Encyclopedia of Evolution.pdf - Online Reading Center

asteroids and comets
one planet, it would be less than half the size of the Moon.
There was brief controversy over whether Pluto was a mere
asteroid rather than a planet. Since Pluto is about as large
as all the asteroids combined, most astronomers still consider
it to be a planet. The largest known asteroid is Ceres,
about 600 miles (1,000 km) in diameter, while the smallest
asteroids are pebbles. Sixteen asteroids exceed 150 miles
(240 km) in diameter. Most asteroids have a nearly circular
orbit around the Sun. Asteroids are classified into three
categories:
• More than three-quarters of the asteroids are carbonaceous
(C-type) asteroids. They are found predominantly in
the outer portion of the asteroid belt. Their composition
is similar to that of the Sun, minus the volatile materials
(such as hydrogen and helium) that have been lost. The
carbonaceous chondrite meteors, such as the meteorite that
fell near Murchison, Australia, in 1869, appear to be Ctype
asteroids. They contain organic molecules similar to
those that may have formed on the primordial Earth and
from which life is believed to have developed (see origin
of life).
• A little less than one-fifth of the asteroids are silicaceous
(S-type) asteroids. They are composed of metallic iron
mixed with iron-magnesium silicates. They are found predominantly
in the inner portion of the asteroid belt. Many
astronomers believe that S-type asteroids are the source of
most meteorites, but this issue remains unresolved.
• The other asteroids are metallic (M-type) asteroids, composed
mostly of metallic iron and found in the middle
region of the asteroid belt.
Comets consist mainly of wet and dry ice (H2O and
CO2) but also contain many organic chemicals that are
similar to those involved in the origin of life. Many comets
revolve around the Sun in extremely slow and very elliptical
circuits, in one of two main groups:
• The Kuiper Belt comets reside (along with some asteroids)
in a band beyond the orbit of Pluto.
• The Oort cloud comets are beyond the Kuiper Belt, many
of them nearly two light-years from the Sun. At this distance,
they are nearly halfway to the nearest star, and the
Sun itself would appear as merely a star.
Solar gravitation is weak, especially in the Oort cloud,
but only the tiniest perturbation in gravity is enough to send
the comets in a path toward the Sun. Comets can die (for
example, when they collide with a planet), but there is a virtually
unlimited supply of them in the Kuiper Belt and Oort
cloud. The tail of the comet consists of water and CO2 molecules
that vaporize as the comet approaches the Sun; therefore,
the tail of the comet points away from the Sun rather
than trailing behind the comet.
A large number of comets and asteroids roamed in the
paths of planets and moons early in the history of the solar
system. Many of these objects collided with planets and
moons until about four billion years ago. The craters from
these impacts can still be seen on the Moon. The influence of
the tremendous gravitational field of Jupiter has profoundly
affected the asteroids of the solar system: Jupiter cleared
many of them out of the plane of the ecliptic and stabilized
a band of asteroids between its orbit and that of Mars that
might otherwise have formed into a planet. Humans have
Jupiter to thank for the fact that asteroid collisions with
Earth are so rare.
Asteroids with orbits that bring them within about 125
million miles (200 million km) of the Sun are considered
NEAs (near-Earth asteroids). Most of these asteroids were
jarred from the asteroid belt by collisions and/or nudged by
interactions of gravitation. The approximately 250 NEAs
that have been found to date probably represent only a tiny
fraction of the total. It is estimated that there may be a thousand
NEAs that are large enough (1 km or more in diameter)
to threaten mass extinction, as at the end of the Cretaceous
period (see Cretaceous extinction) and at the end of the
Permian period (see Permian extinction).
On March 23, 1989, an asteroid one-quarter mile (0.4
km) in diameter came within 400,000 miles (640,000 km) of
the Earth. Its existence had not been previously known. Scientists
estimated that the asteroid and the Earth had passed the
same point in space a mere six hours apart. A similar nearmiss
occurred on February 23, 2004, in which a previously
unknown asteroid, large enough to destroy life on the planet,
appeared near the Earth. An approximately 1,500-foot (500
m) asteroid passed within 250,000 miles (400,000 km) of the
Earth on July 3, 2006. Near misses with smaller asteroids that
came closer to Earth had occurred in 1991, 1993, and 1994.
All three of these asteroids were about 26 feet (8 m) in diameter,
enough to create local devastation but not a threat to
life. If a large NEA appeared on a collision course with Earth,
humans would not be able to send up space cowboys to save
the Earth as in the Armageddon movie, because splitting the
asteroid would only create smaller asteroids whose smaller
impacts may have the same cumulative effect on the planet.
Asteroid and comet collisions have occurred numerous
times during the history of the planet. The most famous
impact occurred 65 million years ago at the end of the Cretaceous
period (see table on page 30). Most of the craters listed
in the table are found on dry land, in regions that are cold or
dry enough that erosion has not erased evidence of them. The
Barringer Crater in Arizona is still easily recognizable because
the impact was recent (50,000 years ago) and the weather is
dry (see figure on page 29). Chicxulub, the crater from the
end of the Cretaceous period, has largely eroded and filled
with sediment and would probably not have been discovered
had scientists not been looking for it.
These impacts have had an important effect on the
course of evolution. While natural selection explains
most evolutionary patterns, asteroid impacts cause largely
random extinctions. Paleontologist David Raup indicates
that mass extinctions are due to bad luck rather than bad
genes. In the period following an impact, a wide world of
possibility is open to many new types of organisms, some of
them with characteristics that would have been eliminated
by competition in a more heavily populated world. In this
way, asteroid impacts have created punctuations of evolutionary
novelty.