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.