Olympus Mons is not only the highest mountain on Mars, but the largest volcanic cone in the whole solar system. Like Mount Fuji, Mount Shasta, and other famous volcanic peaks of our own planet, it is basically a huge pile of lava flows that erupted at one spot, piling upon one another to build a mountain. But Olympus Mons dwarfs all mountains we Earthlings have seen, volcanic or not. Its discovery is a 100-year odyssey of Martian exploration. Big as Olympus Mons is, it is not big enough to have been seen or understood by telescopic observers in the era before the age of spacecraft exploration. Still, something odd was noted there. Tenacious astronomers, peering through their telescopes of glass and brass in the 1800s, kept records of changes they saw on the red planet, and the region of Olympus Mons emerged as an early puzzle. Sometimes observers saw a bright spot there, like a strange white beacon in the broad orange Martian desert of Tharsis. That's why in 1879 Schiaparelli named it Nix Olympica, or Snows of Olympus. The name turned out to be wonderfully prescient, because without any knowledge of the true nature of Mars, Schiaparelli had named Mars's grandest peak after the grandest peak of classical history—snowcapped for much of the year—where the mythic Greek gods had made their everlasting home. Schiaparelli was not necessarily claiming that Nix Olympica was a mountain. He probably chose the Greek peak more for its mythic luminosity. Homer, for example, wrote of Mount Olympus as being bathed in a “white radiance” that glittered like a frosty “plain of snow.” The bright spot called Nix Olympica turned out to be caused by the clouds that frequently form atop Olympus Mons. Transient clouds can form over our own terrestrial mountains in this way and are called orographic clouds. Over mighty Olympus Mons, they can swell to prominence in just a few hours, then dissipate in the thin Martian air—all in response to the ebb and flow of wind and thermal conditions around this towering but now frozen inferno. Here, then, is a single mountain mass three times as high as Mount Everest, with a width that would straddle the whole Himalayan range. Its base is defined by a cliff rising a kilometer (around 3,000 to 4,000 feet) and extending all the way around the mountain. The volcano would approximately cover the state of Missouri.

• published: 26 Mar 2015
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