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Mars is the fourth planet from the sun and sixth largest planet, named for the Roman god of war.
Mars has two small moons, Phobos and Deimos. Phobos is about 21 km (about 13 mi) in diameter across;
Deimos, about 12 km (about 7.5 mi) in diameter across.
Viewed without a telescope, Mars is reddish and varies in brightness. When closest to Earth, Mars is the
second-brightest planet next to Venus, visible in the night sky.
The atmosphere on Mars is composed of carbon dioxide (95%), nitrogen (2.7%), argon (1.6%), oxygen (0.2%),
and small amounts of other gases. Because the atmosphere is thin, daily temperature variations of 100° C (180° F)
are common. Average temperatures do not exceed 240 K (-27° F). Mars is like a very cold, high-altitude desert.
Temperatures are mostly too cold and pressures too low for liquid water, but liquid water may exist just below the
surface in a few places.
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