Mars

The "red planet" is so named because of the color of its surface, which indeed is strikingly red. Simply put, Mars has rusted—iron oxides are responsible for its orange hue.

Mars is smaller than Earth. Its diameter of about 2,111 mi (3,397 km) is a little over half that of Earth, and it is only 10% as massive as our planet. Mars has seasons because the tilt of its axis relative to the plane of its orbit is nearly the same as Earth's. It rotates on its axis once every 24 hours and 40 minutes, so a Martian day is just a little longer than one of ours. The Sun would appear larger in the Martian sky because Mars is half as far from the Sun as the Earth, and its year is 687 (Earth) days long.

Mars's gravity is weaker than Earth's, and the planet has been unable to retain much of an atmosphere. The Martian atmosphere is less than 1% as dense as Earth's, and is made mostly of carbon dioxide, with trace amounts of nitrogen and argon.

Atmospheric carbon dioxide is the source of Mars's polar ice caps. Atmospheres act like giant insulators for planets, preventing heat from radiating away to space. Mars's thin atmosphere holds very little heat—a blazing summer day on Mars might get up to the freezing point of water 32°F (0°C), but at night the temperature plummets well back below 0°F (-18°C). At the poles, temperatures drop well below -100°F (-73°C), sufficiently cold for the carbon dioxide in the atmosphere to freeze. Mars's polar ice caps consist of frozen carbon dioxide with an underlayer of ice.

Although no life has been found on Mars, the planet's surface does have some very Earth-like features. There are enormous volcanoes, the largest of which, Olympus Mons, is almost the size of the entire state of Arizona. Elsewhere are long, eroded channels telling us that at some time in the past water flowed freely on the Martian surface.

Mars surface terrain can be divided into two main areas, the southern highlands (the older part of Mars) and the northern plains, a lower younger region. Dividing these areas is a planet-encircling feature called the global escarpment. The southern highlands are densely cratered and there are two very large impact crater basins there called Hellas and Argyre. There is abundant evidence of river systems draining the southern highlands, and the drainage is mainly toward the northern plains (or lowlands) across the global escarpment. On of the largest vallies in the solar system, Valles Marinaris, cuts across this escarpment, showing where water drained from south to north during a period in Mars history when abundant water was present. The northern lowlands are about 1.6 mi (2.5 km) below the mean radius of Mars and contain evidence of extensive flood-type volcanic flows, as well as river systems and wind-blown dust layers. In the northern plains, two continent-sized upwarped areas occur (Tharsis and Elysium). These are volcanic areas, home of A photograph of the Martian surface taken by one of the Viking landers. The layer of morning frost that can be seen in the photo is less than one one-thousandth of an inch thick. U.S. National Aeronautics and Space Administration (NASA). giant shield volcanoes, including the largest volcano in the solar system, Olympus Mons.