The immense Olympus Mons volcano on Mars (about 23 km tall and 600 km wide) exhibits a somewhat lopsided structure: elongated to the northwest, shortened to the southeast, with corresponding types of faulting (extensional and compressional, respectively) prevalent in each sector.
However, the overall shape of the edifice is characterized by a decrease in slope with increasing distance from the center, somewhat like the surface of a tent that is supported by a single central pole.
In order to uncover the mechanism that generates the distinctive features of the shape of Olympus Mons, Patrick J. McGovern and Julia K. Morgan, Lunar and Planetary Institute, Universities Space Research Association, constructed models of Olympus Mons as collections of particles that slip and slide past each other, controlled by the weight of other particles and the friction value assigned to each.
They find that a low-friction zone at the base of the volcanic pile is required, and lateral variations in basal friction are critical. Friction decreasing outward from the center of the edifice can explain the overall "circus tent" shape, whereas friction decreasing from southeast to northwest accounts for the shortening and elongation of the respective quadrants.
The most likely origin for the basal low-friction zone is in a layer of clay sediment, overpressured by pore water.
This layer would be thicker in the "downhill" (northwest) direction, accounting for the asymmetry. Such a layer likely corresponds to widespread and ancient clay deposits discovered by the Mars Express mission, and may constitute a favored environment for extant thermophilic ("heat-loving") organisms on Mars.
This research was published in Feb 2009 in the journal Geology.
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