Mars moves through our skies in its stately dance, distant and enigmatic, a world awaiting exploration.

—Astronomer Carl Sagan, 1967

Enigmatic means mysterious. Mars has been a mystery to humans for thousands of years. Although we know much about it now, there is still much more to learn. Mars is the fourth planet from the Sun, and the planet most like Earth in the solar system. It is named after the mythical god of war who the Romans called Mars and the Greeks called Ares. Mars is also known as The Red Planet, because it looks reddish from Earth. Mars is a dusty, cold world. The average temperature is minus 64 degrees Fahrenheit. Rays of ultraviolet radiation beat down on the surface continuously. There is no oxygen to breathe, only carbon dioxide.

People on Earth have always been fascinated with the idea of life on Mars. Ancient people could see...