Earth's outer neighbour is a desert planet. Its diameter is only half that of Earth, but its surface area is nevertheless as large as that of all terrestrial continents put together. Today, no water flows on Mars; its icy temperatures do not allow the ice at the North and South Pole to melt. Its volcanos – the tallest in the Solar System, rising up to 24 kilometres – appear extinct. A gigantic system of rifts and valleys, Valles Marineris, bears witness to the intense tectonic activity that was once powered by forces in the interior of the planet. Mars is surrounded by a thin atmosphere in which violent storms blow around the entire planet, scattering the characteristic orange-red dust.