The earth is the only planet that scientists are certain has life .What does the earth have that the other planets don't ? For one thing , the earth has just the right temperature . As _ from the sun , the earth seems to be just the right distance away . The planets that are closer to the sun are so hot that their surfaces bake in the sun . The farthest planets are cold balls . When the earth developed-- which scientists believe may have happened about billions of years ago , many gases covered the earth . The gases caused the earth to be hot . But something wonderful happened . The temperature was just right for thick clouds to form . It rained very hard for a very long time . This gave the earth its oceans . Water made it possible for plants to grow . The plants created oxygen in the atmosphere . Oxygen is the gas that humans and animals breathe . Only one other planet in the solar system seems to be something like the earth . That planet is Mars . Mars is smaller than the earth , and it is quite a bit cooler . But it is not too cold for humans . On some days , the temperatures are as cold as a winter day in the northern United States . If you wore a special spacesuit , you could walk around on Mars . You would have to bring your own air to breathe , though . The air on Mars is too thin to breathe . Mars has the largest volcano in the solar system . It is sixteen miles high . The highest volcano on the earth is five miles high . The most unexpected sight on Mars is dried-up river beds . Scientists believe that Mars was once much wetter that it is now . Does this mean there could have been living things on Mars ? Scientists are not sure , but there has been no sign so far . Which of the following is TRUE ?
The earth has a history of about 4 million years . There is no air on Mars . The largest volcano on Mars is over three times higher than the highest one on the earth Scientists believe there are living things on Mars .