The Mars Dichotomy

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The Mars Dichotomy

• Evidence for Plate Tectonics or Erosion?

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Theories of the Dichotomy Formation

• Giant Impact
• One major collision resurfaced the region.
• Multiple Impacts
• Several large impacts caused resurfacing.
• Ancient Ocean
• An ancient ocean existed in the northern
  hemisphere. The ocean eroded away the surface
  erasing the craters observed in the southern
  hemisphere.
• Plate tectonics
• Erosion
• Out flow channels

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Impacts

• Wilhems and Squyres (1984) suggested a single
  impact hypothesis.
• The geology of the northern lowlands (Vastitas
  Borealis) is not consistent with a one impact
  hypothesis. (Frey et. al., 1986a)
• The lowlands are not radial in shape (Smith et.
  al. 1999), and there is no evidence of a crater
  rim.

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Several Impacts

• The lowland geology could have formed through
  several large impacts.
• Impact basins on mercury and the moon roughly
  follow a the D-? distribution curves,
• Frey and Schultz (1988) concluded that the
  largest impact basins on Mars (Fig 2A) roughly
  follow this curve, but the proposed Borealis
  impact does not.
• But is the 2A curve really a good fit?

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Ocean erosion

• Topographic profiles across the Mars dichotomy
  are not consistent with ancient shoreline
  (Withers and Neumann, 2001).
• Possible shoreline slopes are not orientated in
  the correct direction
• Concluded shorelines were most likely created
  by compressive tectonic stress.
• Suggested volcanism and impact craters.

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Utopia
Utopia
-Inferred shorelines slope down to the
North. -However, the slopes appear to be impact
related.
North facing slopes

• Alba Patera
• -Inferred shorelines
• slope down to the south.
• This is opposite of the expected slope
• direction, if formed by an ocean shore.
• -Slopes are most likely due to volcanic
  compression.

Note shoreline reversal
Alba Patera
South facing slopes
(Withers and Neumann, 2001)
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Plate Tectonics

• The differences between the Northern and Southern
  hemisphere's can be interpreted using analogs to
  earths plate tectonics.
• Crustal thickness, volcanoes, and contraction
  features.
• Sea floor spreading continuously forms new
  crust at rift margins.
• New crust will be smother and thinner then old
  crust.
• Subduction destroys the new crust, which provides
  the fuel for volcanic activity.
• Tharsis?
• Convergence will produce contraction features
  along the plate boundary

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Contraction Features

• Watters (1993) mapped contraction features on the
  surface of Mars.
• Wrinkle ridges, lobate scarps, and high relief
  ridges.
• Wrinkle ridges are regularly spaced landforms
  generally caused by thrust faulting and/or
  folding.
• Wrinkle ridges accounted for 80 of the mapped
  contraction features.
• Watters found that contractional features are
  generally parallel to the dichotomy boundary in
  the eastern hemisphere.
• Suggests the influence of regional stresses
  related to dichotomy formation.
• Whereas the pattern in the western hemisphere
  reflects the Tharsis volcanic province.
• The geometry of the inferred stress was analyzed
  by fitting great circles to each mapped segment,
  and plotting them on a Schmidt net to create a
  Beta diagram (an equal area stereonet
  projection). (Watters, 1993)

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East
West
Above Beta Diagram showing the concentration of
great circle intersections to inferred maximum
principle stress direction. -Note the general E-W
trend of the contraction features. -Two
clusters dominate at Tharsis and Hesperia
Planum. (Watters, 1993)
Wrinkle Ridges
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Mars Tectonics

• The northern hemisphere of Mars was formed during
  sea floor spreading along a ridge axis that
  broke away from Terra Cimmeria.
• Terra Cimmeria then acted as a passive margin.
• Subduction initiated along Arabia Terra and the
  eastern edge of the Tharsis volcanic province.

(Sleep, 1994)
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Not to scale
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Not to scale
Plate motion ceases when the rift margin is
subducted.
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Plates and Margins
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1
2
-Yellow outline shows the plate margin at the
time of the break up.
-The singular plate breaks into 2 plates,
possibly due to subduction angle and different
plate velocities.
(Sleep, 1994)
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4
3
-New plate geometry after the break up. -Note
the transform fault between the two plates.
-Plate geometry at the time plate motion is
inferred to have ceased.
(Sleep, 1994)
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Fast vs. Slow Spreading Rates

• On earth a fast spreading center produces smooth
  topography, and virtually no vertical scarps.
• Slow spreading centers build up topography, and
  have a large number of high angle scarps.

East Pacific Rise
Mid-Atlantic Ridge
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Summary of Martian Tectonics(Sleep, 1994)

• Due to the smooth nature of the northern lowlands
  sea floor spreading must have occurred
  relatively fast.
• A quantitative estimate of a full plate spreading
  rate is 80 mm/yr.
• This is fast, but comparable to places on earth,
  such as the East Pacific Rise.
• The northern lowland crust would have formed
  rather quickly, and plate tectonics may not have
  lasted that long.
• Plate tectonics could aid in cooling the interior
  of Mars.

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Magnetic Stripes

• Magnetic field observations acquired by the MGS
  suggest Mars possessed a periodically reversing
  dynamo

(Acuna et. al., 1999).
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Dynamo

• On earth the dynamo occurs due to convection in
  the outer core. Convection in the core occurs
  because it is being cooled by the mantle.
• Plate tectonics can drive core convection.
• Absence of crustal magnetism near large impacts
  basins suggests the dynamo was only active early
  in Mars history during the Naochian epoch 4
  billion years ago. (Acuna et. al., 1999)
• It is proposed that cessation of plate tectonics
  is linked to the cessation of the Mars dynamo
  (Nimmo and Stevenson, 2000).
• i.e. Plate tectonics allows the planet to cool
  from the inside out.

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Summary

• It is unlikely a giant impact resurfaced the N
  hemisphere of Mars.
• It is possible multiple large impacts resurfaced
  the N hemisphere.
• However, this is not supported geological or
  statistically.
• There is no evidence of an ocean shore line, only
  tectonic features and impact ridges.
• Plate tectonics provides a possible mechanism for
  N hemisphere resurfacing.
• Magnetic stripes are present on Mars, therefore
  a dynamo may have existed early in Mars history.

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Future Work

• Photo mapping of the dichotomy boundary would
  reveal subtle details required for an accurate
  geologic analysis.
• Detailed N-S gravity profiles across the
  dichotomy to analyze the details of the crustal
  thickness variation.
• Send a geologist to Mars to map the boundary!
  Well figure it out.....

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References

• Acuna, M.H. et. al., Global distribution of
  crustal magnetization discovered by the Mars
  Global Surveyor MAG/ER experiment, Science, 284,
  790-793, 1999.
• Frey et. al., The martian crustal dichotomy
  product of accretion and not a specific event?
  (abstract) Lunar and Planet. Sci., 27, 241-242,
  1986a.
• Frey, H. and R.A. Schultz, Large impact basins
  and the mega-impact origin for the crustal
  dichotomy on Mars, Geophys. Res. Lett., 15,
  229-232, 1988.
• Nimmo F. and D.J. Stevenson, Influence of early
  plate tectonics on the thermal evolution and
  magnetic field of Mars, J. Geophys. Res., 105,
  11,969-11,979, 2000.
• Sleep, N.H., Martian plate tectonics, J Geophys.
  Res., 99, 5639-5655, 1994.
• Smith, D.E. et. al., The global topography of
  Mars and implications for surface evolution,
  Science, 284, 1495-1502, 1999.
• Watters, T. R., Compressional tectonics on Mars,
  J. Geophys. Res., 98, 17,049-17,060, 1993.
• Wilhems D.E. and S.W. Squyres, The martian
  hemisphere dichotomy may be due to a giant
  impact, Letters to Nature, 309, 138-140, 1984.
• Withers P. and G.A. Neumann, Enigmatic northern
  plains of Mars, Nature brief communications,
  410, p.651, 2001.

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Outflow channels

• Images from Mars show distinct valley networks
  that flow across the dichotomy from S to N.
• These outflow channels may have carried water (or
  even volcanic materials) into the northern plains
• But it would take a large standing body of water
  to physically form the dichotomy.
• Implies episodic flooding.

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Valles Marineris Linear shape implies it was
formed tectonically.
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Increase in crustal thickness to the south must
have formed by internal processes.However, the
Northern lowlands may have been resurfaced
through water/volcanic outflow.
Arabia Terra
(Zuber, 2001)