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


----------------------------------------
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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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