Mars is the fourth planet from the Sun in our solar system and is named after Mars, the Roman god of war. Mars is also known as the "Red Planet" due to its reddish appearance when seen from Earth. The prefix areo-, from the Greek god of war, Ares, refers to Mars in the same way geo- refers to Earth.

Mars has two moons, Phobos and Deimos, which are small and oddly-shaped. These may be captured asteroids similar to 5261 Eureka, a Mars Trojan asteroid. Mars can be seen from Earth with the naked eye. Its apparent magnitude reaches -2.9, a brightness surpassed only by Venus, the Moon, and the Sun. For much of the year, Jupiter may appear brighter to the naked eye than Mars.

Until the first flyby of Mars by Mariner 4 in 1965, it was hoped, both within and especially perhaps outside scientific circles, especially in the popular media and literary circles, that Mars had ample liquid water. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, and long dark striations that could perhaps even be irrigation channels of liquid water.

These straight line features were shown to in fact not exist when they were subsequently analyzed and explained as optical illusions. Still, of all the planets in our solar system other than Earth, Mars is the most likely to harbor liquid water, and perhaps life, so the myth has had enough influence that even now probes carry packages to attempt to find microscopic life. Mars' rotational period and seasonal cycles are also similar to those of the Earth. It has the highest mountain in the solar system, Olympus Mons, the largest canyon in the solar system, Valles Marineris, and polar ice caps.

Mars is currently host to four orbiting spacecraft: Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. This is more than any planet other than Earth has. It is also home to the two Mars Exploration Rovers (Spirit and Opportunity).