1. The Inner Planets 1. Mercury 2. Venus 3. Earth 4. Mars The inner planets are relatively small and rocky. They are known as “terrestrial” planets.

2. Mercury is the planet closest to the Sun It is not much larger than our moon, and it has no moons of its own.

3. The surface of Mercury is covered with craters. Its interior is composed of nickel and iron. It has a thin atmosphere and, because of this, experiences great temperature extremes. (430 degrees Celsius in the day, -170 degrees Celsius at night!)

4. Exploration of Mercury was conducted by the space probe Mariner 10 in 1974.

5. Venus is the second planet from the sun. It is similar in size to Earth.

6. The atmosphere of Venus is very thick. Because of this, the Sun’s heat gets trapped and causes the planet to heat up. The temperature there has gotten hot enough to melt lead! This is called the greenhouse effect when the atmosphere traps heat.

7. Venus rotates on its axis in the opposite direction from the other planets. It rotates from east to west. This is known as retrograde motion. Venus has no moons.

8. Exploration of Venus is difficult. For example, one spacecraft landed on its surface only to burn up 23 minutes later! However, the Magellan probe in 1990 was able to use radar to generate computer images of Venus.

9. Earth is the third planet from the Sun. 70% of Earth is water.

10. Earth has an atmosphere high in oxygen.

11. Mars is the fourth planet from the Sun. It is about half the size of the Earth. The atmosphere on Mars is mostly carbon dioxide. Mars has two very small moons.

12. Mars has seasons, just like Earth. It is thought that there is frozen water under the surface of Mars.

13. The remote controlled explorer Sojourner explored the surface of Mars in 1997.