1. Chapter 25
Section 3

2. 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.

3. 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.

4. 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.

5. 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.

6. The Terrestrial Planets

7. Mercury
What are the characteristics of Mercury?
Mercury is the smallest of the terrestrial planets, and the closest planet to the sun.

8. 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.

9. 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.

10. Temperature and Atmosphere
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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. Venus
Photographs in visible light only show Venus’s clouds.
Radar is able to penetrate Venus’s atmosphere. Radar data were used to produce this false-color image of a volcano on Venus.

17. Earth
What are the characteristics of Earth?
Earth’s surface has a suitable atmosphere and temperature range for water to exist as a liquid.

18. 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.

19. Earth
Earth is unique among the planets in having liquid-water oceans at its surface.

20. 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.

21. 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.

22. Mars
What are the characteristics Mars?
Mars shows evidence of once having a great deal of liquid surface water.

23. 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.

24. 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. 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.

26. 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.

27. Mars
Mars has ice caps at both poles.

28. Mars
Olympus Mons is the largest volcano in the solar system.

29. Mars
These Martian valleys are thought to have been formed at least in part by large flows of liquid water in the distant past.

30. 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.

31. Mars
This photo of a dark boulder on Martian sand was taken by the Mars Exploration Rover Spirit in Spirit found rocks and minerals that formed in, or were altered by, water.

32. 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.

33. 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.

34. 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.

35. 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.

36. Asteroids
The asteroid Ida was photographed by the Galileo probe on its way to Jupiter.
In 2001, the NEAR Shoemaker space probe landed on the asteroid Eros.

37. 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.

38. Assessment Questions
Which object in the solar system has conditions on its surface most like the conditions on planet Earth?
Mercury
Venus
the moon
Mars

39. Assessment Questions
Which object in the solar system has conditions on its surface most like the conditions on planet Earth?
Mercury
Venus
the moon
Mars ANS: D

40. Assessment Questions
Why do most scientists no longer accept the hypothesis that asteroids are remnants of a shattered planet?
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.

41. Assessment Questions
Why do most scientists no longer accept the hypothesis that asteroids are remnants of a shattered planet?
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