In some places, Earth and Mars look so much alike it's hard to tell which is which. See if you can tell which planet is shown in each of these photos. It's not completely surprising that these planets can look similar. Mars is a close planetary neighbor. Both Earth and Mars are at about the right distance from the Sun to avoid the staggering extremes of hot and cold found on other worlds. Both planets are also rocky and relatively small (unlike more distant planets, which are big and mostly made of gas). Like Earth, Mars has mountains, plains, and canyons. But even though Mars resembles Earth in some ways, it's still very different. Mars is only about half the size of Earth, with about a third of the gravity. And it has less than 1% of Earth's atmosphere - so little that liquid water can't exist on the surface. That's why scientists are looking for signs of life on Mars below its surface.