Seymour Simon has written more than 150 award-winning science books about animals, anatomy, astronomy, earth science, and vehicles!Named after the Roman god of war, Mars has fascinated people for thousands of years. Did you know that . . . A Martian year is 687 Earth days, almost twice as long as a year on Earth. Mars's biggest valley, Valles Marineris, is four times as deep as the Grand Canyon! Scientists think that Mars, now rocky and barren, once had an ocean, rivers, and a thicker atmosphere. The first robot ever to explore the surface of Mars is only about the size of a child's toy wagon.