Mars has fascinated humans throughout history. It appears as a blood-red star in the sky, which led the Romans to name it after their war god. Its motions across the sky helped German astronomer Johannes Kepler (1571-1630) derive his laws of planetary motion, which dictate how celestial bodies move. Two small moons, Phobos and Deimos, were discovered orbiting Mars in 1877. But it is primarily the question of life that has driven scientists to study Mars.

Basic Physical and Orbital Properties

Mars displays a number of Earth-like properties, including a similar rotation period, seasons, polar caps, and an atmosphere. In the 1800s astronomers also noted seasonal changes in surface brightness, which they attributed to vegetation. In 1877 Italian astronomer Giovanni Schiaparelli reported the detection of thin lines crossing the planet, which he called canali, Italian for "channels." But the term was mistranslated into English as "canals," which implies waterways constructed...