The Solar System

558 PART 4^ |^ THE SOLAR SYSTEM

Th en, you can use triangulation to fi nd the altitude, speed, and
direction of the meteor as it moves through the atmosphere,
and work backward to calculate its orbit before it entered
Earth’s atmosphere. Th ese studies confi rm that meteors belong-
ing to showers, as well as some sporadic meteors, have orbits
that are similar to the orbits of comets. In contrast, a few spo-
radic meteors, including all observed meteorite falls, have
orbits that lead back to the asteroid belt between Mars and
Jupiter. From this you can conclude that meteors have a dual
source: Many come from comets, but a few come from the
asteroid belt. Meteors that are big and durable enough to
become meteorites on the ground appear always to come from
the asteroid belt.
It is a Common Misconception that a bright meteor
disappearing behind a distant hill or line of trees probably
landed just a mile or two away. Th is has triggered hilarious
“wild goose chases” as police, fi re companies, and TV crews try
to fi nd the impact site. Almost every meteor you see vaporizes
high above Earth’s surface. Only rarely does a meteor become a
meteorite by reaching the ground, and it can land as much as
100 miles from where you are standing when you see it.

Origin of Meteoroids and Meteorites

Evidence you have already encountered suggests that many mete-
orites are fragments of parent bodies that were large enough to
grow hot from radioactive decay or other processes. Th ey then
melted and diff erentiated to form iron-nickel cores and rocky
mantles. Th e molten iron cores would have been well insulated
by the thick rocky mantles, so that the iron would have cooled
slowly enough to produce big crystals that result in Widmanstätten
patterns. Some stony meteorites that have been strongly heated
appear instead to have come from the mantles or surfaces of such
bodies. Stony-iron meteorites apparently come from boundaries
between stony mantles and iron cores. Collisions could break up
such diff erentiated bodies and produce diff erent kinds of mete-
orites (■ Figure 25-7). In contrast, many chondrites are probably
fragments of smaller bodies that never melted, and carbonaceous
chondrites may be from unaltered bodies that formed especially
far from the sun.
Th ese hypotheses trace the origin of meteorites to planetes-
imal-like parent bodies, but they leave you with a puzzle. Th e
small meteoroids now fl ying through the solar system cannot
have existed in their present form since the formation of the
solar system because they would have been swept up by the
planets in a billion years or less. Th ey could not have survived
traveling in their current orbits for the full 4.6 billion years that
the solar system has existed. Nevertheless, when the orbits of
meteorite falls are determined, those orbits lead back into the
asteroid belt. Th us, all the evidence together indicates that the
meteorites now in museums around the world were produced

■ Figure 25-7

Some of the planetesimals that formed early in the solar system’s history

may have differentiated—that is, melted and separated into layers of dif-

ferent density and composition, as did the Terrestrial planets. The fragmen-

tation of such a planetesimal could produce different types of meteorites.

(Adapted from a diagram by Clark Chapman)

Cratering

collisions

Iron

Silicates

The Origin of Meteorites

A large planetesimal

can keep its internal

heat long enough to

differentiate.

Collisions break up

the layers of different

composition.

Meteorites from

deeper in the

planetesimal were

heated to higher

temperatures.

Fragments from near

the core might have

been melted entirely.

Fragments of the

iron core would fall

to Earth as iron

meteorites.

by asteroid collisions within the last billion years. Astronomers

conclude that nearly all meteors are material from comets, but

meteorites are pieces of shattered asteroids.