PLEASE NOTE:
*
CCNet ESSAY: COSMIC UNCERTAINTY
-------------------------------
From ASTRONOMY NOW, March 2000, pp. 53-55
http://astronomynow.com
IS THE EARTH DUE FOR ANOTHER CATACLYSMIC IMPACT? THE SEARCH FOR
PAST
IMPACTS IS HELPING TO SAFEGUARD OUR FUTURE.
By Benny Peiser
Ever since Sir Isaac Newton, we have learned to see our world
through
the lens of order and stability. In this deeply ingrained view,
the
world works like a giant clockwork according to set a universal
laws
and patterns that can be recognised and understood. Perhaps the
greatest triumph of Newtonian cosmology was the scientific
comprehension of comets. It was the ability to predict their
movements
which laid to rest the ancestral fear of comets as random
portents of
an unpredictable universe. The main message of this optimistic
cosmology was that human society could be assured of the
constancy,
predictability and stability of our world.
In the early 19th century, a new class of extra-terrestrial
bodies, the
asteroids, were discovered. Slowly but gradually they started to
undermine the notion of cosmic optimism. It was not until the
20th
century, however, that it became manifest that some of these
objects
display extremely complex behaviours that are essentially
unpredictable. The recognition that disorder rather than harmony
held
sway in the solar system gradually began to emerge.
FROM ORDER TO CHAOS
The shift that led to the replacement of the concept of an
essentially
benign universe by that of an unpredictable cosmos punctuated by
catastrophes occurred in several phases. One of the most
consequential
transformations happened in the 1920s and 1930s when two
momentous
events changed our view of the universe forever.
In 1927, the public was stunned by the announcement of Leonid
Kulik, a
Russian scientist, that he had discovered the epicentre of a
large
meteorite impact. Kulik confirmed that he had detected the
devastated
area of a meteorite impact near the Tunguska river in Siberia in
1908.
Despite the rather spectacular report, only a handful of
individuals
were interested in the search for extra-terrestrial debris or the
craters some of them had left behind. Harvey Nininger, the
founder
of the scientific study of meteorites, tried in vain to convince
the
scientific world to finance a US expedition to Tunguska. Then, in
late
October 1937, a newly discovered object, asteroid Hermes, passed
Earth
so close that leading astronomers at the time even contemplated
its
collision with our planet.
The portentous event soon became eclipsed by the dark clouds of
impending war. The incident, however, left a powerful impression
on a
small number of researchers. As a direct result of the Hermes
trauma,
Harvey Nininger suggested in 1942 that some episodes of mass
extinctions in the geological record may have been the
consequence of
past asteroid collisions with the Earth.
For the next 40 years or so, the scientific community was rather
unresponsive to Nininger’s hypothesis. The gradualist
paradigm of Lyell
and Darwin continued its unchallenged reign. In the 1960s, Gene
Shoemaker was able to verify the true nature of hypervelocity
impact
structures. But it was not before the early 1980s that the actual
breakthrough to a more realistic view of our cosmic environment
occurred.
The geological (K/T) boundary seperating the Cretaceous from the
Tertiary period is believed to be linked to the impact of a giant
comet or asteroid that triggered the extinction of the dinosaurs
and
many other species around 65 million years ago. The strength of
the K/T
impact theory by Luis Alvarez and his team, and the main reason
for its
eventual acceptance, was based on the extra-terrestrial iridium
sandwiched between the K/T layers. Since then, most scientists
have
accepted the idea that cosmic catastrophes have repeatedly
punctuated
life on Earth. This recognition, however, went with the
assumption that
cosmic-induced cataclysms were restricted to primordial times,
millions
of years before the emergence of Homo sapiens.
US VERSUS UK
Current research on the history of cosmic catastrophes differs
significantly between what might be called the British and the
American
schools of thought. Advocates of the American School are known
for
their cosmic 'optimism' and bordering on what critics have called
cosmic 'naivite'. Their philosophy is characterised by a belief
that
giant impacts triggering global disasters happen very rarely
(every
100,000 to 1,000,000 years on average) and that the flux of such
impactors is more or less constant even over long periods of
time.
Cosmic impacts, according to this view, occur on a random basis
mainly
as a result of single asteroid impacts.
The American School is so convinced of their doctrine, that most
of
their advocates are not even interested in studying the
historical or
environmental records of humankind's more recent past.
The British School, by contrast, is more concerned with cometary
debris which may have led to more recent punctuations. Although
their
focus on historical catastrophism makes them look like
pessimists,
their emphasis on empirical data and the notion that impacts
often
occur in clusters, may ultimately prove that they are actually
cosmic
realists.
The questions raised by this group of researchers are, I believe,
essential for a better understanding of our cosmic environment
and
the latent hazards it poses for the future of our civilisation.
The main questions which need to be addressed are: How many
extra-terrestrial impacts has humankind experienced during the
last
10,000 years? What can be determined about the magnitude and
environmental knock-on effects of these events? Did they have
any pernicious effects on cultural and societal evolution?
It should be obvious that a clarification of these enigmas would
be
extremely helpful for an improved understanding of human
evolution.
More importantly, these answers would be essential for any
effective
measures taken to safeguard the future of civilisation. The
British
School of Coherent Catastrophism argues that cosmic impacts are
often
not the product of chaotic and random processes but should be
understood as the outcome of the disintegration of larger objects
whose
evolution can be reconstructed.
Such episodes of increased cometary/meteoric activity punctuating
the evolution of human cultures are thus looked upon as primary
agencies determining the rise and fall of ancient civilisations.
Both the emergence and the collapse of human cultures, e.g. the
Pleistocene-Holocene transition and the Neolithic Revolution, the
onset and collapse of the Bronze Age civilisations, and even the
collapse of the Roman Empire may be associated with episodes of
increased meteoric activity and multiple impacts that may well
have included incidents of cosmic dust loading.
What is envisaged here are trains of cometary debris which
repeatedly
encounter the Earth at particular periods. Such debris, depending
on
its constitution and hence their cohesive strength, can have
catastrophic effects on the ecological system in a variety of
ways.
COMETARY ENCOUNTERS
The encounters between cometary debris and the Earth are capable
of
producing (although they do not always do so) three types of
natural
disaster:
1) Low or high level multi-megaton explosions of fireballs which
destroy natural and cultural features on the surface of the Earth
by
means of floods, blasts and seismic damage and which can destroy
regions up to the size of a large country;
2) Massive high level influx of cosmic dust high above the
stratosphere
which causes a dramatic drop of global temperature, (this
cosmically
induced climatical disaster can last for decades and consequently
lead
to the suspension of agriculture);
3) Massive high-level influx of cosmic chemicals (associated with
dust)
with as yet incalculable biochemical potentials which may be
harmful to
DNA and can trigger evolutionary mutation.
So how can we find out how many such encounters humankind has
experienced during the Holocene, i.e. in the space of the last
10.000
years or so? In recent years, it has become evident just how
little we
actually know about impact events even for our most recent
history.
The most distinct evidence for the occurrence of a cosmic impact
is the
existence of a hypervelocity impact crater formed on the Earth’s
surface. For the Holocene, 14 such craters have so far been
detected.
While almost all of them are small in size (diameter > 0.5
km), it is
interesting to note that they are not scattered in time. 50% of
them
seem to cluster at around 5000-4000 years ago. Does this batch
indicate
a particular intensive period of impacts at that time?
The impression may or may not turn out to be correct. But there
are
still many uncertainties with any interpretation of this small
sample
of impact scares. Not only is their dating far from certain. What
is
more, for every Holocene impact on land we should expect two
impacts to
have occurred in the oceans. These, however, are almost
impossible to
detect unless they were on a massive scale, causing lasting
impact
configurations on the sea floor.
It is conceivable that many oceanic impacts have occurred during
the
last 10,000 years. Due to the lack of hard evidence, however, it
is
impossible, at least for the time being, to determinate when they
occurred or what magnitude and destruction they may have yielded.
Just as little is known about Tunguska-like impacts during that
period of time. The 1908 Tunguska object also failed to produce a
crater because the impactor detonated some 5 km above ground,
causing devastation due to the huge atmospheric explosion which
yielded energy equivalent to twenty Megatons of TNT.
Undoubtedly, many similar – and even larger –
atmospheric impacts have
occurred during the Holocene. There are, however, at least two
principal problems with this class of extra-terrestrial
punctuations:
1) how often do they occur on average, and 2) how can evidence of
such
past impacts be detected?
There is no consensus within the scientific community regarding
the impact rate of Tunguska-type events. Current estimates range
from once every 300 years to three Tunguska-class impacts per
century. While cosmic optimists expect 'only' 30-40 such events
to
have occurred during the Holocene, others contemplate up to 300
atmospheric impacts in the same space of time. The likelihood of
large-scale atmospheric impact events, however, makes it
extremely
difficult to detect unambiguous evidence for the occurrence of
celestial disasters. As in the case of oceanic impacts, smoking
guns are almost impossible to locate.
DEADLY COSMIC DUST
There may be another, even less understood mechanism that could
have contributed to cosmic disasters. The disintegration of giant
comets does not only yield debris in form of asteroids and meteor
streams, but can lead to the release of huge amounts of cosmic
dust.
When encountered by the Earth, such dust loading can have serious
effects on the global climate.
The 14 Holocene known impact craters, in other words, most
certainly
paint a rather deceptive picture of our past. Yet, the fact that
no
massive impact crater dating to the Holocene has been detected,
has
led to the belief that no hemispheric or global impact disaster
can
possibly have happened. However, this is a widespread delusion.
It
should be obvious by now that the limited focus on impact crater
counts
alone may provide an illusory account.
In view of a cosmic environment that appears to be disorderly and
unstable, we need to re-establish order by obtaining the
necessary
information about all the objects that may come close to Earth.
By
studying past impacts and the current behaviour of near-Earth
objcets (NEOs), we are attempting to take control of our
immediate
cosmic environment.
Copyright 2000, Astronomy Now
-----------------
CCNet-ESSAY is part of the Cambridge Conference Network. It
includes
interesting and thought-provoking essays about our place in space
and
the prospects of a planetary civilisation that is in control of
our
terrestrial and extraterrestrial environment. Contributions to
this
ongoing debate are welcome. To subscribe or unsubscribe from
CCNet,
please contact Benny J Peiser at <b.j.peiser@livjm.ac.uk>.
The fully
indexed archive of the CCNet, from February 1997 on, can be found
at
http://abob.libs.uga.edu/bobk/cccmenu.html