25.3 The Inner Solar System
This imaginative
painting shows a
possible future
human settlement
on Mars. Settlers
might live in
domes to protect
themselves from
the harsh climate
and to provide an
atmosphere for
breathing.
25.3 The Inner Solar System
The Terrestrial Planets
How are the inner planets similar?
The four inner planets are all relatively small
and dense, and have rocky surfaces. Like
Earth, they all have a crust, mantle, and iron
core.
25.3 The Inner Solar System
The Terrestrial Planets
The terrestrial planets are planets similar in
structure to Earth: Mercury, Venus, Earth,
and Mars
• All of the terrestrial planets have rocky crusts.
• Mercury and Mars have surfaces that are
pockmarked by craters. Most craters on Earth
and Venus have disappeared because the
surface is continually being altered.
25.3 The Inner Solar System
The Terrestrial Planets
The terrestrial planets are much warmer on
average than the outer planets.
The inner planets have few (if any) moons
and no rings.
25.3 The Inner Solar System
The Terrestrial Planets
25.3 The Inner Solar System
Mercury
What are the characteristics of Mercury?
Mercury is the smallest of the terrestrial
planets, and the closest planet to the sun.
25.3 The Inner Solar System
Mercury
Mercury is a dense planet with a very large
iron core.
There is no mantle convection within the
planet, and little erosion on its surface.
It takes only 88 Earth days for Mercury to
complete one revolution. This is the shortest
period of revolution of any planet.
25.3 The Inner Solar System
Mercury
Mercury’s surface is
heavily cratered from
meteoroid collisions.
This false-color image
was made by combining
a series of smaller
images taken by the
Mariner 10 space
probe. Orange areas for
which no images are
available are blank.
25.3 The Inner Solar System
Mercury
Temperature and Atmosphere
Mercury’s surface temperature ranges between
two extremes.
• Day temperatures at the equator reach about 430°C.
• At night, the temperature drops as low as –170°C.
The time between sunrise and sunset equals the
time it takes for Mercury to revolve around the
sun, about 88 days.
Mercury has virtually no atmosphere.
25.3 The Inner Solar System
Mercury
Exploring Mercury
Because Mercury is so close to the sun, it’s hard
for astronomers on Earth to see. In 1974–1975,
Mariner 10 took many images of Mercury’s
surface.
The surface looks like that of Earth’s moon.
Mercury’s surface has been largely unchanged for
billions of years.
The Messenger space probe will orbit Mercury in
2011.
25.3 The Inner Solar System
Venus
What are the characteristics of Venus?
Venus’s thick atmosphere is composed
mostly of carbon dioxide, which traps heat
and raises the planet’s temperature.
25.3 The Inner Solar System
Venus
Venus is called the “evening star” or “morning
star” because it is only seen in the west after
sunset or in the east before sunrise.
Except for the moon, Venus is the brightest
object in Earth’s night sky.
Venus rotates slowly in the direction opposite
to which it revolves, and its day is longer than
its year.
25.3 The Inner Solar System
Venus
Venus’s Atmosphere
Venus is difficult to study from space because
clouds hide its surface.
Venus’s atmosphere is so thick that the pressure
at its surface is 90 times that on Earth.
Spacecraft that landed on Venus’s surface in the
1970s found the average surface temperature to
be 460°C.
25.3 The Inner Solar System
Venus
Exploring Venus
The high sulfur content of Venus’s atmosphere
probably results from numerous volcanoes.
The outpouring of lava from volcanoes has
frequently resurfaced the planet, erasing craters
as well.
Though Venus probably has active volcanoes, it
shows no evidence of plate tectonics.
25.3 The Inner Solar System
Venus
A. Photographs in visible
light only show Venus’s
clouds.
B. Radar is able to
penetrate Venus’s
atmosphere. Radar data
were used to produce
this false-color image of
a volcano on Venus.
25.3 The Inner Solar System
Earth
What are the characteristics of Earth?
Earth’s surface has a suitable atmosphere
and temperature range for water to exist as a
liquid.
25.3 The Inner Solar System
Earth
Earth is unique in several important respects.
• The presence of liquid water makes it possible
for Earth to support millions of different species
of living things.
• Water also causes erosion, which has shaped
Earth’s land surface in many ways.
25.3 The Inner Solar System
Earth
Earth is unique
among the planets
in having liquidwater oceans at
its surface.
25.3 The Inner Solar System
Earth
Unlike most smaller planets and moons, Earth
has enough gravity to hold most gas
molecules.
This allows Earth to maintain a thick
atmosphere.
Earth’s atmosphere is not composed mostly
of carbon dioxide, like the atmospheres of
Venus and Mars, because some living
organisms use carbon dioxide and produce
oxygen.
25.3 The Inner Solar System
Earth
The moon, Mercury, and Mars are all so small
that over time they have lost much of their
internal heat and have become geologically
dead.
Earth is large enough that it has not had a
chance to cool down much, so Earth has
moving tectonic plates that continually change
its surface.
25.3 The Inner Solar System
Mars
What are the characteristics Mars?
Mars shows evidence of once having a great
deal of liquid surface water.
25.3 The Inner Solar System
Mars
Mars is the most Earthlike of all of the planets.
• The weathering of iron-rich rocks on its surface
gives the planet a reddish color.
• Though the surface of Mars is very old, as
shown by the presence of many impact craters,
it once had many volcanic eruptions.
25.3 The Inner Solar System
Mars
Mars’s Atmosphere
Mars has a very thin atmosphere, which allows
astronomers to see its surface with a telescope.
• The thin Martian atmosphere and its distance from
the sun make the surface very cold. Temperatures
vary from –140°C to 20°C.
• Mars’s atmosphere is more than 95% carbon dioxide,
similar to the atmosphere of Venus, but much thinner.
25.3 The Inner Solar System
Mars
Water on Mars
There is no liquid water on Mars’s surface now,
but there likely was some in the past. Some
geologists think Mars’s northern lowlands were
once covered by an ocean.
Some water vapor has escaped into space. Some
water is stored in ice caps at the poles, and some
is frozen underground just below the surface.
25.3 The Inner Solar System
Mars
Because Mars had liquid water on its surface
in the past, many scientists wonder if life once
existed there.
Robotic probes have carried out experiments
on the Martian surface to look for signs of life.
None of these probes found clear evidence
for the existence of life on Mars.
25.3 The Inner Solar System
Mars
A.
Mars has ice caps at both poles.
25.3 The Inner Solar System
Mars
B.
Olympus Mons is the largest volcano in the solar system.
25.3 The Inner Solar System
Mars
C.
These Martian valleys are thought to have been formed at
least in part by large flows of liquid water in the distant past.
25.3 The Inner Solar System
Mars
Martian Seasons
Like Earth, Mars has distinct seasons. Mars’s
seasons are caused by the tilt of its axis.
Over the course of a Martian year, Mars’s ice caps
grow and shrink.
The change in seasons also causes large dust
storms that occasionally rage across the surface.
25.3 The Inner Solar System
Mars
This photo of a
dark boulder on
Martian sand was
taken by the Mars
Exploration Rover
Spirit in 2006.
Spirit found rocks
and minerals that
formed in, or were
altered by, water.
25.3 The Inner Solar System
Asteroids
What are asteroids and how were they
formed?
Scientists now hypothesize that asteroids
are remnants of the early solar system that
never came together to form a planet.
25.3 The Inner Solar System
Asteroids
Beyond Mars is a region of the solar system
where small, rocky bodies called asteroids
can be found orbiting the sun.
This region is referred to as the asteroid belt.
Although most asteroids are found in the
asteroid belt, others orbit near Jupiter or
range through the inner solar system.
25.3 The Inner Solar System
Asteroids
Astronomers have discovered more than
10,000 asteroids, and many more are thought
to exist.
Most asteroids are less than 1 kilometer in
diameter; only three are larger than 500
kilometers across.
25.3 The Inner Solar System
Asteroids
The largest asteroids have enough gravity to
have pulled themselves into nearly spherical
or oval shapes while forming.
Smaller asteroids, however, can have very
irregular forms.
Scientists think that some asteroids are like
floating rubble heaps, held together by weak
gravity.
25.3 The Inner Solar System
Asteroids
A. The asteroid Ida was photographed by the
Galileo probe on its way to Jupiter.
B. In 2001, the NEAR Shoemaker space
probe landed on the asteroid Eros.
25.3 The Inner Solar System
Asteroids
In the past, scientists thought that asteroids
were the remnants of a shattered planet.
Scientists now hypothesize that asteroids are
remnants of the early solar system that never
came together to form a planet.
This hypothesis is supported by the lack of
mass in the asteroid belt. All the asteroids
together would form a planetary body only
about 1500 kilometers in diameter.
25.3 The Inner Solar System
Assessment Questions
1. Which object in the solar system has conditions
on its surface most like the conditions on planet
Earth?
a.
b.
c.
d.
Mercury
Venus
the moon
Mars
25.3 The Inner Solar System
Assessment Questions
1. Which object in the solar system has conditions
on its surface most like the conditions on planet
Earth?
a.
b.
c.
d.
Mercury
Venus
the moon
Mars
ANS: D
25.3 The Inner Solar System
Assessment Questions
2. Why do most scientists no longer accept the
hypothesis that asteroids are remnants of a
shattered planet?
a.
b.
c.
d.
Their total mass is much less than the mass of a planet.
They are not as spherical as are all the planets.
Unlike planets, asteroids are made primarily of ice.
There is no known cause for a planet to shatter.
25.3 The Inner Solar System
Assessment Questions
2. Why do most scientists no longer accept the
hypothesis that asteroids are remnants of a
shattered planet?
a.
b.
c.
d.
Their total mass is much less than the mass of a planet.
They are not as spherical as are all the planets.
Unlike planets, asteroids are made primarily of ice.
There is no known cause for a planet to shatter.
ANS: A