Mars: Tharsis
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
One of the most interesting areas on Mars is the Tharsis Plateau, shown here. This region includes a number of great shield volcanoes, including the highest point on Mars.
How do we measure elevations on a planet with no seas? On Mars, we use the elevation where the atmosphere has a pressure about .006 that of Earth. At that pressure, and a temperature near 0 Centigrade (32 F), liquid water, ice and water vapor can all coexist together, although it is usually too cold on Mars for liquid water to exist. This temperature and pressure is called the triple point of water. Mars has much greater elevation differences than Earth. The lowest point on Mars is 8 kilometers below zero elevation and the highest point, Olympus Mons near the left edge, is a whopping 21 kilometers above. There is also a vast rift valley, Vallis Marineris, cutting across the right (east) side of the plateau. At lower right is a huge impact basin, Argyre Planitia. Early in Mars' history, huge floods swept down the flanks of Tharsis, cutting deep valleys that are visible in the figure.
Places on planets are named using Latin geographic terms ("Mons" is Latin for "mountain" and "vallis" means "valley." Planitia" is Latin for "plain" and is used for large flat regions).
This figure appears to violate a fundamental law of contour mapping. Topographic contours never end or cross, yet in many places we see areas differing in elevation by several kilometers separated by only a single contour line. The reason is that many slopes on Mars are steep and the contour lines are too close together to show individually.
The very large mountain left of the Tharsis Plateau is Olympus Mons (Mount Olympus), a giant shield volcano and the highest point on Mars. Early observers on Earth saw this mountain when it was covered with clouds, and assumed they were seeing snow, so they called it "Nix Olympica," or "the snow of Olympus." In the middle of the Tharsis Plateau are three more huge volcanoes, Arsia Mons at the bottom, Pavonis Mons in the middle and Ascraeus Mons at top. The isolated high spot well above (north) of the plateau is Alba Mons, a huge but very low shield volcano. Alba Mons is directly opposite the Hellas Basin, a huge impact crater, and the impact may have fractured the crust of Mars, permitting eruptions of lava.
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Created 21 August 2009, Last Update 15 January 2020