Learn about the history, geology, climate, and exploration of Mars, the fourth planet from the Sun. Discover how Mars is similar to and different from Earth, and why it is called the Red Planet.

Learn all about the Red Planet in our fascinating facts about Mars. How big is Mars? What's the temperature on Mars? Is there water on Mars?

Mars is the second smallest planet in the solar system, only larger than Mercury and slightly more than half the size of Earth. It has an equatorial radius of 3,396 km (2,110 miles) and a mean polar radius of 3,379 km (2,100 miles).

Mars is no place for the faint-hearted. It’s dry, rocky, and bitter cold. The fourth planet from the Sun, Mars, is one of Earth's two closest planetary neighbors (Venus is the other). Mars is one of the easiest planets to spot in the night sky — it looks like a bright red point of light.

Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity. Mars is the only presently known example of a desert planet, a rocky planet with a surface akin to that of Earth's hot deserts.