Page 416 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch15_377-404.indd Page 393 9/3/10 6:17 PM user-f465
tiL12214_ch15_377-404.indd Page 393 9/3/10 6:17 PM user-f465 /Users/user-f465/Desktop

A B
FIGURE 15.18 (A) A stony meteorite. The smooth, black surface was melted by compression of the atmosphere. (B) An iron meteorite that
has been cut, polished, and etched with acid. The pattern indicates that the original material cooled from a molten material over millions of years.

minerals or glass, called chondrules, held together by a fine- ets. They identify the presence of planets by measuring the very
grained cement. The achondrites do not have the chondrules, as slight wobble of a central star and then using the magnitude of
their name implies, but have a homogeneous texture more like this motion to determine the presence of orbiting planets, the
volcanic rocks such as basalt that cooled from molten rock. size and shape of their orbits, and their masses. The technique
The iron meteorites are about one-half as abundant as the works only for larger planets and cannot detect those much
stony meteorites. They consist of variable amounts of iron and smaller than about one-half the mass of Saturn. The technique
nickel, with traces of other elements. In general, there is propor- does not provide a visual image of the planets but only measures
tionally much more nickel than is found in the rocks of Earth. the gravitational effect of the planets on the star.
When iron meteorites are cut, polished, and etched, beautiful The most widely accepted theory of the origin of the solar
crystal patterns are observed on the surface. The patterns mean system is called the protoplanet nebular model. A protoplanet
that the iron was originally molten, then cooled very slowly over is the earliest stage in the formation of a planet. The model can
millions of years as the crystal patterns formed. be considered in stages, which are not really a part of the model
A meteorite is not, as is commonly believed, a ball of fire but are simply a convenient way to organize the total picture
that burns up the landscape where it lands. The iron or rock has (Figure 15.19).
been in the deep freeze of space for some time, and it travels
rapidly through Earth’s atmosphere. The outer layers become
hot enough to melt, but there is insufficient time for this heat to STAGE A
be conducted to the inside. Thus, a newly fallen iron meteorite The first important event in the formation of our solar system
will be hot since metals are good heat conductors, but it will not involves stars that disappeared billions of years ago, long before
be hot enough to start a fire. A stone meteorite is a poor conduc- the Sun was born. Earth, the other planets, and all the mem-
tor of heat, so it will be merely warm. bers of the solar system are composed of elements that were
manufactured by these former stars. In a sequence of nuclear
reactions, hydrogen fusion in the core of large stars results in
15.3 ORIGIN OF THE SOLAR SYSTEM
the formation of the elements up to iron. Elements heavier than
Any model of how the solar system originated presents a problem iron are formed in rare supernova explosions of dying massive
in testing or verification. This problem is that the solar system stars. Thus, stage A of the formation of the solar system con-
originated a long time ago, some 5 billion years ago according to sisted of the formation of elements heavier than hydrogen in
a number of different independent sources of evidence, and that many, many previously existing stars, including the supernovas
there are no other planetary systems either in existence or in the of more massive stars. Many stars had to live out their life cycles
process of being formed that can be directly observed. From the to provide the raw materials of the solar system. The death of
distance at which they occur, even the Hubble Space Telescope each star, including supernovas, added newly formed elements
would not be able to directly observe planets around their suns. to the accumulating gases and dust in interstellar space. Over a
Astronomers have identified more than 460 extra-solar plan- long period of time, these elements began to concentrate in one

15-17 CHAPTER 15 The Solar System 393