Mars

Small and red in colour, Mars is more like Earth than any other planet. People once believed that intelligent beings live on Mars - but space probes have shown that there are no Martians, and no other life on the planet. It is too cold, and the atmosphere is too thin for life to exist.

Mars' surface is dotted with vast dessert, craters and volcanoes. The highest volcano, Olympus Mons, is nearly 30 kilometres high. There is also gash in the surface over 4000 kilometres long and 7 kilometres deep in places. It has been called Mars' Grand Canyon, but its proper name is Mariner Valley. Smaller valleys look as if they have been made by flowing water, so astronomers think that Mars may once have had rivers and seas.

In 1965, Mariner 4 flew past Mars and sent back pictures. Mariner 6 and 7 also flew past, and in 1971, Mariner 9 went into orbit round it. Five years later two Viking craft dropped landers onto the surface. In 1997, the Pathfinder probe landed, carrying a small vehicle called Sojourner, which investegated the srrounding rocks.