Olympus Mons is an enormous shield volcano on the planet Mars. The volcano has a height of over 21 km as measured by the Mars Orbiter Laser Altimeter (MOLA). Olympus Mons is about two and a half times Mount Everest's height above sea level. It is one of the largest volcanoes, the tallest planetary mountain, and the second tallest mountain currently discovered in the Solar System, comparable to Rheasilvia on Vesta. It is often cited as the largest volcano in the Solar System. However, by some metrics, other volcanoes are considerably larger. Alba Mons, northeast of Olympus Mons, has roughly 19 times the surface area, but is only about one third the height. Pele, the largest known volcano on Io, is also much larger, at roughly 4 times the surface area, but is considerably flatter. Additionally, Tharsis Rise, a large volcanic structure on Mars of which Olympus Mons is a part, has been interpreted as an enormous spreading volcano. If this is confirmed, Tharsis would be by far the largest volcano in the Solar System. Olympus Mons is the youngest of the large volcanoes on Mars, having formed during Mars's Hesperian Period. It had been known to astronomers since the late 19th century as the albedo feature Nix Olympica. Its mountainous nature was suspected well before space probes confirmed its identity as a mountain.
