1. Mantle Convection and the Martian Hemispheric Dichotomy John Hernlund

2. 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.”

3. Basic Convection Convection occurs whenever Ra exceeds some critical value

4. How Convection Works

5. Dependence of Critical Ra on Wavelength Short wavelengths are killed off rapidly by thermal diffusion Long wavelengths are more loosely inhibited by viscous resistance to shear

6. 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

7. Zhong and Zuber, 2001

8. 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?

9. 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?

10. 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

11. Zhong and Roberts, 2003 Tharsis is at most 15% supported by plumes beneath it, mostly a flexural feature due to volcanic loading.

12. 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

13. Smith et al., 1999 Round? Hardly!

14. Smith et al., 1999

15. 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.

16. 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.