OLYMPUS MONS

THE TALLEST MOUNTAIN IN THE SOLAR SYSTEM IS A MASSIVE shield volcano that has been dormant for tens, perhaps hundreds of millions of years.

Once, though, its mighty outpourings of lava dwarfed everything else on the planet. Over time, they built a mountain three times taller than Everest, with a base the size of the state of Iowa.

The edges of that base are rugged cliffs of basalt more than a kilometer high. The summit of the mountain, where huge calderas mark the vents that once spewed molten rock, stands some twenty-seven kilometers above the supporting plain: 27,000 meters, more than 88,000 feet. For comparison, Mt. Everest is 8848 meters high, 29,028 feet.

Olympus Mons is so tall that, on Earth, its summit would poke high above the troposphere—the lowest layer of air, where weather phenomena take place—and rise almost clear of the entire stratosphere. On Mars, however, the atmosphere is so thin that the atmospheric pressure at Olympus Mons’ summit is only about one-tenth lower than the pressure at ground level.

At that altitude, the carbon dioxide that forms the major constituent of Mars’ atmosphere can freeze out, condense on the cold, bare rock, covering it with a thin, invisible layer of dry ice.