Transcript Convection and the Hemispheric Dichotomy: Any Links, or Just B.S.?

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
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Convection occurs whenever Ra
exceeds some critical value
How Convection Works
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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
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., 1999
Round? 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
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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.