( figure 15.18 The planets drawn to scale. )diameter of Neptune (the smallest Jovian planet) is three times larger than the diameter of Earth or Venus. Further, Neptune's mass is 17 times greater than that of Earth or Venus (figure 15.18). Other properties that differ include densities, chemical compositions, orbital periods, and numbers of satellites. Variations in the chemical composition of planets are largely responsible for their density differences. Specifically, the average density of the terrestrial planets is about five times the density of water, whereas the average density of the Jovian planets is only 1.5 times that of water. Saturn has a density only 0.7 times that of water, which means that it would float if placed in a large enough tank of water. The outer planets are also characterized by long orbital periods and numerous satellites. Internal Structures Shortly after Earth formed, the segregation of material resulted in the formation of three major layers defined by their chemical composition—the crust, mantle, and core. This type of chemical separation occurred in the other planets as well. However, because the terrestrial planets are compositionally different than the Jovian planets, the nature of these layers differs between these two groups (figure is. i 9). The terrestrial planets are dense, having relatively large cores of iron and iron compounds. From their centers outward, the amount of metallic iron decreases while the amount of rocky silicate minerals increase. The outer cores of Earth and Mercury are liquid, whereas the cores of Venus and Mars are thought to be partially molten. This difference is attributable to Venus and Mars having lower internal temperatures than those of Earth and Mercury. Silicate minerals and other lighter compounds make up the mantles of the terrestrial planets. Finally, the silicate crusts of terrestrial planets are relatively thin compared to their mantles. The two largest Jovian planets, Jupiter and Saturn, have small metallic inner cores consisting of iron compounds at extremely high temperatures and pressures. The outer cores of these two giants are thought to be liquid metallic hydrogen, whereas the mantles are comprised of liquid hydrogen and helium. The outermost layers are gases and ices of hydrogen, helium, water, ammonia, and methane—which account for the low densities of these planets. Uranus and Neptune also have small metallic cores but their mantles are likely hot dense water and ammonia. Above their mantles, the amount of hydrogen and helium increases, but exists in much lower concentrations than those of Jupiter and Saturn. The Atmospheres of the Planets The Jovian planets have very thick atmospheres composed mainly of hydrogen and helium, with lesser amounts of water, methane, ammonia, and other hydrocarbons. The Jovian atmospheres are so thick that they do not show a clear boundary between "atmosphere" and "planet." By contrast, the terrestrial planets, including Earth, h ...

1. (
figure 15.18
The planets drawn to scale.
)diameter of Neptune (the smallest Jovian planet) is three times
larger than the diameter of Earth or Venus. Further, Neptune's
mass is 17 times greater than that of Earth or Venus (figure
15.18).
Other properties that differ include densities, chemical
compositions, orbital periods, and numbers of satellites.
Variations in the chemical composition of planets are largely
responsible for their density differences. Specifically, the
average density of the terrestrial planets is about five times the
density of water, whereas the average density of the Jovian
planets is only 1.5 times that of water. Saturn has a density only
0.7 times that of water, which means that it would float if
placed in a large enough tank of water. The outer planets are
also characterized by long orbital periods and numerous
satellites.
Internal Structures
Shortly after Earth formed, the segregation of material resulted
in the formation of three major layers defined by their chemical
composition—the crust, mantle, and core. This type of chemical
separation occurred in the other planets as well. However,
because the terrestrial planets are compositionally different than
the Jovian planets, the nature of these layers differs between
these two groups (figure is. i 9).
The terrestrial planets are dense, having relatively large cores
of iron and iron compounds. From their centers outward, the
amount of metallic iron decreases while the amount of rocky
silicate minerals increase. The outer cores of Earth and Mercury
are liquid, whereas the cores of Venus and Mars are thought to
be partially molten. This difference is attributable to Venus and
Mars having lower internal temperatures than those of Earth and
Mercury. Silicate minerals and other lighter compounds make

2. up the mantles of the terrestrial planets. Finally, the silicate
crusts of terrestrial planets are relatively thin compared to their
mantles.
The two largest Jovian planets, Jupiter and Saturn, have small
metallic inner cores consisting of iron compounds at extremely
high temperatures and pressures. The outer cores of these two
giants are thought to be liquid metallic hydrogen, whereas the
mantles are comprised of liquid hydrogen and helium. The
outermost layers are gases and ices of hydrogen, helium, water,
ammonia, and methane—which account for the low densities of
these planets. Uranus and Neptune also have small metallic
cores but their mantles are likely hot dense water and ammonia.
Above their mantles, the amount of hydrogen and helium
increases, but exists in much lower concentrations than those of
Jupiter and Saturn.
The Atmospheres of the Planets
The Jovian planets have very thick atmospheres composed
mainly of hydrogen and helium, with lesser amounts of water,
methane, ammonia, and other hydrocarbons. The Jovian
atmospheres are so thick that they do not show a clear boundary
between "atmosphere" and "planet." By contrast, the terrestrial
planets, including Earth, have relatively meager atmospheres
composed of carbon dioxide, nitrogen, and oxygen. .
Two factors explain these significant differences—solar heating
(temperature) and gravity (figure 15.20). These variables
determine what planetary gases, if any, were captured by
planets during the formation of the solar system and which were
ultimately retained.
During planetary formation, the inner regions of the developing
solar system were too hot for ices and gases to condense. In
contrast, the Jovian planets formed where temperatures were
low and solar heating of planetesimals was minimal. This
allowed water vapor, ammonia, and methane
4311
Our Solar System: An Overview

3. (
432
CHAPTER 15
The Nature of the Solar System
figure 15.19
Comparison of the internal structures of the planets.
) (
Uranus Neptune
Key
Visible clouds Gaseous hydrogen Ices (water/methane) Rocky
core
) (
Key Visible clouds Gaseous hydrogen Liquid hydrogen Metallic
hydrogen Rocky core
Saturn
)
Jupiter
431
Our Solar System: An Overview
(
Gravity
figure 15.20
The factors that explain why some bodies have thick atmos
pheres, whereas others are airless include solar heating
(temperature) and gravity. Airless worlds are comparatively
warm and have weak gravity, whereas bodies with significant
atmospheres have weak heating and strong gravity.

4. )to condense into ices. Hence, the gas giants contain large
amounts of these volatiles. As the planets grew, the largest
Jovian planets, Jupiter and Saturn, also attracted large
quantities of the lightest gases, hydrogen and helium.
How did Earth acquire water and other volatile gases? It seems
that early in the history of the solar system, gravitational tugs
by the developing protoplanets sent planetesi-mals into very
eccentric orbits. As a result, Earth was bombarded with icy
objects that originated beyond the orbit of Mars. This was a
fortuitous event for organisms that currently inhabit our planet.
Mercury, our Moon, and numerous other small bodies lack
significant atmospheres even though they certainly would have
been bombarded by icy bodies early in their development.
Airless bodies develop where solar heating exceeds a certain
level, which depends on the strength of the body's gravity
(Figure 15.20). Simply stated, less massive planets have a better
chance of losing their atmosphere because gas molecules need
less speed to escape their weak gravities. Comparatively warm
bodies with small surface gravity, such as our Moon, are unable
to hold even heavy gases such as carbon dioxide and nitrogen.
Mercury holds trace amounts of gas.
The slightly larger terrestrial planets Earth, Venus, and Mars
retain some heavy gases including water vapor, nitrogen, and
carbon dioxide. However, their atmospheres are miniscule
compared to their total mass. Early in their development, the
terrestrial planets probably had much thicker atmospheres. Over
time, however, these primitive atmospheres
433Our Solar System: An Overview
(
High-speed
) (
FIGURE
15.21
Formation of an impact

5. crater.The
energy of the rapidly mov
ing meteoroid is transformed into heat energy and
compressional
waves.The
re
bound of the compressed rock causes debris to be ejected from
the crater. Heat melts some material, producing glass beads.
Small secondary craters are formed by the material "splashed"
from the impact crater. (After
e. m.
Shoemaker)
)gradually changed as certain gases trickled away into space.
For example, Earth's atmosphere continues to leak hydrogen and
helium (the two lightest gases) into space. This phenomenon
occurs near the top of Earth's atmosphere where air is so
tenuous that nothing stops the fastest moving ions from flying
off into space. The speed required to escape a planet's gravity is
called escape velocity. Because hydrogen
is the lightest gas, it most easily reaches the speed needed to
o vercome Earth's gra vity.
*
Planetary Impacts
Planetary impacts have occurred throughout the history of the
solar system. On bodies that have little or no atmosphere, such
as the Moon and Mercury, even the smallest pieces of
interplanetary debris (meteorites) can produce microscopic
cavities on individual mineral grains. By contrast, large impact
craters are the result of collisions with massive bodies, such as
asteroids and comets.
Planetary impacts were considerably more common in the early
history of the solar system than they are today, with the
heaviest bombardment occurring 3.8 to 4.1 billion years ago.
Following that period, the rate of cratering diminished
dramatically and now remains essentially constant. Because

6. weathering and erosion are almost nonexistent on the Moon and
Mercury, their cratered past is clearly evident.
On larger bodies, thick atmospheres may cause the impacting
objects to break up and /or decelerate. For example, Earth's
atmosphere causes meteoroids with masses of less than 10
kilograms (22 pounds) to lose up to 90 percent of their speed as
they penetrate the atmosphere. Therefore, impacts of low-mass
bodies produce only small craters on Earth. Earth's atmosphere
is much less effective in slowing large bodies—fortunately, they
make very rare appearances.
The formation of a large impact crater is illustrated in figure
15.21. The meteoroid's high-speed impact compresses the
material it strikes, causing an almost instantaneous rebound that
ejects material from the surface. Craters excavated by objects
that are several kilometers across often exhibit a central peak,
such as the one in the large crater in figure 15.22. Much of the
material expelled, called ejecta, lands in or near the crater,
where it accumulates to form a rim. Large meteoroids may
generate sufficient heat to melt
DID YOU KNOW?
Although it was long suspected, it was not until recently that
the presence of extrasolar planets has been verified.
Astronomers have found these bodies by measuring the telltale
wobbles of nearby stars.The first apparent planet outside the
solar system was discovered in 1995, orbiting the star 51
Pegasi, 42 light-years from Earth. Since that time, numerous
Jupiter-size bodies have been identified, nost of them
surprisingly close to the stars they orbit some of the impacted
rock. Samples of glass beads produced in this manner, as well
as rocks consisting of broken fragments welded together by the
heat of impacts, have been collected from the Moon, allowing
planetary geologists to learn about such events.
433
Our Solar System: An Overview

7. 462 CHAPTER 16 Beyond Our Solar System
RUNG
433
Our Solar System: An Overview
(
March 10, 1935
) (
figure
16.6 This false-color image from three of NASA's Great
Observatories provides one example of a star that died in a fiery
supernova blast Called Cassiopeia A, this supernova remnant is
located
10,000
light-years away in the constellation Cassiopeia. At the center,
visible only as a tiny turquoise dot, is the leftover corpse of the
now-dead star, called a neutron star. (NASA)
) (
May 6, 1935
figure
16
.5
Photographs of Nova
Herculis
(a nova in the constellation Hercules), taken about two months
apart, showing the decrease in brightness. (Courtesy of Lick
Observatory)
)
Early in the twentieth century, Einar Hertzsprung and Henry
Russell independently studied the relationship between the true
brightness (absolute magnitude) of stars and their temperatures.
From this research each developed a graph, now called a

8. Hertzsprung-Russell diagram (H-R diagram), that displays these
intrinsic stellar properties. By studying H-R diagrams, we can
learn a great deal about the relationships among the sizes,
colors, and temperatures of stars.
To produce an H-R diagram, astronomers survey a portion of
the sky and plot each star according to its luminosity
(brightness) and temperature (figure 16./). Notice that the stars
in Figure 16.7 are not uniformly distributed. Rather, about 90
percent of all stars fall along a band that runs from the upper-
left corner to the lower-right corner of the H-R diagram. These
"ordinary" stars are called main-sequence stars. As shown in
Figure 16.7, the hottest main-sequence stars are intrinsically the
brightest, and the coolest are intrinsically the dimmest.
The luminosity of the main-sequence stars is also related to
their mass. The hottest (blue) stars are about 50 times more
massive than the Sun, whereas the coolest (red) stars are only
1/10 as massive. Therefore, on the H-R diagram, the main-
sequence stars appear in decreasing order, from hotter, more
massive blue stars to cooler, less massive red stars.
Note the location of the Sun in Figure 16.7. The Sun is a yellow
main-sequence star with an absolute magnitude of about 5.
Because the magnitude of a vast majority of main-sequence
stars lie between -5 and 15, and because the Sun falls midway in
this range, the Sun is often considered an average star.
Just as all humans do not fall into the normal size range, some
stars are clearly different than main-sequence stars. Above and
to the right of the main sequence in the H-R diagram (Figure
16.7) lies a group of very luminous stars called giants, or, on
the basis of their color, red giants. The size of these giants can
be estimated by comparing them with stars of known size that
have the same surface.temperature. We know that objects
having equal surface temperatures radiate the same amount of
energy per unit area. Therefore, any difference in the brightness
of two stars having the same surface temperature is attributable
to their relative sizes.
For example, a red main-sequence star and another red star that

9. is 100 times more luminous radiate the same amount of energy
per unit area. Therefore, in order for the
433
Our Solar System: An Overview
(
3C
) (
Idealized
Hertzsprung
-Russell diagram on which stars are plotted according to
temperature and absolute magnitude.
) (
Hertzsprung
-Russell Diagram
463
)
more luminous star to be 100 times brighter than the less
luminous star, it must have 100 times more surface area. Stars
with large radiating surfaces appear in the upper-right position
of an H-R diagram and are appropriately called giants.
Some stars are so large that they are called supergiants.
Betelgeuse, a bright red supergiant in the constellation Orion,
has a radius about 800 times that of the Sun. If this star were at
the center of our solar system, it would extend beyond the orbit
of Mars, and Earth would find itself inside the star! Other red
giants that are easy to locate are Arcturus in the constellation
Bootes and Antares in Scorpius.
In the lower-left portion of the H-R diagram, the opposite
situation arises. These stars are much fainter than main-

10. sequence stars of the same temperature, and by using the same
reasoning, they must be much smaller. Some probably
approximate Earth in size. This group has come to be called
white dwarfs.
Soon after the first H-R diagrams were developed, astronomers
realized their importance in interpreting stellar evolution. Just
as with living things, a star is born, ages, and dies. Owing to the
fact that almost 90 percent of the stars lie on the main sequence,
we can be relatively certain that stars spend most of their active
years as main-sequence stars.
464 CHAPTER 16 Beyond Our Solar System
DID YOU KNOW?
In a few billion years, the Sun will exhaust the remaining
hydrogen fuel in its core, an event that will trigger hydrogen
fusion in the surrounding shell. As a result, the Sun's outer
envelope will expand, producing a red giant that is hundreds of
times larger and more luminous. The intense solar radiation will
cause Earth's oceans to boil, and the solar winds will drive away
Earth's atmosphere. In another billion*years, the Sun will expel
its outermost layer, producing a spectacular planetary nebula,
while its interior will collapse to form a dense, small (planet-
size), white dwarf. Because of its small size, the Sun's energy
output will be less than I percent of its current level. Gradually,
the Sun will emit its remaining thermal energy, eventually
becoming a cold, nonluminous body.
Only a few percent are giants, and perhaps 10 percent are white
dwarfs. After a brief discussion of interstellar matter, we will
come back to stellar evolution and the life cycle of stars.
INTERSTELLAR MATTER
Lying between the stars is "the vacuum of space." However, it

11. is far from a perfect vacuum, for it is populated with
accumulations of dust and gases. The name applied to these
concentrations of interstellar matter is nebula (nebula = cloud).
If this interstellar matter is close to very hot (blue) stars, it will
glow and is called a bright nebula (figure 16.8). The two main
types of bright nebulae are known as emission nebulae and
reflection nebulae.
Emission nebulae are gaseous masses that consist largely of
hydrogen. They absorb ultraviolet radiation emitted by
embedded or nearby hot stars. Because these gases are under
very low pressure, they reradiate, or emit, this energy as visible
light. This conversion of ultraviolet light to visible light is
known as fluorescence, an effect you observe daily in
fluorescent lights.
Reflection nebulae, as the name implies, merely reflect the light
of nearby stars (figure 16.9). Reflection nebulae are likely
composed of relatively dense clouds of large particles called
interstellar dust. This view is supported by the fact that atomic
gases with low densities could not reflect light sufficiently to
produce the glow observed.
figure 16.8 TheTrifid Nebula, in the constellation

12. Sagittarius.This colorful nebula is a cloud consisting mostly of
hydrogen and helium gases.These gases are excited by the
radiation of the hot, young stars within and produce a reddish
glow. (Courtesy of National Optical Astronomy Observatories)