552 PART 4^ |^ THE SOLAR SYSTEM
there but because they are easy to recognize. No Earth rocks
are on top of the Antarctic ice cap; the nearest native rocks are
buried under the ice. Any rock you fi nd there must have fallen
from space. (For similar reasons, another good place to fi nd
meteorites is the Sahara desert, where deep layers of sand keep
Earth rocks completely out of sight.) Th e slow fl ow of the
Antarctic ice cap from the center of the continent toward the
ocean concentrates meteorites in areas of especially good hunt-
ing where the moving ice runs into mountain barriers, slows
down, and evaporates. Teams of scientists travel to Antarctica
and ride snowmobiles in systematic sweeps across the ice each
South Pole summer to recover meteorites (■ Figure 25-1).
A four- to eight-person team can fi nd a thousand meteorites
during a single two-month fi eld season. After 25 years of work,
more than half of the 40,000 meteorites in human hands are
from Antarctica.
Meteorites that are seen to fall are called falls; a fall is known
to have occurred at a given time and place, and thus the meteor-
ite is well documented. A meteorite that is discovered on or in
the ground, but was not seen to fall, is called a fi nd. Such a
meteorite could have fallen thousands of years ago. Th e distinc-
tion between falls and fi nds will be important as you analyze the
diff erent kinds of meteorites.
Meteorites can be divided into three broad composition
categories. Iron meteorites (■ Figure 25-2a) are solid chunks of
iron and nickel. Stony meteorites (Figure 25-2b) are silicate
masses that resemble Earth rocks. Stony-iron meteorites (Figure
25-2c) are mixtures of iron and stone. Carbonaceous chon-
drites (pronounced kon-drite; Figure 25-2d) are a special type of
stony meteorite.
Iron meteorites are easy to recognize because they are heavy,
dense lumps of metal—a magnet will stick to them. Th at explains
an important statistic. Iron meteorites make up 66 percent of
fi nds (■ Table 25-1) but only 6 percent of falls. Why? Because an
iron meteorite doesn’t look like an ordinary rock. If you trip over
one on a hike, you are more likely to recognize it as something
odd, carry it home, and show it to the local museum. Also, some
stony meteorites deteriorate rapidly when exposed to weather;
irons are made of stronger material and generally survive longer.
Th at means there is a selection eff ect that makes it more likely
that iron meteorites will be found (How Do We Know?
25-1). Th e fact that only 6 percent of falls are irons shows that
iron meteoroids, although easier to fi nd on Earth, are relatively
rare in space.
When iron meteorites are sliced open, polished, and etched
with acid, they reveal regular bands called Widmanstätten pat-
terns (pronounced Veed-mahn-state-en) (■ Figure 25-3). Th ose
patterns are caused by certain alloys of iron and nickel that
formed crystals as the molten metal cooled and solidifi ed long
ago. Th e size and shape of the bands indicate that the molten
metal cooled very slowly, no faster than 20 degrees Kelvin per
million years.When they shall cry “PEACE, PEACE”
then cometh sudden destruction!COMET’S CHAOS?—What Terrible
events will the Comet bring?
— FROM A PAMPHLET PREDICTING THE END OF THE WORLD
BECAUSE OF THE APPEARANCE OF COMET KOHOUTEK IN 1973O
ne afternoon in 1954, while Mrs. E. Hulitt Hodges
of Sylacauga, Alabama, lay napping on her living
room couch, an explosion and a sharp pain jolted her
awake. Analysis of the brick-sized rock that smashed through the
ceiling and bruised her left leg showed that it was a meteorite.
Mrs. Hodges is the only person known to have been injured by
a meteorite. Coincidentally, she lived right across the street from
the Comet Drive-In Th eater.
Meteorites arrive from space all over the Earth every day,
although not as spectacularly as the one that struck Mrs. Hodges.
You will learn in this chapter that meteorites are fragments of
asteroids, and that asteroids, as well as their icy cousins the com-
ets, carry precious clues about conditions in the solar nebula
from which the sun and planets formed. Because you cannot
easily visit comets and asteroids, you can begin by learning about
the pieces of those bodies that come to you.
Meteoroids, Meteors,
and MeteoritesYou learned some things about meteorites in Chapter 19
when you studied evidence for the age of the solar system. Th ere
you saw that the solar system includes small particles called mete-
oroids. Some of them collide with Earth’s atmosphere at speeds of
10 to 70 km/s. Friction with the air heats the meteoroids enough
so that they glow, and you see them vaporize as streaks across the
night sky. Th ose streaks are called meteors (“shooting stars”). If a
meteoroid is big enough and holds together well enough, it can
survive its plunge through the atmosphere and reach Earth’s sur-
face. Once the object strikes Earth’s surface, it is called a meteorite
(“-ite” being the Greek root for “rock”). As you will learn later in
this chapter, the largest of those objects can blast out craters on
Earth’s surface, but such big impacts are extremely rare. Th e great
majority of meteorites are too small to form craters.
What can meteorites and meteors tell you about the origin
of the solar system? To answer that question, you can consider
their compositions and orbits.
Composition of Meteorites and Meteors
One of the best places to look for meteorites turns out to be
certain parts of Antarctica—not because more meteorites fall
25-1