Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the “Red Planet” because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

Mars/Earth comparison:
Average Distance from Sun: 142 million miles (228.5 million km) / Earth: 93 million miles (150 million km)
Average Speed in Orbiting Sun: 14.5 miles per second (23.34 km per second) / Earth: 18.5 miles per second (29.77 km per second)
Diameter: 4,220 miles (6,791 km) / Earth: 7,926 miles (12,755 km)
Tilt of Axis: 25 degrees / Earth: 23.5 degrees
Length of Year: 687 Earth Days / Earth: 365.25 Days
Length of Day: 24 hours 37 minutes / Earth: 23 hours 56 minutes
Gravity: 0.375 that of Earth
Average Temperature: -81 degrees F (-62.78 degrees C) / Earth: 57 degrees F (13.88 degrees C)
Atmosphere: mostly carbon dioxide, some water vapor / Earth: nitrogen, oxygen, argon, others
Number of Moons: 2 / Earth: 1

The planet Mars has two permanent polar ice caps. During a pole’s winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO₂ ice (dry ice). When the poles are again exposed to sunlight, the frozen CO₂ sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.
The caps at both poles consist primarily of water ice. Frozen carbon dioxide accumulates as a comparatively thin layer about one meter thick on the north cap in the northern winter only, while the south cap has a permanent dry ice cover about 8 m thick. Photo: windows2universe.org