Artist's concept of the northwest side of the Olympus Mons volcano on Mars - stock illustration

The northwest side of Olympus Mons' 20,000 foot scarps cast long shadows into the water mist and dust09/14/2009filled atmosphere over the plains of the Tharsis Bulge. In the sky immediately above Olympus Mons' caldera are, left to right, Mars' satellites Deimos and Phobos. Nearly as large as the state of Arizona and three times the height of Mount Everest, Olympus Mons is the largest volcano, and mountain, in the Solar System. It is thought to be very old, though its last eruption may have been as (geologically) recent as 40 million years ago. Olympus Mons may yet still be an active volcano.