Comets and Asteroids - Tsunami - page 165

photosynthesis for several years, induced acid rain due to
atmospheric reaction of nitrogen and oxygen, and depleted
ozone in the stratosphere.

tsunami that were occurring at regular intervals in circum-
oceanic regions.
These probabilities also assume a random but constant
flux of objects intersecting the Earth. However, the
appearance of comets and asteroids in the inner solar sys-
tem, and their impact with the Earth, is clustered in time in a
phenomenon astronomers call coherent catastrophism
(Asher et al. 1994 ; Verschuur 1996 ). This is logical when it
is realised that cometary disintegration leads to the pro-
duction of objects 10-100 m in size, with some kilometer-
sized objects orbiting about the Sun within the confines of a
narrow stream or trail. These objects tend to be clustered
within this stream. Resonant interaction by Jupiter and the
inner planets upon a stream such as the Taurid complex
periodically allows objects within the stream to intersect the
Earth's orbit, leading to multiple bombardments of Tung-
uska-sized objects (or larger) over periods of one to four
centuries. In terms of global catastrophes, it is not the
random impact of celestial objects greater than 1 km in
diameter occurring on average every hundred thousand
years that is important, but rather the occurrence of clusters
of Tunguska-sized objects during epochs of high activity.
The latter can affect civilizations deleteriously through
direct impact, the generation of tsunami, or modifications to
the atmosphere leading to sudden periods of global cooling.
Some measure of coherence in meteorites and comets
can be obtained from Chinese, Japanese, and European
records of meteor, comet, and fireball sightings gathered
over the last 2000 years (Rasmussen 1991 ; Hasegawa
1992 ). The accumulated record, up to the beginning of the
nineteenth century when scientific observations began in
earnest, is plotted in Fig. 9.3 . The Asian records are the
most complete, with European sightings accounting for less
than 10 % of the record over the last 1000 years. The comet
observations from Asia are also plotted in Fig. 9.3 . A quasi-
cyclic pattern is evident in the records that can be linked to
the dominance of the Taurid complex in the inner solar
system. Peak occurrences of cosmic input to the atmosphere
occurred between 401 and 500, 801 and 900, 1041 and
1100, 1401 and 1480, 1641 and 1680, and 1761 and 1800.
These intervals have been shaded. The first period corre-
sponds with the last extended epoch of nodal intersection
with the Taurid complex, while the prominent fluxes in the
eleventh and fifteenth centuries correspond to nodal inter-
sections with parent objects in the complex. The fifteenth
century represents the last phase of coherent catastrophism
associated with the Taurid complex. In addition, there is a
preference for sightings to occur in July-August and
October-November. Some astronomers believe that many
of these peaks were responsible for climate changes and
direct impacts that have affected the course of human his-
tory (McCafferty and Baillie 2005 ; Baillie 2006 ).

9.2.2 How Frequent Have Comet and Asteroid

Impacts Been?

Since hominids evolved, between 200 and 500 extraterres-
trial bodies several hundred meters in size have impacted
with the Earth (Asher and Clube 1993 ; Asher et al. 1994 ).
About 70 % of NEOs are asteroids, of which 50 % are
derived originally from comets. The best approximation of
the probability of impact with the Earth of various sizes of
objects is presented in Fig. 9.2 (Verschuur 1996 ). There are
large uncertainties, spanning an order of magnitude, on the
return period of these objects. Observations of the more
frequent, smaller iron meteorites and those that would
explode in the atmosphere—termed bolides—are also
uncertain because historical records beyond a few centuries
are very limited globally. The range in kinetic energy pre-
sented in Fig. 9.2 reflects the range in speed of objects
striking the Earth—typically between 10 and 45 km s -1 .
Astronomical observations indicate that one to three near
Earth objects with a diameter of 1 km could impact the
Earth every 100000-200000 years. Note that this is three
times more frequent than NASA's estimate. An object 50 m
in diameter crashes into the Earth every century, while a
Tunguska-sized object of 60 m diameter occurs every
200-300 years. These estimates are also more frequent that
those obtained by NASA. Simulations have been made of
the number of type of impacts that could randomly be
expected over a period of 10000 years. Over this time span,
there could be 110 impact events, 285 Tunguska style air-
bursts over land, 680 over the ocean, and 12 ocean impacts
that could produce ocean-wide tsunami. Only four events
would be big enough to leave a crater on land, and all of
these would have a high probability of being eroded or
buried. Of the Tunguska-sized events, one or two impacts
would have been equivalent to 500-1,000 megatons of
TNT. Their impacts must have affected global climate and
the course of human history. All of the Tunguska-sized
objects that strike the ocean could have generated signifi-
cant tsunami. Gerrit Verschuur ( 1996 ) makes the interesting
comment that, historically, few population centers existed
around the shores of the Atlantic or Pacific Oceans. The
cradles of civilization emerged in relatively sheltered river
valleys around smaller seas—the Mediterranean, Red Sea,
and Persian Gulf, or in mountain regions such as the Andes
and the South Indian highlands. Our ancestors may have
been wiser than we are and avoided coastal areas because of
the

potentially devastating effects of asteroid-generated