Mars - Tharsis region and Valles Marineris, illustration. Tharsis is a volcanic plateau in Mars' western hemisphere. It is home to several volcanoes, including Olympus Mons (over the limb of the planet, not shown), the largest volcanic feature in the Solar System. At the top can be seen Ascraeus Mons, with Pavonis Mons below it to the left. These volcanoes range in size from 350 to 450 kilometres (km) across and they each have a height of approximately 15 km. At the bottom of the image is the easternmost portion of Valles Marineris, the largest canyon system on Mars - indeed, anywhere in the Solar System. It dwarfs the Earth's Grand Canyon. North is at the top.