CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 405
Technically, the word meteor refers to the streak of light in
the sky. In space, before its fi ery plunge, the object is called a
meteoroid, and any part of it that survives its fi ery passage to
Earth’s surface is called a meteorite. Most meteoroids are specks
of dust, grains of sand, or tiny pebbles. Almost all the meteors
you see in the sky are produced by meteoroids that weigh less
than 1 gram. Only rarely is a meteoroid massive and strong
enough to survive its plunge and reach Earth’s surface.
Th ousands of meteorites have been found, and you will learn
more about their various types in Chapter 25. Meteorites are
mentioned here for one specifi c clue they can give you concern-
ing the solar nebula: Meteorites can reveal the age of the solar
system.
The Age of the Solar System
According to the solar nebula theory, the planets should be about
the same age as the sun. Th e most accurate way to fi nd the age
of a rocky body is to bring a sample into the laboratory and ana-
lyze the radioactive elements it contains.
When a rock solidifi es, it incorporates known percentages of
the chemical elements. A few of these elements have forms, called
isotopes (see Chapter 7), that are radioactive, meaning they
gradually decay into other isotopes. For example, potassium-40,
called a parent isotope, decays into calcium-40 and argon-40,
called daughter isotopes. Th e half-life of a radioactive substance
is the time it takes for half of the parent isotope atoms to decay
into daughter isotope atoms. Th e abundance of a radioactive
substance gradually decreases as it decays, and the abundances of
the daughter substances gradually increase (■ Figure 19-7). Th e
half-life of potassium-40 is 1.3 billion years. If you also have
information about the abundances of the elements in the original
rock, you can measure the present abundances and fi nd the age
of the rock. For example, if you study a rock and fi nd that only
50 percent of the potassium-40 remains and the rest has become
a mixture of daughter isotopes, you could conclude that one half-
life must have passed and that the rock is 1.3 billion years old.
Potassium isn’t the only radioactive element used in radioac-
tive dating. Uranium-238 decays with a half-life of 4.5 billionComet’s orbit■ Figure 19-5
Comets orbit the sun in long, elliptical orbits and become visible when the
sun’s heat vaporizes its ices and pushes the gas and dust into a tail point-
ing away from the sun. (a) A comet may remain visible in the evening or
morning sky for weeks as it moves through the inner solar system. Although
comets are moving rapidly along their orbits, they are so distant that, on
any particular evening, a comet seems to hang motionless in the sky. Comet
Hyakutake is shown here near Polaris in 1996. (Mike Terenzoni) (b) A comet in
a long, elliptical orbit becomes visible when the sun’s heat vaporizes its ices
and pushes the gas and dust away in a tail. (Celestron International)