2001 Mars Odyssey

One of five planets known to ancients; Mars was Roman god of war, agriculture and the state
Reddish color; at times the third brightest object in night sky after the Moon and Venus

Physical Characteristics

Average diameter 6,780 kilometers (4,217 miles); about half the size of Earth, but twice the size of Earth's Moon
Same land area as Earth
Mass 1/10th of Earth's; gravity only 38 percent as strong as Earth's
Density 3.9 times greater than water (compared to Earth's 5.5 times greater than water)
No planet-wide magnetic field detected; only localized ancient remnant fields in various regions

Orbit

Fourth planet from the Sun, the next beyond Earth
About 1.5 times farther from the Sun than Earth is
Orbit elliptical; distance from Sun varies from a minimum of 206.7 million kilometers (128.4 million miles) to a maximum of 249.2 million kilometers (154.8 million miles); average distance from Sun, 227.7 million kilometers (141.5 million miles)
Revolves around Sun once every 687 Earth days
Rotation period (length of day in Earth days) 24 hours, 37 min, 23 sec (1.026 Earth days)
Poles tilted 25 degrees, creating seasons similar to Earth's

Environment

Atmosphere composed chiefly of carbon dioxide (95.3%), nitrogen (2.7%) and argon (1.6%)
Surface atmospheric pressure less than 1/100th that of Earth's average
Surface winds up to 40 meters per second (80 miles per hour)
Local, regional and global dust storms; also whirlwinds called dust devils
Surface temperature averages -53°C (-64°F); varies from -128°C (-199°F) during polar night to 27°C (80°F) at equator during midday at closest point in orbit to Sun

Features

Highest point is Olympus Mons, a huge shield volcano about 26 kilometers (16 miles) high and 600 kilometers (370 miles) across; has about the same area as Arizona
Canyon system of Valles Marineris is largest and deepest known in solar system; extends more than 4,000 kilometers (2,500 miles) and has 5 to 10 kilometers (3 to 6 miles) relief from floors to tops of surrounding plateaus
"Canals" observed by Giovanni Schiaparelli and Percival Lowell about 100 years ago were a visual illusion in which dark areas appeared connected by lines. The Mariner 9 and Viking missions of the 1970s, however, established that Mars has channels possibly cut by ancient rivers

Moons

Two irregularly shaped moons, each only a few kilometers wide
Larger moon named Phobos ("fear"); smaller is Deimos ("terror"), named for attributes personified in Greek mythology as sons of the god of war