Our nearest neighbour is a geologist’s dream. Gravity is similar to but weaker than Earth’s, meaning familiar landscapes but taller cliffs, deeper canyons and more imposing mountains. The most impressive feature is Olympus Mons, the biggest volcano in the Solar System. It’s so vast it pokes right through the thin atmosphere into space. Circling Mars are two moons, Deimos and Phobos (the ‘Dogs of War’) and at least six man-made satellites.