Mars, the fourth planet in the Solar System, is one of the brightest objects in the night sky. It is invisibly red, even to the naked eye. This rust color is caused by exactly that: rust. The red sands are derived from oxidized iron, and this fiery appearance led the planet to be named after the Roman god of war.
Mars orbits the Sun at a distance of about 1.5 astronomical units (about 225 million km). It is about 11% of the mass of the Earth with a radius just over half of the Earth's. Its thin atmosphere, combined with its distance from the Sun, makes the planet cold, with average temperatures near the equator of - 50 ' C.
This picture was taken when Mars was close to the Earth, only 103 million km away, one of the near 'oppositions' that occurs every two years and gives us our best views of the planet. It is immediately apparent that Mars is quite cloudy. The number of clouds in the Martian atmosphere is usually quite low, and their presence indicates that the temperature of the planet has fallen recently allowing water vapour to freeze our of the atmosphere and form clouds. The planet's north polar ice cap, which is tilted towards the Earth, is clearly visible at the top of the picture. This ice cap is made of huge deposits of water ice, very similar to the Earth's polar caps. The southern polar cap on Mars, however, is very much colder as it is pointed away from the Sun. This cap is mostly frozen carbon dioxide. The red spot on the left-hand edge of the picture is the huge volcano Ascraeus Mons. protruding through the clouds. Beneath this is a darker patch, the Vallcs Marinciis, a huge rift valley system that is 5000 km long and up to 500 km wide in places. It is thought that this huge scar on the surface of Mars may be connected to the mysterious upheaval that created the Tharsis Bulge, an area of ground that has been pushed as far as 10 km above the surrounding crust.
The picture was taken on 25 February 1995 with the small, high-resolution planetary camera secrion of WF/PC2.