Mars is the fourth planet (行星) from the Sun. Scientists are now watching it. And now three little spacecrafts (宇宙飞船) are studying it. They are taking pictures. Three more are traveling around in the sky, high above the Mars. All the six are working to find out the planet’s past. Did anything ever live on the Mars? All the living things can’t live without on the Mars? All the living things can’t live without water. Is there any water on the Mars? On