted to
irrigate cotton fields. Lake Chad in central Africa is one-twentieth the
size it was 35 years ago, also because of excessive irrigation.

A billion people lack adequate clean water, according to Peter Gleick,
director of the Pacific Institute for Studies in Development, Environment
and Security in Oakland, Calif.

Water-borne diseases kill 10,000 to 20,000 children every day, said Gleick,
author of a report "The World's Water, 2000-2001."

"Half the world's population has water service inferior to the ancient
Greeks and Romans," Gleick said.

People all over the globe are abandoning small towns and villages and
jamming into metropolitan areas, especially in poorer, Third World
countries.

By 2015, population experts predict, there will be 28 "megacities," each
with more than 10 million people. The Tokyo region is already home to more
than 26 million people. Bombay, India, is expected to grow from 18 million
to 26 million; Los Angeles from 13.1 million to 14.1 million; New York City
from 16.6 million to 17.4 million.

To be sure, modern cities enjoy more advanced technologies than ancient
metropolises. Nevertheless, the problems of crowding, pollution, crime and
sanitation that overwhelmed societies in the past threaten to do so again,
especially in less fortunate parts of the world.

"The lessons from history, or prehistory, are usually inconvenient and
painful to deal with and easy to ignore," Scarborough said.

Copyright 2001, Philadelphia Inquirer, 25 March 2001

MODERATOR'S NOTE: There seems to be no end to doom-and-loom stories which
link current environmental concerns with natural disasters that led to the
collapse of ancient civilisations. Although the industrial and technological
revolutions during the last 150 years have steadily  increased the living
standards and life expectancy of most of the world's population, some U.S.
researchers believe that ancient societies collapsed due to problems similar
to ours: "crowding, pollution, crime and sanitation overwhelmed societies in
the past." However, the most important factor for civilisation collapse
isn't even mentioned once: all collapses of ancient societies which I have
studied are directly correlated with significant climatic downturns.
Regardless of the unresolved questions as to the triggers of such
regressions, it is during these periods of climatic cooling that most of the
societal collapses occurred. The often *abrupt* deterioration of the
temperatures consequently led to general crop failure, starvation, migration
and warfare. Contrast these natural disasters with the *political* and
*economic* problems many mismanaged countries face today: "overcrowded
cities, water shortages and electricity brownouts" are almost entirely due
to failed economic policies, a severe lack of political accountability,
democratic transparency and high levels of corruption. The claim that "the
complexity caused by high technology could be mankind's undoing" is perhaps
the most ridiculous given that it is those societies that drive and
encourage the development of highly complex technologies that have the least
environmental, social or economic problems. BJP

==========
(5) "IS THERE A DOOSMDAY ASTEROID?"

From Duncan Steel <D.I.Steel@salford.ac.uk

Dear Benny,

North American CCNet members might like to know of a new documentary about
the impact hazard:

"Is There a Doomsday Asteroid?"  Scheduled to air on Friday, March 30 on PAX
TV's series, "Encounters with  the Unexplained." Info available via
http://www.paxtv.com

Duncan Steel

=========
(6) EMPAND ASTEROID ENTRY

From Phil Bland <P.A.Bland@open.ac.uk>

Benny,

On EMP and asteroid entry, see also ELECTROPHONIC SOUNDS FROM THE RE-ENTRY
OF THE MOLNIYA 1-67 SATELLITE OVER AUSTRALIA: CONFIRMATION OF THE
ELECTROMAGNETIC LINK.  A. Verveer, P. A. Bland and A. W. R. Bevan,
Meteoritics and Planetary Sciences 35, A163-A164 (2000).

best wishes,

Phil

=============
* ABSTRACTS *
=============

(7) PERIODICAL MASS EXTINCTIONS AND RECOVERY

Prokoph A, Fowler AD, Patterson RT: Periodically forced self-organization in
the long-term evolution of planktic foraminifera CANADIAN JOURNAL OF EARTH
SCIENCES 38: (2) 293-308 FEB 2001

Wavelet transform and other signal analysis techniques suggest that the
planktic foraminiferal (PF) long-term evolutionary record of the last 127 Ma
can be attributed to complex periodic and nonlinear patterns. Correlation of
the PF extinction pattern with other geological series favors an origin of
the similar to 30 Ma periodicity and self-organization by quasi-periodic
mantle-plume cycles that in turn drive episodic volcanism, CO2-degassing,
oceanic anoxic conditions, and sea-level fluctuations. Stationary similar to
30 Ma periodicity and a weak secular trend of similar to 100 Ma period are
evident in the PF record, even without consideration of the mass extinction
at the K-T boundary. The 27-32 Ma periodicity in the impact crater record
and lows in the global sea-level curve, respectively, are similar to6.5 Ma
and similar to2.3 Ma out of phase with PF-extinction data, although major
PF-extinction events correspond to the bolide impacts at the K-T boundary
and in late Eocene. Another six extinction events correspond to abrupt
global sea-level falls between the late Albian and early Oligocene.
Self-organization in the PF record is characterized by increased radiation
rates after major extinction events and a steady number of baseline species.
Our computer model of long-term PF evolution replicates this SO pattern. The
model consists of output from the logistic map, which is forced at 30 Ma and
100 Ma frequencies. The model has significant correlations with the relative
PF-extinction data. In particular, it replicates singularities, such as the
K-T event, nonstationary 2.5-10 Ma periodicities, and phase shifts in the
similar to 30 Ma periodicity of the PF record.

Addresses:
Fowler AD, Univ Ottawa, Ottawa Carleton Geosci Ctr, POB 450, Ottawa, ON K1N
6N5, Canada.
Univ Ottawa, Ottawa Carleton Geosci Ctr, Ottawa, ON K1N 6N5, Canada.
Univ Ottawa, Dept Earth Sci, Ottawa, ON K1N 6N5, Canada.
Carleton Univ, Ottawa Carleton Geosci Ctr, Ottawa, ON K1S 5B6, Canada.
Carleton Univ, Dept Earth Sci, Ottawa, ON K1S 5B6, Canada.

Copyright © 2001 Institute for Scientific Information

=============
(8) A SHORT DURATION OF THE K/T BOUNDARY EVENT

Mukhopadhyay S, Farley KA, Montanari A: A short duration of the
Cretaceous-Tertiary boundary event: Evidence from extraterrestrial helium-3
SCIENCE  291: (5510) 1952-1955 MAR 9 2001

Analyses of marine carbonates through the interval 63.9 to 65.4 million
years ago indicate a near-constant flux of extraterrestrial helium-3, a
tracer of the accretion rate of interplanetary dust to Earth. This
observation indicates that the bolide associated with the
Cretaceous-Tertiary (K-T) extinction event was not accompanied by enhanced
solar system dustiness and so could not have been a member of a comet
shower. The use of helium-3 as a constant-flux proxy of sedimentation rate
implies deposition of the K-T boundary clay in (10 +/- 2) x 10(3) years,
precluding the possibility of a Long hiatus at the boundary and requiring
extremely rapid faunal turnover.

Addresses:
Mukhopadhyay S, CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA.
CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA.
Osservatorio Geol Coldigioco, I-62020 Frontale Di Apiro, Italy.

Copyright © 2001 Institute for Scientific Information

===========
(9) AMINO ACIDS IN THE K/T BOUNDARY SEDIMENTS

Brisman K, Engel MH, Macko SA: Distribution, stereochemistry, and stable
isotope composition of amino acids in K/T boundary sediments PRECAMBRIAN
RESEARCH  106: (1-2) 59-77 FEB 1 2001

The Cretaceous/Tertiary (K/T) boundary, with an age of 65 million years
before present, is associated with the mass extinctions of dinosaurs and
other terrestrial and marine organisms. Evidence for the K/T boundary is
reported in both marine and non-marine stratigraphic units. The presence of
two exotic amino acids, alpha -aminoisobutyric acid and isovaline, has been
reported from Stevns Klint, Denmark sediments. Two hypotheses have been
proposed for the occurrence of alpha -Aiba and I-val in the Stevns Klint
sediments: (1) they are of extraterrestrial origin; and (2) they are the
by-products of the thermal alteration of coal. To test these hypotheses,
samples were obtained from three K/T boundary sites, two (Starkville South
and Raton Pass) located in the non-marine Raton Basin of Colorado and New
Mexico and one from the marine sequence at Stevns Klint, Denmark. The
samples were analyzed for amino acid distributions and stereochemistry, bulk
stable C and N isotope compositions and stable C isotope compositions of
individual amino acids at the Starkville South site. Protein and non-protein
amino acid components are present in the K/T boundary sediments at nmol/g
levels and are primarily of the L-configuration. The non-protein amino
acids, alpha -Aiba and I-val, were detected in several of the sediment
samples based on retention times and verified through coinjections of
authentic standards. Protein and non-protein amino acid components of the
Murchison meteorite are enriched in C-13 relative to terrestrial materials.
The delta C-13 values of individual amino acids in the Starkville South
samples are, however, depleted in C-13 in all samples, with values ranging
from - 13(0)/(00) for glycine to -31 (0)/(00) for L-isoleucine. In contrast,
the delta C-13 value for glycine in Murchison is + 22(0)/(00). Also, bulk
organic delta C-13 and delta N-15 values for the K/T boundary sediments are
substantially depleted in C-13 and N-15 relative to bulk values for the
Murchison meteorite. It is therefore suggested that the amino acids in these
K/T boundary sediments are primarily of terrestrial, biogenic origin. (C)
2001 Elsevier Science B.V. All rights reserved.

Addresses:
Macko SA, Univ Virginia, Dept Environm Sci, Charlottesville, VA 22903 USA.
Univ Virginia, Dept Environm Sci, Charlottesville, VA 22903 USA.
Univ Oklahoma, Sch Geol & Geophys, Norman, OK 73019 USA.

===========
(10) MORE ON THE CHICXULUB IMPACT CRATER

Pope KO, Rejmankova E, Paris JF: Spaceborne imaging radar-C (SIR-C)
observations of groundwater discharge and wetlands associated with the
Chicxulub impact crater, northwestern Yucatan Peninsula, Mexico GEOLOGICAL
SOCIETY OF AMERICA BULLETIN 113: (3) 403-416 MAR 2001

Analyses of spaceborne imaging radar-e (SLR-C) data and field data from the
northwestern Yucatan Peninsula, Mexico, demonstrate that spaceborne
multifrequency polarimetric radars are excellent tools for characterizing
patterns of wetland flooding, Seasonal flooding can be detected in most
types of forest and marsh in the radar backscatter magnitude and phase data
of both L and C band. Field observations made in the wet and dry seasons
concurrent with the space missions and chemical analyses of floodwaters
confirm that flooding is the product of discharge from the Yucatan aquifer,
which consists of a fresh-water lens floating on seawater, This discharge
controls the distribution of wetlands. Therefore, vegetation and flooding
patterns, mapped with SIR-C imagery, provide valuable information on the
hydrogeology of the region.
Radar-image maps of wetlands and flooding indicate that there are three
major zones of groundwater discharge that correlate with structures of the
buried Chicxulub crater--zone 1 with the peak ring, zone 2 with the crater
rim, and zone 3 with the exterior ring. Zone 1 has sulfate-poor discharge,
unlike the sulfate-rich discharge in zones 2 and 3, The highest discharge is
in zone 3, where the buried crater is closest to the surface. This
groundwater-discharge pattern can be explained by tidal pumping of fresh
water to the surface through high permeability zones developed in the
Tertiary carbonates overlying crater faults and escarpments.

Addresses:
Pope KO, Geo Eco Arc Res, 3220 N St NW, Suite 132, Washington, DC 20007 USA.
Geo Eco Arc Res, Washington, DC 20007 USA.
Univ Calif Davis, Div Environm Studies, Davis, CA 95616 USA.
Calif State Univ Monterey Bay, Spatial Informat Visualizat & Anal Resource
Ctr, Seaside, CA 93955 USA.

Copyright © 2001 Institute for Scientific Information

===========
(11) DETECTION OF SMALL COMETS WITH A GROUND-BASED TELESCOPE

Frank LA, Sigwarth JB: Detection of small comets with a ground-based
telescope
JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS  106: (A3) 3665-3683 MAR 1
2001

The Iowa Robotic Observatory (IRO) located in the Sonoran desert near
Sonoita, Arizona, was used for an optical search for small comets in the
vicinity of Earth during the period October 1998 through May 1999. The
previous reports of detection of the small comets with an optical telescope
were based on the search with the Spacewatch Telescope during November 1987,
January 1988, and April 1988. The searches with both of these telescopes
required that their fields of view be moved in a special manner across the
celestial sphere in order to maximize the dwell times of the comet images on
a small set of pixels of the telescopes' array detectors. There were
sightings of nine small comets in the set of 1500 usable images which were
gained with the IRO. The possibility that these events were spurious owing
to random fluctuations of the responses in the sensors was eliminated in two
different ways. The first method was the use of a shutter to provide either
two or three trails for the same small comet. In the two-trail mode, no
events were seen with three trails, and for the three-trail mode, no events
were seen with two trails. The second assurance that the trails were not due
to noise was provided by a rigorous determination of the signals S and
signal-to-noise ratios (S/N) in the trails due to random statistical
fluctuations, or "random trails," and the subsequent comparison with these
parameters for the small-comet sightings. The diameter of the primary mirror
of the IRO is significantly smaller than that of the Spacewatch Telescope,
and thus the uncertainties in the number densities of the small comets are
greater with the IRO. However, within a factor of 2 or 3, the average number
densities of small comets detected with the IRO are similar to those
observed previously with the Spacewatch Telescope, that is, about
10-(18)/m(3).

Addresses:
Frank LA, Univ Iowa, Dept Phys & Astron, 212 Van Allen Hall, Iowa City, IA
52242 USA.
Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA.

Copyright © 2001 Institute for Scientific Information

============
(12) ANALYSIS OF COMA DUST PROPERTIES IN COMET HALE-BOPP

Kerola DX, Larson SM: Analysis of coma dust optical properties in Comet
C/1995 O1 (Hale-Bopp) II. Effects of polarization ICARUS 149: (2) 351-356
FEB 2001

Efforts to apply a single-scattering polarized radiative transfer code to
interpret photopolarimetric measurements of coma dust optical properties in
Comet Hale-Bopp corroborate previous photometrically derived conclusions
concerning the predominance of small sized particles in Hale-Bopp's coma.
Calculations of the degree of linear polarization (DP) as a function of
observation phase angle (alpha) produced by prolate spheroidal crystalline
olivine particles with effective radii (a = 0.216 mum) are compatible with
the comet's measured polarization in standard filters at lambda = 0.4845 and
0.684 mum. Our rudimentary "trade-off" studies highlight the extreme
sensitivity of DP to dust particle size and shape. A combination of viewing
geometry effects in association with enhanced multiple scattering might
provide a quantitative explanation of the negative polarization for 0
degrees less than or equal to alpha less than or equal to 20 degrees seen in
Hale-Bopp and other comets. (C) 2001 Academic Press.

Addresses:
Kerola DX, Univ Arizona, Lunar & Planetary Lab, Tucson, AZ 85721 USA.
Univ Arizona, Lunar & Planetary Lab, Tucson, AZ 85721 USA.

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*

CCNet SPECIAL: COMETARY IMPACTS AND ICE-AGES
--------------------------------------------


"It is therefore cometary impacts that we must thank for the equable
spell of climate in which human history and civilisation has prospered
so spectacularly. The renewal of ice-age conditions would render a
large fraction of the world's major food-growing areas inoperable, and so
would inevitably lead to the extinction of most of the present human
population. Since bolide impacts cannot be called up to order, we must look
to a sustained greenhouse effect to maintain the present advantageous world
climate. This might imply the ability to inject effective greenhouse gases
into the atmosphere, the opposite of what environmentalists are
erroneously advocating."
-Fred Hoyle and Chandra Wickramasinghe, Cardiff University,
March 2001


COMETARY IMPACTS AND ICE-AGES

By Fred Hoyle and Chandra Wickramasinghe
School of Mathematics, Cardiff University, Cardiff CF2 4YH, UK

Abstract: Cycles of glaciation with an average period of ~100kyr are
mediated by impacts of cometary bolides. Ice-age conditions are dry and
dusty with low rates of precipitation. Comets in the mass range 10^15 -
10^16 g impacting the oceans could release enough water vapour into the
atmosphere to enhance a depleted greenhouse effect. The energy deposited in
the oceans would also warm the surface layers, thus starting up an
evaporation-precipitation cycle which ushers in warmer interglacial
interludes. The latter have neutral stability and are necessarily
short-lived, eventually drifting back to glacial conditions on timescales of
~10kyr.

Keywords:  Comets, impacts, ice-ages, climate, greenhouse effect, Holocene

1. Introduction

Despite the great cost and effort that has gone into modelling the
terrestrial climate system our understanding of glacial cycles and their
forcing mechanisms remains rudimentary to say the least. Studies of the last
glacial period which commenced about 115k years ago show that during this
time the Earth's mean temperature underwent many sharp oscillations leading
up to an eventual degalaciation about 10kyr ago (Allen et al, 1999). Earlier
ice-age cycles which are also documented seem to occur with a mean period of
~100kyr, the characteristics of each cycle being generally similar to one
another.

Fig. 1 Progress through the Younger Dryas period of parameters that
characterise the mean temperature of the Earth. (From Severinghaus et al
(1998))

Some years ago Milankovitch (1941) attributed ice-age cycles to
perturbations in the orbit of the Earth and the consequent changes of solar
heating in the Northern hemisphere which has most of the land mass. But this
model, at any rate in its original form, is deficient for a number of
reasons. During the ice-ages the whole Earth was cooled, including the
tropics. This is proved by glaciers extending down to about 10,000 feet on
tropical mountains, mountains which at present do not hold glaciers, such as
the mountains on the island of Hawaii. 

The need for the whole Earth to be appreciably cooled, in our view, disposes
of the Milankovitch theory of small oscillations of the tilt of the Earth's
rotation axis to the plane of the ecliptic (a period of 41,000yr), and of
small oscillations in the eccentricity of the Earth's orbit (period of
100,000yr). A third effect that is also discussed in this context involves
the precession of the Earth's axis of spin with a period of 23,000yr,
leading to a drift of solstices. None of these effects produces any change
in the amount of solar energy incident on the Earth and so could not lead to
widespread cooling. Oscillations of tilt merely produce slight latitude
variations in the incidence of solar energy, which are in any case much
smaller than the transport in latitude of heat by atmospheric storms and
ocean currents. Indeed the transport of oceanic heat towards the poles gives
a far larger effect that would easily buffer slight latitude variations of
insolation. Oscillations in eccentricity of the Earth's orbit produce small
shifts of solar energy between one geographical hemisphere and the other,
and so should tend to cool one hemisphere and warm the other. But ice-ages
occur contemporaneously in both hemispheres, not alternatively, a problem
that was already well-understood more than half a century ago. 

A variant of the Milankovitch model involves oscillations of the inclination
of the Earth's orbital plane to the invariable plane of the planets. Over
the past million years the inclination has varied from about 0.5 degree to
about 3 degrees, and during times of low inclination the Earth would acquire
more interplanetary debris leading to atmospheric dusting episodes with a
potential for climatic catastrophes (Muller and MacDonald, 1997).

Considerable attention has been focussed in recent years on the closing
stages of the last ice age. Studies of Greenland ice cores, including O^18
/O^16 isotope measurements show two major jumps in mean temperature
occurring one at 14.6ky and another at 11.5kyr, the latter marking the end
of the Younger Dryas cold interval and the beginning of the Holocene. Figure
1 shows a plot of several parameters relating to Greenland ice drills that
are all related to mean temperature.  There is a growing body of evidence to
support the view that a rise of temperature of ~15C occurred with remarkable
speed, perhaps with a few decades (Severinghaus, et al, 1998). The
coincidence of this event with a similar phenomenon discovered in ice-cores
from Antarctica (Steig et al, 1998) point strongly to a global rather than
hemisphere event that triggered the final deglaciation leading up to the
Holocene. These facts would not be consistent with the requirements of the
Milankovitch theory, although they may be in accord with the class of model
discussed by Muller and MacDonald (1997).

In the rest of this article we discuss the general conditions that are
needed for maintaining ice ages as well as for triggering their
terminations. We shall argue that comet impacts play an important role in
these processes.

2. The Greenhouse Effect

The most important factor that controls the Earth's climate is the
greenhouse effect. The greenhouse effect raises the Earth's temperature by
about 40oC above what it would otherwise have been. Without the greenhouse
effect the Earth would be locked into a permanent ice-age. This fact gives
the lie to those constantly seeking to persuade the public that the
greenhouse effect is a bad thing greatly to be feared. The reverse is true.
The greenhouse effect is an exceedingly good thing, without which those of
us who happen to live in temperate latitudes would be buried under several
hundreds of metres of ice.

Water vapour and carbon dioxide are the main greenhouse gases. Carbon
dioxide produces essentially the whole of its effect through absorption at
infrared wavelengths from about 13.5Mum to 17.5Mum. Because the blocking by
carbon dioxide over this interval is large, the band having steeply-falling
wings, additions of carbon dioxide have only a second-order influence on the
greenhouse effect and are inconsequential compared to the major factors
which control the Earth's climate. The blocking effect of water vapour rises
all the way from 17.5Mum to almost 100Mum.

The wavelength 13.5Mum is important in two respects. In the energy
distribution of radiation emitted at ground and sea-level it marks the
halfway point, one-half of the energy being at wavelengths shorter than
13.5Mum and one-half at wavelengths longer. It also marks a division in the
effectiveness of the blocking of greenhouse gases. Shortward of 13.5Mum the
blocking is comparatively weak, longward of 13.5Mum it is strong, excepting
for a partial window from 17.5Mum to about 20Mum. This is clearly
illustrated in Figure 2. Shortward of 13.5Mum there is a broad weak
absorption from water vapour with its minimum in the region of 10mm,
together with narrow bands from O(3) and CH(4). Of these, some current fuss
is being made about CH(4). But blocking by methane is somewhat shortward of
8mm, which is so far out on the short wavelength tail of the Earth's
reradiated spectrum as also to be of no great consequence. Thus the Planck
maximum for a reradiated spectrum of, say, an effective temperature 290K is
at 17.6Mum with respect to energy, and at 12.7Mum with respect to maximum
photon emission.

Thus methane makes its contribution in a region of the reradiated spectrum
where there is only 10 percent of the energy, for which reason fluctuations
in atmospheric methane can produce only minor effects, like those produced
by fluctuations of CO(2). The gas that can produce major effects, and
towards which one must therefore look for an understanding of large shifts
of the Earth's climate, is water vapour.

Without the greenhouse effect the Earth's mean temperature, averaged with
respect to latitude, between day and night and between land and sea, is
given by the formula

T= [1.37 x 10^6 (1-A)/ac]^l/4,

where 1.37 x 10^6erg cm^-2 s^-1 is the solar energy flux outside the Earth,
A is an averaged value for the Earth's albedo, c is the velocity of light,
and a is the radiation density constant, equal to 7.565 x 10^-15 erg
cm^-3deg^-4. Thus for an albedo of 0.4 one would have T=245K, very cold
indeed.

Fig.2. Schematic diagrams to illustrate Planck curves for 295K (solid curve)
and for 245K (dashed curve) compared with the infrared blocking actions of
CO(2) and H(2)O.

It is known from model calculations of stellar atmospheres that the
situation becomes complex and difficult when opacity sources are highly
wavelength dependent, as they are for the terrestrial greenhouse effect. The
same must arise here so that it seems desirable to seek an approximation
with the virtue of physical rectitude rather than to set up a supposedly
accurate computation in which approximations of uncertain physical validity
are nevertheless made in the end. Owing to the fortunate circumstance that
the wavelength 13.5Mum has the special properties described above, such a
useful approximation lies immediately to hand. Suppose the half of the
reradiated energy longward of 13.5Mum to be completely blocked by the heavy
opacity of the greenhouse gases and suppose the half shortward of 13.5Mum to
be completely free to escape. Then it is easy to see that the greenhouse
effect must raise the Earth's mean temperature by 2^1/4 above what it would
otherwise be, about 292K instead of 245K, a result agreeing very well with
experience. One can see that the weak blocking which actually takes place
shortward of 13.5mm is approximately compensated by the partial window from
17.5Mum to 20Mum. With a first approximation that is evidently close to the
truth it is possible to calculate the effects of changing individual
greenhouse gases as fluctuations from this first approximation, thereby
keeping close contact with physical reality. To this end we proceed to a
second order heuristic computation of radiation transfer assuming a static
plane parallel atmosphere with Rosseland mean opacities calculated for each
of the relevant greenhouse gases. Fig. 3 shows the results for a "standard"
atmosphere with a fixed albedo in which the CH(4) optical path is varied in
the three curves.

Fig. 3 Mean surface temperature calculated from a simplified static
radiation transfer model with constant albedo. The three curves show the
effects of varying the methane content as a fraction f of that used in a
standard model of the present day atmosphere. The abscissa is the column
density of precipitable water vapour. (The parameters d, D refer to the
lengths of a column of unit cross-section of a given gas at NTP that
contains the same number of molecules of this gas as a column extending
through the entire atmosphere.)

Each curve plots mean surface temperature as a function of the water vapour
column density measured in cm per cm^2 of precipitable water. We note that
reducing the water content appreciably to only a few millimetres of
precipitable water weakens the greenhouse, dropping the Earth's mean
temperature (for the same A) to about 280K, which corresponds closely to
what is required for ice-age conditions. The conclusion is therefore that
reducing the average water content of the atmosphere to between a third and
a tenth of its present-day value, while maintaining the albedo, would
produce an ice-age.

3. Ice-age Conditions

Ice-age conditions were dry, dusty and cold. The great deposits of loess,
wind-blown soil, in E. Europe and China, imply a climate that was dusty in
the lower atmosphere, a situation implying in turn a low precipitation rate.
Low precipitation is not a handicap to the accumulation of large glaciers,
which will grow even at annual precipitation rates as little as a few
centimetres per year, provided the temperature is low enough to prevent
summer melting.

If we were to imagine an atmospheric state appropriate to an ice age being
brought about today, evaporation from the relatively warm surface layers of
the ocean would quickly re-supply water vapour to a typical amount of 1 cm
of precipitable water per cm2 and the cooling due to a reduced greenhouse
effect would quickly be gone. Thus it is the heat of the ocean which saves
us from the possibility of an immediate onset of ice-age conditions.
Reckoning the heat of the ocean as being the energy content above freezing
point, which can be thought of as available heat, almost all is contained in
a surface layer with depth no greater than a few hundred metres, the amount
being equivalent to a supply of sunlight over a time interval of a few
years, say 3 to 5 years.  It is because the ocean has this back storage of
heat that we do not drop almost immediately into an ice-age.

In distant geological periods the heat storage in the oceans was
considerably greater than it is at present. Today the ocean bottom waters
are close to freezing, whereas only 50 million years ago the bottom
temperature was about 15o C and the available oceanic heat was then
equivalent to a 50 year supply of sunlight. The difference has been caused
by drifting continents, especially by the positioning of Antarctica and
Greenland at or close to the poles. Melt water from Arctic glaciers has
gradually filled the lower ocean with water close to freezing, greatly
reducing the margin of safety against the onset of ice-age conditions. This
is why the past million years has been essentially a continuing ice-age,
broken occasionally by short-lived interglacials. It is also why those who
have engaged in uncritical scaremongering over an enhanced greenhouse effect
raising the Earth's temperature by a degree or two should be seen as both
misguided and dangerous. The problem for the present swollen human species
is of a drift back into an ice-age, not away from an ice-age. Manifestly, we
need all the greenhouse we can get, even to the extent of the British Isles
becoming good for the growing of vines.

The present-day situation is best seen as one of neutral equilibrium unlike
an ice-age which is a position of stable equilibrium. The present-day
situation is one in which over relatively short intervals the world climate
stays the way it is, but over longer intervals can be subject to drift.
Looking through climatic records for the recent millennium the drift over a
century or two is by 1-2oC. Drift from the present-day down by 10o C into an
ice-age requires an excess of about ten downward steps over upward steps,
say each step of 1o C. With a century between steps,  random shifts would
bring on the next ice-age in an interval of about 10,000 years, the typical
length of an interglacial. Without some artificial means of giving positive
feedback to the climate, such an eventual drift into ice-age conditions
appears inevitable.

4. The Albedo

All this is on the assumption of a fixed albedo, a point which now requires
consideration. The remarkable feature of the Earth's albedo is that
atmospheric water does not lift A close to unity. If only a small fraction
of even a very dry atmosphere were to condense into tiny ice crystals, this
would happen. The mass "exclusion" coefficient, through the scattering back
into space of sunlight, produced by dielectric crystals with radii of a few
tenths of a micrometre, is about 3000 cm^2 g^-1 (Hoyle and Wickramasinghe,
1991). Thus a condensation of only 0.1 percent of the water in a very dry
atmosphere with only 1 mm of precipitable would yet contribute about 0.3 to
A. Essentially no water must be condensed into ice crystals if A is to be
appreciably less than unity. Otherwise the Earth would appear from the
outside as an intensely bright white planet with an albedo even higher than
Venus, while below the haze of ice crystals it would be exceedingly cold at
ground-level.

The saving grace is that ice crystals do not form in supersaturated water
vapour except at very low temperatures, below say -50oC. For the Earth's
emission into space of radiation at wavelengths longer than 20mm we can
think of a photosphere at which the optical depth out into space is of order
unity. If only radiation were involved in determining the water vapour
temperature at this photosphere the temperature would be of order 290
Pi^-1/4 where is the optical depth from ground level up to the photosphere,
suitably averaged at wavelengths longer than 20mm. In a typical atmosphere
would be about 10 Pi, leading to a photospheric temperature for water vapour
(and hence for surrounding air) of as little as 163K, i.e. -110oC, far below
that needed for ice crystal formation. The circumstance that ice crystals do
not form profusely except under special conditions in Antarctica shows that
calculating for radiation alone cannot be correct. A convective transport of
energy from ground-level to the water-vapour photosphere is required. This
cannot be carried by air movements but must come from the upward transport
of the latent heat of condensation of the water vapour itself. To keep the
photospheric water vapour temperature above -50oC, and so to prevent ice
crystal formation, the transport of water vapour must be such as would lead
to an annual precipitation rate of about 50 cm. For comparison, the
present-day world-wide average of the precipitation rate is about 80 cm of
rain, sufficient to prevent ice crystal formation, but not by a wide margin.

Let the world climate drift downward, however, sufficiently for the surface
layers of the ocean to cool to the point where an annual average rainfall of
50 cm cannot be maintained and the consequent formation of an atmospheric
haze of ice crystals would plunge the Earth immediately back into an
ice-age.

5. Emergence from an Ice-age

The cooling of the ocean over the past 50 million years eventually made an
ice-age the norm of the Earth's climate, as it has been throughout most of
the Pleistocene, with brief changes only during interglacials lasting for
times of about 10,000 years or less. Left to itself, it is hard to see how
anything internal to the Earth could ever break the stable grip of an
ice-age. Thus to understand the cause of interglacials we must look to
external catastrophic events. With 10 kilometre-sized cometary impacts, such
as occurred at KT boundary, taking place at an average rate of one per
100myr, it would be reasonable to expect the impact frequency of 1
kilometre-sized comets to occur at a rate of one every 100kyr. This is on
the assumption of an equipartition of mass between fragments of a giant
comet of the type considered by Clube et al (1996). The average frequency of
one per 100kyr is remarkably close to the mean length of the glacial cycle.

The impact of a kilometre-sized cometary object (or a slightly smaller
asteroid) into a major ocean appears essential to the ending of an ice-age.
An object of mass 10^16g would have sufficient energy to throw up some
10^20g of water into the stratosphere, immediately creating a powerful
greenhouse effect as the water spread around the world to give some 10g of
precipitable water per cm^2. Such a greenhouse effect lasting for some
months, and at a lesser level for several years, would produce a sufficient
warming of the surface waters of the ocean to jerk the Earth almost
discontinuously out of a long drawn-out ice-age into the beginning of an
interglacial. We have already noted that this happened at the very end of
the Younger Dryas to herald the present Holocene period.

A sharply defined renewal of the water vapour greenhouse would inevitably
have a knock-on effect on other greenhouse gases. The sudden warming of the
Earth following an impact would have the effect of unlocking the vast store
of methane, ~10^20 - 10^21 g, that lies trapped in permafrost and as methane
clathrate hydrates near continental margins and on the ocean floor (Gold,
1987; Mac Donald, 1990). The amount of methane released in this way would be
subject to a negative feedback, however. Methane burning near the surface
(from impact energy) would produce CO2 , whereas CH4 lofted to the
stratosphere at heights above 50km would undergo photodissociative reactions
leading eventually to the production of H(2)O. The H(2)O would then be
available to condense into submicron-sized ice crystals (such as occur in
noctilucent clouds) and hence to an enhancement of A and a consequent
reduction of the surface temperature. Methane release following impact would
therefore have the effect of preventing a runaway greenhouse effect.

The 18^O/16^O analysis of Greenland ice cores shows that an immense melting
of glacier ice began abruptly about 14.5kyr ago. The jumps shown in Figure 1
are also matched by similar effects in the South Polar region with major
temperature rises of some 12o C occurring over a timespan of only a few
decades (Steig et al, 1998). On a more restricted geographical scale, fossil
insect records show that the summer temperature in Britain rose by 10oC or
more in as little as 50 years (Coope, 1970) on at least two occasions during
the Younger Dryas, an essentially decisive indication of a catastrophic
event as its cause. It is therefore cometary impacts that we must thank for
the equable spell of climate in which human history and civilisation has
prospered so spectacularly.

The renewal of ice-age conditions would render a large fraction of the
world's major food-growing areas inoperable, and so would inevitably lead to
the extinction of most of the present human population. Since bolide impacts
cannot be called up to order, we must look to a sustained greenhouse effect
to maintain the present advantageous world climate. This might imply the
ability to inject effective greenhouse gases into the atmosphere, the
opposite of what environmentalists are erroneously advocating.

6. Conclusions

Ice-age conditions are dry and cold, the local temperature being reduced
over the entire Earth.  The high atmosphere probably had a haze of small ice
crystals while the lower atmosphere was dusty. Such conditions were stable,
capable of persisting until a large bolide hit one of the major oceans. The
water then thrown high into the stratosphere provided a large temporary
greenhouse effect, but sufficient to produce a warming of the world ocean
down to a depth of a few hundred metres. It is this warming that maintains
the resulting interglacial period. The interglacial climate possesses only
neutral equilibrium however. It experiences random walk both up and down,
until a situation arises in which the number of steps downward become
sufficient for the Earth to fall back into the ice-age trap. Thereafter only
a further large bolide impact can produce a departure from the grey, drab
ice-age conditions. This will be so in the future unless Man finds an
effective way to maintain a suitably large greenhouse effect.

References

Allen, J.R.M. et al.: 1999. Nature, 400, 740-743
Clube, S.V.M., Hoyle, F., Napier, W.M. and Wickramasinghe, N.C.: 1996. ApSS,
245, 43-60
Coope, G.R.:1970. Ann.Rev.Entomology, 15, 97-120
Gold, T.: 1987. Power from the Earth  (J.M. Dent, Lond.)
Hoyle, F. and Wickramasinghe, N.C.: 1991 Earth, Moon & Planets, 52, 161-170
MacDonald, G.J.: 1990. Climate Change, 16, 247
Milankovitch, M. 1941. Royal Serbian Acad. Sp. Publ. 132, 1941
Muller, R.H. and Mac Donald, G.J.: 1997. Science, 397, 215-218
Severinghaus, J.P. et al: 1998. Nature, 391, 141-146
Steig, E.J. et al: 1998.  Science, 282, 92-95

Cardiff Astrobiology Preprint 02, Oct. 2000, Published in ASTROPHYSICS AND
SPACE SCIENCE 275: (4) 367-376 MAR 2001. The permission by the authors to
post their paper on CCNet is kindly acknowledged.

--------------------------------------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
--------------------------------------------------------------------
The CCNet is a scholarly electronic network. To subscribe/unsubscribe,
please contact the moderator Benny J Peiser <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 fully
indexed archive of the CCNet, from February 1997 on, can be found at
http://abob.libs.uga.edu/bobk/cccmenu.html
DISCLAIMER: The opinions, beliefs and viewpoints expressed in the articles
and texts and in other CCNet contributions do not  necessarily reflect the
opinions, beliefs and viewpoints of the moderator of this network.

CCCMENU CCC for 2000