1. THE INNER PLANETS The four planets that are closest to the sun are called the inner planets. They are all small and rocky. They are sometimes called the terrestrial planets.

3. The inner planets: note the spin and axis angles of each planet

4. Earth The Earth has an atmosphere that extends for about 100 kilometers. It is 20% oxygen,78% nitrogen and a few other gases. 70% of the surface is covered with water. Earth has three main layers, a crust, a mantle, and a core. The crust is the hard rocky surface. The mantle is under the crust and is made of liquid rock.

5. The Earth as seen from the moon

6. The Earth

7. The inner core is most likely solid iron and nickel.

8. MERCURY Mercury is closest to the sun. It has no moons. It is a little larger than our moon. The surface has many flat plain-like areas and craters. It has a very thin atmosphere of sodium and other gases. Since it has little atmosphere and is so close to the sun its temperatures range from 450º C to -170º C.

9. The evening sky

10. Mercury up close

11. Mercury, really close

12. VENUS Venus is about the size and in many ways the same structure as Earth. There are, however, many ways in which it differs. It takes Venus 7.5 Earth-months to revolve around the sun. It takes Venus 8 Earth-months to rotate on its axis. On Venus a day is longer than a year. Venus also rotates in a direction that is opposite of all the other planets. That is called retrograde rotation.

13. Venus’ atmosphere is very thick. Every day is cloudy. The atmosphere is 90 times greater than ours. If you were on the surface you would be crushed. The atmosphere is almost all carbon dioxide with some sulfuric acid as well. A human could not survive. The carbon dioxide traps the sun’s energy so the temperature gets up to 460ºC and stays there. That is called the greenhouse effect. Venus is mostly rock with volcanoes and lava flow.

15. Venus: shrouded with clouds composed of sulfuric acid

16. Radar can see through the clouds to the surface of Venus

17. Venus rotates “backward” compared to the other inner planets. Note other differences between the planets’ motions.

18. MARS Mars is called the red planet because it has an iron oxide surface that is red. It has a very thin atmosphere that is mostly carbon dioxide. There is some solid water at the north pole of Mars. This is covered with solid carbon dioxide in the winter. Mars is tilted on its axis so it has seasons. During the change of seasons many dust storms exist. Mars has two very small moons called Phobos and Deimos.