What makes Earth suitable for life? About 75 percent of Earth is covered with liquid water, which organisms use for life processes. Earth’s place in space makes it just the right temperature for the organisms that live on it. Earth’s atmosphere is rich in oxygen, a gas needed by many of its organisms. 1. Explain What makes Earth suitable for life?

Do the characteristics of the other inner planets or their moons make them suitable for life? Mercury is the closest planet to the Sun. Because it has no atmosphere, its temperature varies widely—from 430ºC to about –170ºC. There is no water on Mercury. Venus is the second planet from the Sun. Its atmosphere is poisonous and causes the planet to be very hot. There are traces of water vapor in the atmosphere of Venus, but no water has been found on its surface. (contd.)

Earth’s moon has no atmosphere Earth’s moon has no atmosphere. While there are water molecules on the moon, they exist in very small amounts. Mars is the fourth planet from the Sun. Temperatures on its surface range from –87ºC to –5ºC. Its atmosphere is mostly carbon dioxide. Mars has water ice near its poles. Mars has two very small moons. Both moons are airless and covered with craters. No water has been found on either moon. 2. Analyze Besides Earth, can any of the inner planets or their moons support life? Explain.

Do the characteristics of the outer planets or their moons make them suitable for life? Jupiter, the fifth planet from the Sun, has an average temperature of –148ºC. Its atmosphere is made of a mixture of hydrogen and helium. At least one of Jupiter’s moons might have liquid water. Saturn, the sixth planet from the Sun, is colder than Jupiter. It is surrounded by rings made mostly of water ice. Its atmosphere is mostly hydrogen and helium. (contd.)

Uranus, the seventh planet from the Sun, has an average temperature of –216ºC. Its hydrogen-helium atmosphere contains some methane and traces of water and ammonia. Some of its moons might be made of water ice. Neptune, the eighth planet, has a thick atmosphere made of hydrogen, helium, and methane. Neptune’s atmosphere retains some heat, making it slightly warmer than Uranus. Some water exists near the bottom of Neptune’s atmosphere. 3. Analyze Can any of the outer planets or their moons support life? Explain.