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Planetary scientists have often puzzled over the origins of the Martian landscape, including its so-called hemispheric dichotomy, a pronounced difference in the thickness of the planet's crust between its northern lowlands and highlands of the south. Another curious feature is an elevated region near the equator called the Tharsis Rise, capped by volcanic peaks located in a relatively straight line.
A new study seeks to unify those features by showing how thickness variations in the planet's outer layers may have given birth to the Tharsis Rise. The new model proposes that even though the Red Planet may not have Earth's multiple plates, its single fused outer shell, or lithosphere, may still rotate relative to the planet's center. The study's results were published this week in the online edition of Nature Geoscience.
Study author Shijie Zhong, a geophysicist at the University of Colorado at Boulder, says such a rotation, driven by a single plume of hot material welling up from within and by differences in the thickness of the lithosphere (that some planetary scientists believe was caused by a massive asteroid hit early in the planet's history), may explain the migration of the Tharsis volcanism from its suspected origin to its current location. Such a volcanic migration resulted in the formation of the Hawaiian Island chain here on Earth, where multiple lithospheric plates float independently.
"Earth has plate tectonics, so the plates can move around with respect to the mantle," Zhong says. "For Mars or any other terrestrial planet, the conventional view is that you cannot have this kind of plate motion." This new proposal, Zhong says, hypothesizes that "there is a unique mode of plate motion that is possible—essentially a whole lithospherical shell, a very stiff, very strong layer, and that layer actually can rotate."
This would help to explain the apparent movement of the volcanic activity that formed the Tharsis Rise. "For Mars, observationally we know the Tharsis volcanism really started in the so-called highlands, this thickened crust area, and then with time migrated toward the equator" before stabilizing somewhat about four billion years ago, Zhong says. "This particular observation has been known for quite a while with no clear explanation." (Some theorists have speculated that Mars's surface, including the layout and location of the Tharsis Rise, is the product of plate tectonics that have since fallen dormant.)
Francis Nimmo, an earth and planetary scientist at the University of California, Santa Cruz, who wrote an accompanying analysis of Zhong's paper, finds the lithospheric-rotation proposal compelling. "I do find it a feasible hypothesis," he says. "And to me, one of the most interesting aspects is not just that it makes a nice explanation for what's happening on Mars, but it may actually be a universal process." In other words, bodies such as the moon and Mercury may also have more interesting interior lives than we give them credit for.
"Normally we think of planets that don't have plate tectonics as being rather static and not very interesting," Nimmo says. "What this mechanism is showing is that even though it's a single plate, that single plate can still move bodily over the stuff underneath. And that is, to my knowledge, a completely new thought."
