Mars is the 4th planet from the Sun, and one of the 5 planets visible with the unaided eye. When Mars is closest to the Earth, it becomes one of the brightest stars in the sky, appearing bright and red. Because of this bright red color, ancient peoples associated Mars with blood and war. The ancient Romans named Mars after their god of War, the same persona as the Greek god Ares. The symbol for Mars also represents the god of War.

Mars orbits the Sun at an average distance of 228 million km, which is about 1.5 times the distance of the Earth to the Sun. Earth and Mars can get as close as 54.6 million km apart, making Earth the closest planet to Mars.

The size of Mars makes it the third largest terrestrial planet, after Earth and Venus. The diameter of Mars is 6,792 km across. This is about half the diameter of Earth. The mass of Mars is even lower; the planet has a mass of 6.42 x 1023 kg, which gives it a mass of only 10% compared to Earth. This lower mass gives Mars a much lower gravity. If you could stand on the surface of Mars, you would experience about 38% the force of gravity that you experience on Earth. The volume of Mars is only 15% the volume of Earth.

We know that Mars is a small rocky body once thought to be very Earth-like. Like the other "terrestrial" planets - Mercury, Venus, and Earth - its surface has been changed by volcanism, impacts from other bodies, movements of its crust, and atmospheric effects such as dust storms. It has polar ice caps that grow and recede with the change of seasons; areas of layered soils near the Martian poles suggest that the planet's climate has changed more than once, perhaps caused by a regular change in the planet's orbit. Martian tectonism - the formation and change of a planet's crust - differs from Earth's. Where Earth tectonics involve sliding plates that grind against each other or spread apart in the seafloors, Martian tectonics seem to be vertical, with hot lava pushing upwards through the crust to the surface. Periodically, great dust storms engulf the entire planet. The effects of these storms are dramatic, including giant dunes, wind streaks, and wind-carved features.