Thursday, January 22, 2009

Single Crusty Mars

Mars may be mythologically known as the Red Planet, but its topography can be as captivating as its celestial glow. Several striking features stand out with only a glance at a topographic map of Mars: the odd distribution of land on its surface and the equatorial string of giant volcanoes known as the Tharsis Rise. Since Mars has no plate tectonics, how these unique features formed has been a longstanding mystery. The answer, scientists now say, may be that instead of many plates sliding across the Red Planet’s surface as we have on Earth, the whole crust of crust moves as a single “shell.”

Earth’s surface consists of multiple tectonic plates, and their motion has produced an easily recognizable crustal dichotomy: Most of its land mass is in the northern hemisphere. Mars, on the other hand, does not have tectonic plates constantly shifting around its surface. Yet it does have its own crustal dichotomy, with most of its highlands concentrated in the southern hemisphere and lowlands predominating in the northern hemisphere.

The Tharsis Rise presents another mystery that plate tectonics can’t explain. Located more or less along the Martian equator, the Tharsis Rise contains four of the largest volcanoes in the solar system. Three of these, Arsia Mons, Pavonis Mons and Ascraeus Mons, fall into a neat line. Scientists have long hypothesized that these mountains formed because of a mantle plume, similar to the one that created Hawaii.

There are two standing theories for the asymmetry of the landmass in the northern and southern hemispheres, says Shijie Zhong of the University of Colorado in Boulder. Either the asymmetry was caused by processes in the interior of Mars, or there was a giant impact in the northern hemisphere that then spewed ejecta into the southern hemisphere. Proponents of the internal, or endogenic, theory have suggested that Mars’ crust rotates like a shell around the planet’s interior.

Using 3-D models, Zhong and his collaborators proved that the “differential rotation” of a single plate was easily possible — provided that two necessary conditions were in place. First, you need “degree-one convection,” which entails one source of upwelling in a single hemisphere (in this case Mars’ northern hemisphere), so the other hemisphere will be devoid of volcanic activity. Also necessary is lateral variation, or the physical movement of the plate, which is often facilitated when there is lots of melt present.

Although this is difficult to test directly, Zhong’s shell model of Mars is “probably right,” says Francis Nimmo, a geologist at the University of California in Santa Cruz. “The one plume has extracted lots of melt, and it has a big effect,” Nimmo says.

Mars’ “shell tectonics,” therefore, could not only produce a crustal dichotomy, but could also explain the volcanoes of the Tharsis Rise — essentially, they would be the result of this single large source of upwelling.


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