Mars

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.

The atmosphere of Mars is quite different from that of Earth. It is composed primarily of carbon dioxide with small amounts of other gases. Martian air contains only about 1/1,000 as much water as our air, but even this small amount can condense out, forming clouds that ride high in the atmosphere .

Local patches of early morning fog can form in valleys. At the Viking Lander 2 site, a thin layer of water frost covered the ground each winter.

There is evidence that in the past a denser martian atmosphere may have allowed water to flow on the planet. Physical features closely resembling shorelines, gorges, riverbeds and islands suggest that great rivers once marked the planet.