PLEASE NOTE:
*
CCNet, 11 October 1999
------------------------
QUOTE OF THE DAY
"Extended witch-hunts took place at the
various peaks of the
Little Ice Age because part of society held
the witches
directly responsibile for the high frequency
of climatic
anomalies and the impacts thereof. The
enormous tensions created
in society as a result of the persecution of
witches demonstrate
how dangerous it is to discuss climatic change
under the aspects
of morality."
-- W.
Behringer: Climatic change and witch-hunting: The
impact of the Little Ice Age on mentalities, CLIMATIC
CHANGE 1999
(1) SOME CORRECTIONS REGARDING THE VREDEFORT IMPACT CRATER
Andrew Glikson <andrew.glikson@anu.edu.au>
(2) ASTEROID BRAILLE
Robert Clements <Robert.Clements@dva.gov.au>
(3) ON THE ORBITAL EVOLUTION OF NEAR-EARTH ASTEROIDS
L. Liu*) & J. Ji, NANJING UNIVERSITY
(4) PRELIMINARY INVESTIGATION OF ORBITAL EVOLUTION OF NEAs
L. Liu*) & J. Ji, NANJING UNIVERSITY
(5) NUMERICAL STUDY OF NEA ORBITS
L. Liu*), J.H. Ji, X.H. Liao, NANJING
UNIVERSITY
(6) THE SOCIAL DIMENSION OF CLIMATE INSTABILITY
C. Pfister*) & R. Brazdil, UNIVERSITY OF
BERN
(7) CLIMATE CHANGE IN 16th CENTURY EUROPE
C. Pfister*) et al., UNIVERSITY OF BERN
(8) ASTRONOMY & WEATHER OBSERVATIONS IN 16th CENTURY EUROPE
C. Pfister et al., UNIVERSITY OF BERN
(9) EUROPEAN TREE RINGS & 16th CENTURY CLIMATE
K.R. Briffa*) et al., UNIVERSITY OF EAST
ANGLIA
(10) CLIMATE CHANGE AND WITCH-HUNTS
W. Behringer, UNIVERSITY OF YORK
(11) HOLOCENE IMPACTS
Owen Fox <oefox@labyrinth.net.au>
(12) METRIC & IMPERIAL
Jonathan Shanklin <jdsh@mail.nerc-bas.ac.uk>
=============
(1) SOME CORRECTIONS REGARDING THE VREDEFORT IMPACT CRATER
From Andrew Glikson <andrew.glikson@anu.edu.au>
"SOUTH AFRICAN CRATER IDENTIFIED AS WORLD'S LARGEST IMPACT
STRUCTURE"???
(CCNet 7.10.99)
from Andrew Glikson
Research School of Earth Science,
Australian National University
Canberra, ACT 0200
andrew.glikson@anu.edu.au
I read in CCNet 7.10.99 a report citing "South African
geologists
have identified a crater in central Free State province to be the
oldest and largest in the world caused by the impact of a comet
or an
asteroid, scientists said." and "the Vredefort crater,
which is
between 250 and 300 kilometers in diameter, was long thought to
be of
volcanic origin." (quoted from SpaceDaily, 5 October 1999).
These cited "news" are formulated in terms giving an
impression as if
a recent discovery has been made - yet the history of the
investigation of Vredefort structure goes back at least 50 years!
An impact origin for the Vredefort Structure has been suggested
as
early as 1947 by R.A. Daly (J. Geol. 55:125-145), and uniquely
diagnostic shatter cone structures were reported by R.B.
Hargraves
(1961; Trans. Geol. Soc. S. Africa 64:147-154), R.S. Dietz (1961;
J.
Geol 69:499-516) and W.I. Manton (1965; Ann N.Y. Acad. Sci.
123:1017-1048). An extraterrestrial impact has been questioned by
L.O. Nicolaysen and colleagues of the University of
Witwatersrand,
who suggested an origin by "cryptoexplosion" triggered
by hydrostatic
pressure buildup (for example, Nicolaysen and Ferguson, 1990,
Tectonophysics 171:303-335). However, apart for the shatter
cones,
the identification of Vredefort as of extraterrestrial impact
origin
has been confirmed years ago by a wide range of observations,
including coesite and stishovite (high-pressure quartz
polymorphs)
(Martini, 1978, Nature 272:715-717), planar deformation features
in
quartz (Grieve et al., 1990; Tectonophysics 171:185-200), Iridium
anomalies in granophyre/pseudotachylite (French and Nielsen,
1990,
Tectonophysics 171, 287-301), and Re/Os anomalies (Koeberl et
al.,
1996, Geology 24:913-916). This was followed by a
reinterpretation of
the Vredefort as a 180-300 km-diameter multiring structure
(Therriault et al., 1993, LPS XXIV, 1419-1420) and U-Pb zircon
dating
at 2024+/-5 (Kamo et al., 1995, Proc. Geol. S. S. Africa,
566-569).
The jury is still out whether Vredefort is the largest known
impact
structure known to date on Earth, in view of the 340 km-diameter
circular magnetic and gravity anomaly associated with the late
Jurassic Morokoweng impact structure in the south Kalahari
(Corner et
al., 1997, EPSL 146:351-364; Hart et al., 1997, EPSL 147:25-35)..
Unfortunately, the inherently unreviewed nature of Internet
bulletins
results in possible inaccurate information creeping into this
media.
In order to minimize this risk and maintain minimum scientific
standards, I suggest that CCNet requests authors to provide
relevant
scientific references to accompany their reports, allowing
readers to
substantiate their accuracy.
----------------
MODERATOR'S NOTE: The AFP story covered by SpaceDaily (5 October)
and
posted by CCNet (7 October) about the latest research on the
Vredefort
impact crater was, I guess, mainly based on a press release
provided by
the geologists involved in this impact study. As is sometimes the
case,
it is not science journalists but ambitious researchers who are
tempted
to use sensationalist (and sometimes even misleading) language in
press
releases in order to draw attention to their work. While it is
true
that the unreviewed nature of most Internet science networks
often lead
to inaccurate information being circulated, the openness of the
WWW
also allows for quick and effective self-regulatory messures so
that
false or misleading information can be immediately corrected. In
fact,
I have come to the conclusion that this new form of open peer
review is
not only more democratic but, more importantly, much more
effective in
assessing the soundness of scientific data and argumentation.
Benny J Peiser
==============
(2) ASTEROID BRAILLE
From Robert Clements <Robert.Clements@dva.gov.au>
Chasing some other sources, I t h i n k I now know
what kind of
information SpaceWarn was reading; but the results have become
appallingly garbled in the retelling. The following quote - the
only
one I've found to date - is from JPL Universe, August 6, 1999:
"Successful DS1 flyby sheds light on asteroid mystery",
by John G.
Watson:
> Apart from flyby findings, project scientists have
determined that
> Braille is one of the asteroids that drift in & out of
Earth's
> orbit over eons & that it will return to Earth's
vicinity within a
> few thousand years.
... although where SpaceWarn got the idea about the thing
miraculously
burning up in the atmosphere has still escaped all my sources....
Assuming that this (quasi-?) resonant orbit interpretation is
correct,
reading the unspeakable gibberish that SpaceWarn has put up on
the www
(under the authority of NASA & Goddard, to boot!) is
astonishingly
emabarrassing; & suggests someone seriously needs their
keester
structurally compromised. I must apologise for repeating the
problem
so publically; but on the other hand: I cann't help wondering
what
other superficially intelligent garbage is out there?...
All the best,
Robert Clements <Robert.Clements@dva.gov.au>
==============
(3) ON THE ORBITAL EVOLUTION OF NEAR-EARTH ASTEROIDS
L. Liu*) & J. Ji: On the orbital evolution of near-Earth
asteroids
CHINESE ASTRONOMY AND ASTROPHYSICS, 1999, Vol.23, No.3,
pp.273-276
*) NANJING UNIVERSITY,DEPT ASTRON,NANJING 210093,PEOPLES R CHINA
The choice of dynamical model and algorithm in the calculation of
the
orbital evolution of near-earth asteroids is elucidated from a
dynamical point of view, and illustrated with the results of
asteroid-earth encounter over the next century for six near-earth
asteroids. Copyright 1999, Institute for Scientific Information
Inc.
=============
(4) PRELIMINARY INVESTIGATION OF ORBITAL EVOLUTION OF NEAs
L. Liu*) & J. Ji: A preliminary investigation of the orbital
evolution of near-Earth asteroids. CHINESE ASTRONOMY AND
ASTROPHYSICS, 1998, Vol.22, No.2, pp.218-234
*) NANJING UNIVERSITY,DEPT ASTRON,NANJING 210093,PEOPLES R CHINA
We use an improved explicit symplectic algorithm and a nested
RKF7(8)
integrator to make a numerical study of the orbital evolution of
86
numbered near-Earth asteroids. We give their status of evolution
on
times scales of 10(3)-10(4) yr and their minimum distances to
some of
the major planets. Some of the minimum distances to Earth can be
less
than 0.01 AU, or even less than the distance of the Moon.
Copyright
1999, Institute for Scientific Information Inc.
=============
(5) NUMERICAL STUDY OF NEA ORBITS
L. Liu*), J.H. Ji, X.H. Liao: A numerical study of the orbits of
near
earth asteroids with symplectic algorithm. CHINESE ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.23, No.1, pp.108-119
*) NANJING UNIVERSITY,DEPT ASTRON,NANJING 210008,PEOPLES R CHINA
We use an improved explicit symplectic algorithm and the nested
RKF7(8) integrator to study numerically the orbital evolution in
43
numbered near-Earth asteroids. Post-newtonian effect is included
in
the dynamical model, and we consider particularly the prospect of
applying symplectic algorithm in the orbital study of asteroids
and
its effectiveness in case of close approach to a major planet.
Copyright 1999, Institute for Scientific Information Inc.
=============
(6) THE SOCIAL DIMENSION OF CLIMATE INSTABILITY
C. Pfister*) & R. Brazdil: Climatic variability in
sixteenth-century
Europe and its social dimension: A synthesis. CLIMATIC CHANGE,
1999,
Vol.43, No.1, pp.5-53
*) UNIVERSITY OF BERN,HIST INST,UNITOBLER,CH-3000 BERN
9,SWITZERLAND
The introductory paper to this special issue of Climatic Change
summarizes the results of an array of studies dealing with the
reconstruction of climatic trends and anomalies in
sixteenth-century
Europe and their impact on the natural and the social world.
Areas
discussed include glacier expansion in the Alps, the frequency of
natural hazards (floods 111 central and southern Europe and
storms on
the Dutch North Sea coast), the impact of climate deterioration
on
grain prices and wine production, and finally, witch-hunts.
The documentary data used for the reconstruction of seasonal and
annual precipitation and temperatures in central Europe (Germany,
Switzerland and the Czech Republic) include narrative sources,
several types of proxy data and 32 weather diaries. Results were
compared with long-term composite tree ring series and tested
statistically by cross-correlating series of indices based on
documentary data from the sixteenth century with those of
simulated
indices based on instrumental series (1901-1960). It was shown
that
series of indices can be taken as good substitutes for
instrumental
measurements. A corresponding set of weighted seasonal and annual
series of temperature and precipitation indices for central
Europe
was computed from series of temperature and precipitation indices
for
Germany, Switzerland and the Czech Republic, the weights being in
proportion to the area of each country. The series of central
European indices were then used to assess temperature and
precipitation anomalies for the 1901-1960 period using transfer
functions obtained from instrumental records. The statistical
analysis of these series of estimated temperature and
precipitation
anomalies yielded features which are similar to those obtained
from
instrumental series. Results show that winter temperatures
remained
below the 1901-1960 average except in the 1520s and 1550s.
Springs
fluctuated from 0.3 degrees C to 0.8 degrees C below this
average.
Summer climate was divided into three periods of almost equal
length.
The first was characterized by an alternation of cool and warmer
seasons. The second interval was 0.3 degrees C warmer and between
5
and 6% drier than in the 1901-1960 period. It is emphasized that
this
warm period included several cold extremes 111 contrast to the
recent
period of warming. Summers from 1560 were 0.4 degrees C colder
and 4%
more humid. Autumns were 0.7 degrees C colder in the 1510s and
20%
wetter in the 1570s. The deterioration of summer climate in the
late
sixteenth century initiated a second period of enlarged glaciers
in
this millennium (the first having been in the fourteenth century)
which did not end until the late nineteenth century. An analysis
of
forcing factors (solar, volcanic, ENSO, greenhouse) points only
to
some volcanic forcing. In order to understand circulation
patterns in
the sixteenth century in terms of synoptic climatology, proxy
information was mapped for a number of anomalous months. Attempts
to
compare circulation patterns in the sixteenth century with
twentieth-century analogues revealed that despite broad
agreements in
pressure patterns, winters with distinct northeasterly patterns
were
more frequent in the sixteenth century, whereas the declining
summer
temperatures from the mid-1560s seem to be associated with a
decreasing frequency of anticyclonic ridging from the Azores'
center
of action towards continental Europe. The number of severe storms
on
the Dutch North Sea coast was four times greater in the second
half
of the century than in the first. A more or less continuous
increase
in the number of floods over the entire century occurred in
Germany
and the Czech lands. The Iberian peninsula and the Garonne basin
(France) had the greatest number of severe: floods in the 1590s.
The analysis of the effects of climate on rye prices in four
German
towns involved a model that included monthly temperatures and
precipitation values known to affect grain production. The
correlation with rye prices was found significant for the entire
century and reached its highest values between 1565 and 1600.
From
the 1580s to the turn of the century wine production slumped
almost
simultaneously in four regions over a distance of 800 kilometers
(Lake Zurich to western Hungary). This had far-reaching
consequences
for the Habsburg treasury and promoted a temporary shift in
drinking
habits from wine to beer. Peasant communities which were
suffering
large collective damage from the effects of climatic change
pressed
authorities for the organization of witch-hunts. Seemingly most
witches were burnt as scapegoats of climatic change. Copyright
1999,
Institute for Scientific Information Inc.
================
(7) CLIMATE CHANGE IN 16th CENTURY EUROPE
C. Pfister*) et al.: Documentary evidence on climate in
sixteenth-century Europe.. CLIMATIC CHANGE, 1999, Vol.43, No.1,
pp.55-110
*) UNIVERSITY OF BERN,HIST INST,UNITOBLER,CH-3000 BERN
9,SWITZERLAND
The known documentary climatic evidence from six European
countries -
Switzerland, Germany, the Czech Republic, ancient Hungary, Italy
and
Spain - is presented and classified in this article and then
further
analyzed in subsequent papers included in this volume. The
sixteenth
century witnessed an increase in the number and variety of
sources in
Switzerland, Germany and the Czech Republic as well as in the
western
and northern parts of ancient Hungary (present Slovakia). In
northern
Italy, the relevant sources are more abundant and widespread than
in
central Europe, but they have hardly been explored. Town
chronicles
written by members of the literate elite comprise the basic type
of
evidence in central Europe (including northern Italy and
Hungary).
This kind of source reports exceptional climatic events (e.g.
anomalies and natural disasters) along with their impact on the
environment and on society. Documentary data are the only
evidence
known to exist for reconstructing time series of natural
disasters
prior to the twentieth century. In order to document the extreme
character of an event, chroniclers frequently referred to
features in
the cryosphere, biosphere or hydrosphere that were known to be
more
accurate yardsticks of temperature and precipitation patterns
than
subjective impressions. When records of such events are compiled
with
the description of some of the known effects, the results can be
transformed into a severity index. Whereas chroniclers usually
focused upon extreme events, long, continuous and seemingly
homogeneous series of different kinds of proxy data are drawn
from
administrative records. Most of them are connected to the timing
of
certain kinds of agricultural work (haymaking, beginning of grain
harvest or vintage) or to the amount and quality of agricultural
production (per hectare yield of vineyards, sugar content of
wine,
etc.). In most cases the timing of these works was found to be
directly related to temperature patterns over the preceding
months
and weeks. All the Iberian peninsula towns, which had an
institutionalized municipal authority, have preserved documents
generated from the late Middle Ages. These records frequently
contain
references to floods and meteorological anomalies such as
droughts
and long wet spells. They also include mention of the system of
rogations, those religious rites performed in a standardized way
within the Spanish world with a view to putting an end to an
alleged
meteorological stress. The data for Switzerland, Hungary and
Spain as
well as much of the data for Germany are stared in the
EURO-CLIMHIST
database set up at the Institute of History at the University of
Barn. At present, EURO-CLIMHIST comprises some 600,000 data for
the
period from AD 750 to the beginning of the period of instrumental
networks. About 120,000 records for Germany are currently stored
in a
data bank called HISKLID located at the Department of Geography
of
the University of Wurzburg. The database for the Czech Republic
includes records for the time-span AD 975-1900 and is housed with
the
Department of Geography of Masaryk University in Brno. Data on
Italy
were collected with different purposes and are stored in two data
hanks, the CNR-ICTIMA (climatic data and natural disasters) and
the
SGA (extreme events). Copyright 1999, Institute for Scientific
Information Inc.
============
(8) ASTRONOMY & WEATHER OBSERVATIONS IN 16th CENTURY EUROPE
C. Pfister et al.: Daily weather observations in
sixteenth-century
Europe. CLIMATIC CHANGE, 1999, Vol.43, No.1, pp.111-150
*) UNIVERSITY OF BERN,HIST INST,UNITOBLER,CH-3000
BERN,SWITZERLAND
Thirty-two weather diaries written in astronomical calendars in
central Europe in the late fifteenth and sixteenth centuries are
presented and discussed. Systematic weather observations were
promoted by the rise of planetary astronomy and its application
in
astro-meteorology. The practice of keeping weather diaries spread
from Cracow (Poland) to Ingolstadt (Germany) and from there to
other
universities. The data obtained from these sources provided the
backbone for setting up series of precipitation indices for
Poland,
Germany and Switzerland. Monthly statistics of days with
precipitation, snowfall and frost were computed by counting the
relevant entries in the most important diaries. The results were
compared with either those obtained from instrumental
measurements in
the same place or with those from modern instrumental
measurements in
a neighbouring place. The final results show that autumn was
considerably colder in the early sixteenth century. April was
considerably drier and July was wetter during the period
1508-1531
than during 1901-1960. In order to highlight the impact of
weather
patterns on grain prices in a year of crisis, the timing of wet
and
dry spells in southern Poland and southern Germany is compared
for
the year 1529. Winters became 1.7 degrees C colder from 1564 to
1576
and the month of July tended to be wetter than in 1901-1960.
Details
noted in the diaries kept between 1585 and 1600 by the
astronomers
Brahe (near Copenhagen) and Fabricius (in the Ostfriesland region
of
northwestern Germany) closely agree. It rained more often in June
and
July and temperatures dropped. The winter months were more
frequently
dominated by winds from easterly directions, the frequency of
snowfall was higher and a deficit occurred in precipitation. This
points to a higher frequency of high pressure in the
Fennoscandian
area with cold air advection from the east or northeast.
Copyright 1999, Institute for Scientific Information Inc.
===========
(9) EUROPEAN TREE RINGS & 16th CENTURY CLIMATE
K.R. Briffa*) et al.: European tree rings and climate in the 16th
century. CLIMATIC CHANGE, 1999, Vol.43, No.1, pp.151-168
*) UNIVERSITY OF EAST ANGLIA,CLIMAT RES UNIT,NORWICH NR4
7TJ,NORFOLK,ENGLAND
We present a selective review of tree-ring variability and
inferred
climate changes in Europe during the 16th century. The
dendroclimatological evidence is assessed within the context of
the last 500 years and some interpretational problems are
discussed.
The tree-ring evidence is compared with various
non-dendroclimatic
evidence. The body of evidence shows that a large region of mid
and
northern Europe experienced a sharp cooling at around 1570/80
that,
at least in the north, marked a shift towards a prolonged period
of
cool conditions. This region had its southern boundary in the
Alps
and there is little evidence for a major cooling in southern
Europe.
Copyright 1999, Institute for Scientific Information Inc.
=============
(10) CLIMATE CHANGE AND WITCH-HUNTS
W. Behringer: Climatic change and witch-hunting: The impact of
the
Little Ice Age on mentalities. CLIMATIC CHANGE, 1999, Vol.43,
No.1,
pp.335-351
UNIVERSITY OF YORK,YORK YO1 5DD,N YORKSHIRE,ENGLAND
In addition to objective climatic data, subjective or social
reactions can also serve as indicators in the assessment of
climatic
changes. Concerning the Little Ice Age the conception of
witchcraft
is of enormous importance. Weather-making counts among the
traditional abilities of witches. During the late 14(th) and
15(th)
centuries the traditional conception of witchcraft was
transformed
into the idea of a great conspiracy of witches, to explain
'unnatural' climatic phenomena. Because of their dangerous
nature,
particularly their ability to generate hailstorms, the very idea
of
witches was the subject of controversial discussion around 1500.
The
beginnings of meteorology and its emphasis of 'natural' reasons
in
relationship to the development of weather must be seen against
the
background of this demoniacal discussion. The resurgence of the
Little Ice Age revealed the susceptibility of society. Scapegoat
reactions may be observed by the early 1560s even though
climatologists, thus far, have been of the opinion that the
cooling
period did not begin until 1565. Despite attempts of containment,
such as the calvinistic doctrine of predestination, extended
witch-hunts took place at the various peaks of the Little Ice Age
because a part of society held the witches directly responsibile
for
the high frequency of climatic anomalies and the impacts thereof.
The
enormous tensions created in society as a result of the
persecution
of witches demonstrate how dangerous it is to discuss climatic
change
under the aspects of morality. Copyright 1999, Institute for
Scientific Information Inc.
==============
(11) HOLOCENE IMPACTS
From Owen Fox <oefox@labyrinth.net.au>
Benny,
As an interested observer I would like to add the following
jigsaw
pieces to the discussion about impacts in the last 10,000 years:
1. In his book 'Impact', Gerrit Verschuur refers to an impact
site at
Aorounga in Chad that is 12.6 km in diameter and is estimated at
4000
years old. (p167)
2. In the same book reference is made of an impact S/E of
Tasmania
around 10,000 years ago. In support of an impact in this area I
recall a news item here in Victoria, Australia about two years
ago
describing some academic research that identified the Shoalhaven
river valley (about 200km south of Sydney) as having been
subjected
to massive tidal wave action some 10,000 years ago. I'm sorry I
don't
have any references for this but others may already know of the
paper
concerned or be able to search for it.
Regards,
Owen Fox
-----
MODERATOR'S NOTE: Recent in situ research on the Aorounga impact
craters in Chad, Africa, has revealed that the tentative dating
of this
chain of craters by French geologists in the 1980s (~12.000 -
6000 BP)
is far off the mark. BP
==============
(12) METRIC & IMPERIAL
From Jonathan Shanklin <jdsh@mail.nerc-bas.ac.uk>
>The researchers also found that the boundary between floating
ice and
>ice grounded on the sea floor has withdrawn by an average of
about 100
>metres annually for the last 7,600 years, a total of about
12,800
>kilometres (800 miles) since the last ice age. That average
is similar
>to the current rate, and there is no indication that the
retreat is
>slowing.
It looks as if conversion between metric and imperial is still a
problem..... The diameter of the Earth is about 12,800 km!
Regards,
Jon Shanklin
j.shanklin@bas.ac.uk
British Antarctic Survey, Cambridge, England
http://www.nbs.ac.uk/public/icd/jds
----------------------------------------
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
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*
HOW AN ASTEROID IMPACT CAUSES EXTINCTION
POEM OF THE DAY
Lost in Space!
How many Near Earth Objects have been found this week?
How many will collide with Earth each thousand years?
How many of them are out there still to seek?
How much should be spent to allay or confirm our fears?
What would be the consequences of an ocean fall?
Would coastal cities be destroyed and lost?
Is the worst case so destructive it would kill us all?
Could any nation in the world survive the cost?
When an object newly found is lost from view,
Should we be anxious that it got away,
Its magnitude too faint, large apertures too few,
Perhaps only to return another day?
Anyhow, give credit to Australia's Rob McNaught
Using the old 'Forty Inch' at Siding Spring!
He found the minor planet that was sought
And a better orbit shows it's not a deadly thing.
11.10.99
Malcolm Miller
<stellar2@actonline.com.au>
HOW AN ASTEROID IMPACT CAUSES EXTINCTION
From EXPLOREZONE, 11 October 1999
http://www.explorezone.com/columns/space/1999/october_extinctions.htm
While the impact destroys one side of the planet, shockwaves
converge
to devastate the opposite side of Earth. Then, of course, there's
that
interminable winter to contend with.
How an asteroid impact causes extinction
By Michael Paine for explorezone.com
Imagine: NASA scientists announce they have detected a
10-mile-wide
asteroid on a collision course with the Earth. They calculate it
will
hit Southeast Asia in two weeks. There is no chance of Bruce
Willis
being sent on a beefed-up Space Shuttle to blow up the asteroid.
Earthlings will have to ride out the impact.
The world economy grinds to a halt as people take to the hills.
Anarchy
sets in, civilisation breaks down. Accusations fly over the lack
of
warning -- where was Spaceguard, the proposed international
search
effort for large asteroids?
People in Brazil feel less vulnerable than most of the world's
population. They are on the opposite side of the Earth from the
predicted impact point. But one hour after the impact Brazilians
notice
some brilliant meteors. Then more meteors. Soon the sky gets
brighter
and hotter from the overwhelming number of meteors. Within a few
minutes trees ignite from the fierce radiant heat. Millions of
fragments of rock, ejected into space by the blast, are making a
fiery
return all over the planet.
Only people hiding underground survive the deadly fireworks
display.
Within three hours, however, massive shock waves from the impact
travel
through the Earth's crust and converge on Brazil at the same
time. The
ground shakes so violently that the ground fractures and molten
rock
spews from deep underground. Maybe Brazil wasn't the best place
to be
after all.
The survivors of the firestorms, tsunami and massive earthquakes
emerge
to a devastated landscape. Within a few days the Sun vanishes
behind a
dark thick cloud -- a combination of soot from the firestorms,
dust
thrown up by the impact and a toxic smog from chemical reactions.
Photosynthesis in plants and algae ceases and temperatures
plummet. A
long, sunless Arctic winter seems mild compared to the new
conditions
on most of the planet.
After a year or so the dust settles and sunlight begins to filter
through the clouds. The Earth's surface starts warming up. But
the
elevated carbon dioxide levels created by the fires (and, by
chance,
vaporisation of huge quantities of limestone at the impact site)
results
in a runway greenhouse effect. Those creatures that managed to
survive
the deep freeze now have to cope with being cooked.
Many species of plants and animals vanish. The few hundred
thousand
human survivors find themselves reverting to a Stone Age
existence.
Is it fiction?
Computer modelling of asteroid impacts and climatic effects
suggest
that this devastating sequence of events could happen.
Fortunately it
is extremely unlikely that this scenario will occur in our time.
No
known asteroids are on a collision course with Earth. However,
every 50
to 100 million years the Earth collides with a comet or asteroid
of
sufficient size to cause planet-wide devastation.
Evidence of past impacts has only been recognised in recent
decades and
is pointing to the conclusion that big impacts have caused major
disruptions to the development of life on Earth.
The end of the dinosaurs
In 1980 Scientists Luis and Walter Alvarez claimed they had found
evidence of a huge impact event 65 million years ago. This age
corresponded with the demise of the dinosaurs at the end of the
Cretaceous Period. The evidence included a worldwide layer of
clay with
high levels of the rare element iridium, usually the signature of
an
impact.
The search was on for a giant crater associated with this impact.
Hopes
weren't high because in 65 millions years the Earth's surface has
changed dramatically - nearly all of the present ocean floor is
younger than 50 million years.
Some great scientific detective work pointed to an impact
somewhere in
Central or North America. Finally, in 1990, the buried remains of
a
150-mile-diameter crater were discovered near the town of
Chicxulub on
theYucatan Peninsula in Mexico. A crater this size would have
been
blasted out by a 10-mile-wide comet or asteroid colliding with
the
Earth at some 50,000 mph. The "smoking gun" had been
found.
Impact, volcanoes or both?
The debate continues on whether the Chicxulub impact caused the
mass
extinction at the end of the Cretaceous Period or whether it was
one of
a sequence of disasters. The Deccan Traps of India are the
remnants of
a massive upwelling of molten rock from deep within the Earth 65
million years ago. The toxic fumes and dust from the eruption
have been
put forward as a possible alternative cause of climate change
that led
to the extinction of the dinosaurs.
A possible link between impacts and volcanism became evident in
1974
when the Mariner 10 spacecraft flew past the innermost planet
Mercury.
The planet was found to be covered with impact craters like the
Moon.
One giant impact crater on Mercury was particularly interesting.
Directly opposite the impact point, on the other side of the
planet
(called the "antipodal point") was a region of highly
disrupted terrain
with no evidence of an impact. The shock waves from the impact on
one
side of Mercury had travelled around the surface and met
simultaneously
at the antipodal point to create the chaotic features. Similar
features
have since been detected on several moons of the giant planets.
Astronomer Duncan Steel has suggested that the same occurred with
the
Chicxulub impact and that the shock waves caused the Deccan
Traps.
Taking into account millions of years of continental drift, this
region
would have been at the antipodal point to Mexico at the time of
the
impact. Although the eruption may have contributed to the
suffering, it
now seems more likely that the Deccan Traps were just a
consequence of
the catastrophic initial event, the Chicxulub impact.
More craters are there to be found
As a tourist destination, impact craters on Earth are virtually
unknown. Americans might know of Meteor Crater in Arizona.
Australians
planning an Outback tour may have heard of Wolfe Crater in
Western
Australia. American geologist/astronomer Gene Shoemaker (of Comet
Shoemaker-Levy 9 fame) spent many "holidays" touring
the Australian
Outback looking for impact craters. He and his wife Carolyn
helped to
identify some of the twenty or so Australian impact craters that
are
now known. Tragically, in 1997, Gene died in a car crash during
one of
these searches.
It is only in the past few decades that scientists have learnt
how to
clearly identify impact craters on the surface of the Earth. In
that
time they have found more than 250 craters. Most are heavily
disguised
by siltation, erosion and vegetation.
There are relatively few places on Earth where any geological
features
can be expected to survive beyond tens of millions of years.
Impact
craters have been found on most of the rare ancient landforms. In
several cases the estimated age of a large crater appears to
match
that of a mass extinction event, as told in the fossil record.
Although
the picture is still fuzzy -- due to the time scales involved --
massive impacts by comets and asteroids deserve serious
consideration
as an explanation for some of these extinction events.
Michael Paine is a mechanical engineer based in Sydney Australia.
He
has an amateur interest in astronomy and maintains the web pages
of the
Planetary Society Australian Volunteers.
Rocks from Space appears the second Monday of each month, only in
explorezone.com.
Copyright 1999, EXPLOREZONE
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