The Solar System

CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 553

Chondrites look like dark gray, granular rocks (Figure

25-2d). Th e classifi cation of meteorites has become quite com-

plicated, and there are many types of chondrites, but in general

they contain some volatiles including water and organic (car-

bon) compounds. A few chondrites appear to have formed in

the presence of liquid water.

Most types of chondrites also contain chondrules, small

round bits of glassy rock only a few millimeters across. To be

glassy rather than crystalline, the chondrules must have cooled

from a molten state quickly, within a few hours. One hypothesis

is that chondrules are bits of matter from the inner part of the

solar nebula, near the sun, that were blown outward by winds or

jets to cooler parts of the nebula where they condensed and were

later incorporated into larger rocks. Another hypothesis is that

the chondrules were once solid bits of matter that were melted

by shock waves spreading through the solar nebula and then

resolidifi ed. Th e presence of chondrule particles inside chon-

drite meteorites indicates that those rocks have never been

melted since they formed because melting would have destroyed

the chondrules.

Among the chondrites, the carbonaceous chondrites are rare

but quite important. Th ese dark gray, rocky meteorites are

Th e Widmanstätten pattern tells you that the metal in
iron meteorites was once molten and must have been well
insulated to cool so slowly. Such slow cooling indicates a loca-
tion inside bodies at least 30 km in diameter. (In comparison,
a small lump of molten metal exposed in space would cool in
just a few hours.) On the other hand, the iron meteorites do
not show eff ects of the very high pressures that would exist
deep inside a planet. Evidently, iron meteorite material formed
in the cooling interiors of planetesimal-sized objects, not plan-
ets. You will fi nd this is one important clue to the origin of
meteorites.
A small fraction of falls are meteorites made of mixed iron
and stone (Figure 25-2b). Th ese stony-iron meteorites appear
to have solidifi ed from both molten iron and rock—the kind
of environment you might expect to fi nd deep inside a plane-
tesimal at the boundary between a liquid metal core and a
rocky mantle.
In contrast to irons and stony-irons, stony meteorites
(Figure 25-2c) are common among falls (Table 25-1). Although
there are many diff erent types of stony meteorites, you can clas-
sify them into two main categories depending on their physical
properties and chemical content, chondrites and achondrites.

Large or small, meteorites are

sealed airtight and refrigerated

until they can be studied.

a

b

■ Figure 25-1

Braving bitter cold and high winds, teams of scientists riding snowmobiles search for meteorites that fell long ago

in Antarctica and are exposed as the ice evaporates. When a meteorite is found, it is photographed where it lies,

assigned a number, and placed in an airtight bag. Thousands of meteorites have been collected in this way, includ-

ing rare meteorites from the moon and Mars. (Courtesy Monika Kress)