Comets and Asteroids - Tsunami - page 163

9

Comets and Asteroids

9.1 Introduction

Many asteroids orbit close to the Earth and in a signifi-
cant number of cases intersect the Earth's orbit (Steel 1995 ).
Four groups of asteroids have near Earth orbits (Steel 1995 ;
Lewis 1999 ). The Apollos cross the Earth's orbit but spend
most of their time just beyond it. Their orbital period around
the Sun is greater than 1 year. The Amors orbit further out,
crossing the orbit of Mars as well as that of the Earth. These
objects have unstable orbits, and they are affected the most
by the gravity of Jupiter. The Atens spend most of their time
inside Earth's orbit. Their orbital period about the Sun is
less than 1 year. These latter objects were only discovered
in the 1970s. A fourth group of asteroids is suspected. These
asteroids lie close to the Earth and are thought to originate
from debris remaining from asteroid impacts with the moon.
Most of the Earth-crossing asteroids have short-lived orbits
about the Sun, suggesting that they too originated from the
breakup of comets. The population of near Earth objects
(NEOs) is replenished discontinuously over time. This
chapter looks at the formation of near Earth objects, the
probability of their impact with the Earth, the effect of
resulting tsunami, and evidence for their occurrence in
recent times.

There are two main classes of celestial objects: asteroids
and comets that can cross the Earth's orbit and eventual
impact with the Earth (Steel 1995 ). Comets consist mainly
of ice with some stony or iron material ranging in size from
sand grains to boulders hundreds of meters in diameter.
They enter the inner solar system from the Kuiper belt lying
outside the orbit of Neptune or from the Oort belt lying
beyond the outer boundary of the solar system (Asher et al.
1994 ; Steel 1995 ; Verschuur 1996 ). The gravitational
attraction of Jupiter—and Saturn—can capture a comet as
large as 200 km in diameter and bring it into an orbit in the
inner solar system. This capture occurs about once every
200000 years. These comets tend to disintegrate over suc-
cessive orbits, producing a large number of smaller active
comets and inactive asteroids. There are three types of
comets: short, intermediate, and long period. Short period
comets orbit the Sun with periods of less than 20 years.
They are called Jupiter family comets because they come
under the gravitational influence of Jupiter and have
unstable orbits. Intermediate-period or Halley-type comets
have orbital periods between 20 and 200 years. Long-per-
iod comets have orbital periods greater than 200 years and
tend to appear in the inner solar system only once. As
comets approach the Sun, they slowly disintegrate as ice is
vaporized and discharged with other gases in a tail that can
extend millions of kilometers into space. There is growing
evidence that comets are responsible for much of the debris
orbiting the inner solar system. The best-known comet to
impact with the Earth occurred at Chicxulub on the
Yucatan Peninsula of Mexico at the Cretaceous-Tertiary
boundary (Verschuur 1996 ; Alvarez 1997 ). This event
threw up large volumes of sediment into the stratosphere,
attenuated solar radiation significantly for months, if not
years, and led to the extinction of the dinosaurs Fig. 9.1 .
Our present era is under the influence of a large comet that
entered the inner solar system within the last 20000 years
(Asher et al. 1994 ).

9.2 Near Earth Objects

9.2.1 What Are They?

The Earth is known to cross through at least 12 comet debris
trails that form prominent meteorite showers. The most
important comet stream is the Taurid complex. It is theo-
rized that a large comet, about 100 km in diameter, initially
entered the inner solar system 20000 years ago and began to
break up about 14000 years ago (Asher et al. 1994 ; Steel
1995 ). Further disintegration events occurred around 7500
and 2600 BC. The latter breakup happened during the
Bronze Age and was associated with asteroid strikes within
the next few centuries that impacted dramatically upon
civilizations. Debris in the Taurid complex consists of a