Olympus Mons - General Description

General Description

As a shield volcano, Olympus Mons resembles in its morphology the large volcanoes making up the Hawaiian Islands. The edifice is about 600 km (370 mi) wide. Because the mountain is so large, with complex structure at its edges, allocating a height to the structure is difficult. It stands 21 km (13 mi) above the Mars global datum, and its local relief, from the foot of the cliffs which form its margin to the northwest to its peak, is nearly 22 km (14 mi) (a little over twice the height of Mauna Kea as measured from its base on the ocean floor). The total elevation change from the plains of Amazonis Planitia, over 1,000 km (620 mi) to the northwest, to the summit approaches 26 km (16 mi). The summit of the mountain has six nested calderas (collapse craters) forming an irregular depression 60 km (37 mi) × 80 km (50 mi) across and up to 3.2 km (2.0 mi) deep. The volcano's outer edge consists of an escarpment, or cliff, up to 8 km (5.0 mi) tall, a feature unique among the shield volcanoes of Mars. Olympus Mons covers an area approximately the size of Arizona.

Being a shield volcano, Olympus Mons has a very low profile. The average slope on the volcano's flanks is only 5°. Slopes are highest near the middle part of the flanks and grow shallower toward the base, giving the flanks a concave upward profile. The shape of Olympus Mons is distinctly unsymmetrical. Its flanks are shallower and extend out further from the summit in the northwestern direction than they do to the southeast. The volcano's shape and profile have been likened to a "circus tent" held up by a single pole that is shifted off center.

Because of the size of Olympus Mons and its shallow slopes, an observer standing on the Martian surface would be unable to view the entire profile of the volcano, even from a great distance. The curvature of the planet and the volcano itself would obscure such a synoptic view. Similarly, an observer near the summit would be unaware of standing on a high mountain, as the slope of the volcano would extend beyond the horizon, a mere 3 kilometers away.

The typical atmospheric pressure at the top of Olympus Mons is 72 pascal, about 12% of the average Martian surface pressure of 600 pascal. Both are exceedingly low by terrestrial standards. By comparison, the atmospheric pressure at the summit of Mount Everest is 32,000 pascals, or about 32% of Earth's sea level pressure. Even so, high-altitude orographic clouds frequently drift over the Olympus Mons summit, and airborne Martian dust is still present. Although the average Martian surface atmospheric pressure is less than one percent of Earth's, the much lower gravity on Mars increases the atmosphere's scale height; in other words, Mars's atmosphere is expansive and does not drop off in density with height as sharply as Earth's.

Olympus Mons is an unlikely landing location for automated space probes in the near future. The high elevations preclude parachute-assisted landings because of insufficient atmospheric thickness to slow the spacecraft down. Moreover, Olympus Mons stands in one of the dustiest regions of Mars. A mantle of fine dust covers much of the terrain, obscuring the underlying bedrock (rock samples might be hard to come by). The dust layer would also likely cause severe maneuvering problems for rovers.

• Mars Global Surveyor image of clouds over Tharsis. Olympus Mons is in the upper left.

• Topographical map of Olympus Mons

• Olympus Mons (top) and the Hawaiian island chain (bottom), at the same scale.

• Comparison of Olympus Mons with the highest mountains on Earth.