Mantle Convection and the Martian Hemispheric Dichotomy

John Hernlund

Chandrasekhar, 1961

“It has been suggested that at an early stage in the history of its formation, the Earth was a nearly homogeneous fluid sphere with convective motions of the type we have just described; and, further, that we can infer the existence, at one time, of such motions from the division of the Earth’s surface into a land and an ocean hemisphere. This division of the Earth’s surface reflects a higher deposition of sial in one hemisphere than in the other; and the advocates of the convection hypothesis see in this the systematic difference in temperature in two hemispheres which would accompany convective

motions belonging to the pattern l=1.”

Basic Convection

Convection occurs whenever Ra exceeds some critical value

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How Convection Works

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Dependence of Critical Raon Wavelength

• Short wavelengths are killed off rapidly by thermal diffusion

• Long wavelengths are more loosely inhibited by viscous resistance to shear

Ways to induce longer wavelengths

• Compressibility: material thinks the domain is deeper than it really is

• Small core: effectively increases aspect ratio in sphere

• Depth-dependent viscosity: a greasy layer allows material to shoot across more easily than to penetrate the viscous part in many places

Zhong and Zuber, 2001

Some Problems

• Temperature dependent viscosity shifts things to small wavelengths

• Perhaps we need plate tectonics to organize the large scale flow

• Tharsis is out of phase with the dichotomy: where does it come from?

Cessation of Plate Tectonics?• Lenardic et al., 2004

propose insulation feedback weakens mantle, lowers stress, no longer can break the lithosphere into plates

• Perhaps Mars was in a state of “continent” assembly when plate tectonics shut down?

The Berkeley Group Approach

• Wenzel et al., 2004 suggest compositional layering of mantle organizes the flow to keep plumes underneath Tharsis, and enforced by hemispheric dichotomy

• Some very basic problems with their model

Zhong and Roberts, 2003• Tharsis is at

most 15% supported by plumes beneath it, mostly a flexural feature due to volcanic loading.

Why not an impact?

• Impact should be primordial feature…really old, round, and fairly sharp boundaries

• Zuber et al., 2000: much of the edge seems to be younger deposits

• Smith et al., 1999: shape is not circular

• Smith et al., 1999: variation in topo is smooth globally, not sharp

Smith et al., 1999Round? Hardly!

Smith et al., 1999

Some Reasonable Conclusions

• Convection can develop l=1 patterns. We see that on Earth too, with the assembly of super-continents

• Plate tectonics makes sense in helping to organize this pattern

• Did insulation kill it while assembled? Perhaps…that’s as good an idea as any.

Some References

• Chandrasekhar, Hydrodynamic and Hydromagnetic Stability, 1961.

• Lenardic, Nimmo, Moresi; Growth of the hemispheric dichotomy and the cessation of plate tectonics on Mars, JGR, 2004.

• Wenzel, Manga, Jellinek; Tharsis as a consequence of Mar’s dichotomy and layered mantle, GRL, 2004.

• Zhong, Roberts; On the support of the Tharsis Rise on Mars, EPSL, 2003.

• Zhong, Zuber; Degree-1 mantle convection and the crustal dichotomy on Mars, EPSL, 2001.

• Zuber et al.; Internal structure and early thermal evolution of Mars from MGS Topo and Gravity, Science, 2000.

• Smith et al.; The global topography of Mars and the implications for surface evolution, Science, 1999.