1. THE INNER PLANETS

2. The four inner planets are small and dense and have rocky surfaces
The four inner planets are small and dense and have rocky surfaces. The inner planets are often called “terrestrial planets,” from the Latin word “terra” which means “Earth.”

3. Mercury
Mercury is the smallest terrestrial planet and the planet closest to the sun. It is not much larger than Earth’s moon and has no moons of its own. The inside of Mercury is most likely made up of the dense metal iron.

4. Mercury has many flat plains and craters on its surface
Mercury has many flat plains and craters on its surface. Mercury also has virtually no atmosphere due to its high daytime temperatures. The side of Mercury that faces the sun can reach temperatures of 430 degrees Celsius. However, because Mercury has no atmosphere, at night its heat escapes into space causing temperatures to drop to -170 degrees Celsius.

5. Venus
Venus is similar in mass and size to Earth that it is known as “Earth’s twin.” Although Venus and Earth have similar internal structures and densities, they are very different in other ways.

6. Venus’s atmosphere is so thick that it is always cloudy there
Venus’s atmosphere is so thick that it is always cloudy there. These clouds are mostly made of sulfuric acid. The pressure of Venus’s atmosphere is 90 times greater than Earth’s, which means you would be crushed by it if you were to stand on Venus’s surface. You could not breathe on Venus because its atmosphere is mostly carbon dioxide (which is what WE breathe out!).

7. At 460 degrees Celsius, Venus has a surface temperature hot enough to melt lead. Its surface has many volcanoes and broad plains formed by lava flows.

8. Earth
Earth has 3 main layers: the crust, mantle, and core. The crust includes the solid, rocky surface. Under the crust is the mantle, a layer of hot molten rock. Earth has a dense core made of mainly iron and nickel. The outer core is liquid, but the inner core is solid.

9. Earth is unique in our solar system in having liquid water at its surface. About 70 percent of Earth’s surface is water.

10. Earth has enough gravity to hold on to most gases
Earth has enough gravity to hold on to most gases. These gases make up Earth’s atmosphere, which extends more than 100 kilometers above its surface. Although some other plates also have atmospheres, Earth is the only one that is rich in oxygen. The rest of its atmosphere is mostly nitrogen.

11. Mars
Mars is called the “red planet” because it has a reddish color in the sky. The red color is caused by the breakdown of iron-rich rocks, which causes a rusty dust that covers Mars’s surface.

12. Mars’s atmosphere is more than 95 percent carbon dioxide (similar to Venus’s atmosphere, but much thinner), and contains few clouds. Because Mars has a transparent atmosphere, its surface is easily viewed from Earth through a telescope. Mars has temperatures that range from -140 degrees Celsius to 20 degrees Celsius.

13. Scientists think that a large amount of liquid water flowed on Mars’s surface in the distant past. For this to be true, Mars must have been much warmer and had a thicker atmosphere at that time. Today, liquid water cannot exist on Mars for very long. Since Mars’s atmosphere is so thin, liquid water would quickly turn into gas. Much of the water on Mars exists in its polar ice caps.

15. The surface of Mars is barren and covered with craters like the moon
The surface of Mars is barren and covered with craters like the moon. Some regions of Mars have giant volcanoes. Astronomers see signs that lava flowed from the volcanoes in the past, but the volcanoes are no longer active.