Mars' valleys and volcanoes
Pictures sent back by spacecraft such as ESA's Mars Express show that Mars has many large impact craters. Most of these are to the south of the equator. They seem to have been made by meteorites crashing onto the surface billions of years ago. The largest crater is about 1800 km across – large enough to swallow half of Europe.
However, the surface of Mars has changed during its lifetime. One obvious feature is the huge system of valleys – the Valles Marineris - near the planet's equator. About 5000 km long, they would stretch all the way from Paris to New York.
The valleys seem to have been formed by cracking in the planet's surface, when the rocky crust stretched and pulled apart. The valleys are now so wide that a person standing on one edge would not be able to see the other side.
Not far to the west are five enormous volcanoes. Most impressive of all is Olympus Mons, the largest volcano in the Solar System. It is wider than England and three times higher than Mount Everest, the highest mountain on Earth. None of these volcanoes are active at the present time.