Mars Facts

Mean distance from Sun: 1.524 AU (228,000,000 km/141,700,000 mi)
Diameter: 6,792 km (4,220 mi)
Length of year: 687 days
Rotation period: 24 hr 37 min
Mean orbital velocity: 24.14 km/sec (15 mi/sec)
Inclination of axis: 25.2°
Mean density: 3.95 grams/cm°
Inclination to ecliptic: 1.85°
Number of observed satellites: 2

Comparisons with Earth
Average distance from Sun: 1.52 x Earth
Diameter: 0.532 x Earth
Mass: 0.108 x Earth
Density: 0.7 x Earth

World of Change

Mars has always intrigued us, but only in recent decades have we come to know our smaller neighbor as a world of great complexity. Telescopic views suggested to early observers that its climate changed seasonally. But we now know that even greater changes have occurred over billions of years. Great volcanoes have risen. Floodwaters have raced across the surface. Giant impacts have altered the landscape.

Today, wind is the only major force of change, but the rocks and soil and landscape still hold clues to the planet's past. Why has Mars changed so much over time? Did life evolve there when conditions were more favorable? Mars continues to challenge each new generation of explorers, and understanding Mars may help us discover our own future in the solar system.

Earth and Mars

Teeming with complex life and covered by vast oceans, Earth is unique among the planets of our solar system. Yet it shares common features and processes with Mars. Both planets have polar ice caps, their axes of rotation tilt at similar angles, and their days are almost the same length. The surfaces of both planets have been shaped by water, wind, and volcanism.