Mantle Convection and the Martian Hemispheric Dichotomy

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Mantle Convection and the Martian Hemispheric
Dichotomy

• John Hernlund

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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
Earths surface into a land and an ocean
hemisphere. This division of the Earths 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 l1.
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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

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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

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Zhong and Zuber, 2001
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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?

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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?

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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

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Zhong and Roberts, 2003

• Tharsis is at most 15 supported by plumes
  beneath it, mostly a flexural feature due to
  volcanic loading.

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Why not an impact?

• Impact should be primordial featurereally 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

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Smith et al., 1999
Round? Hardly!
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Smith et al., 1999
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Some Reasonable Conclusions

• Convection can develop l1 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?
  Perhapsthats as good an idea as any.

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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 Mars 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.