Mars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance as seen from Earth.

A terrestrial planet, Mars has a thin atmosphere and surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the solar system, and of Valles Marineris, the largest canyon. In addition to its geographical features, Mars’ rotational period and seasonal cycles are likewise similar to those of the Earth.

Until the first flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on the planet. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents, while long, dark striations were interpreted by some observers as irrigation channels for liquid water. These straight line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in our solar system, Mars is the most likely, other than Earth, to harbor liquid water, and perhaps life.

Mars has half the radius of Earth and only one-tenth the mass, being less dense, but its surface area is only slightly less than the total area of Earth's dry land.
While Mars is larger and more massive than Mercury, Mercury has a higher density. This results in a slightly stronger gravitational force at Mercury's surface. The red-orange appearance of the Martian surface is caused by hematite, or rust.

Mars has two tiny natural moons, Phobos and Deimos, which orbit very close to the planet and are thought to be captured asteroids.