Origin of The Martian Hemispheric Dichotomy: The Case for Impacts

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Origin of The Martian Hemispheric Dichotomy The
Case for Impacts

• David Galvan
• ESS 250

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The Facts

• Lowlands average 3-4 km lower than southern
  highlands. North Pole 6km lower than South Pole.
• Southern highland surface more heavily cratered,
  older (Noachian)
• Northern surface much smoother, displays far
  fewer craters, implies a resurfacing event
  somewhat later in Mars history (Hesperian)

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How do we explain this?

• Endogenic Processes
• Mantle Convection
• Requires long-wavelength pattern of heat loss
• Upwelling in one hemisphere, downwelling in other
• Problems Mars has a relatively large core,
  difficult to produce long-wavelength pattern.
• But, for plausible conditions for early Mars,
  mantle convection could account for a dichotomy
  developed over 1 Gyr from early Noachian into
  Hesperian. (Zhong Zuber, 2001)
• Plate Tectonics
• Transform faults, subduction zones along
  dichotomy boundary
• Problems
• Relatively little evidence.
• Difficult to line up expected geological features
  using a transform fault model (A.D. Fortes,
  1999)
• Exogenic Processes
• Impacts!

Zhong Zuber, 2001
A.D. Fortes, 1999
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Anatomy of an Impact
1. Object impacts crust of planet
2. Impactor releases its kinetic energy,
compressing and melting the crust. Rocks, dust,
and vaporized particles are ejected.
Compressional shock is sent downward through
crust.
3. Shock wave fractures crust, then rebounds
toward surface as a rarefaction wave, uplifting
the crater floor and rim. May cause faulting,
which leads to concentric sets of rings.
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Observing remnants of Impacts

• An impact will lead to the following evidentiary
  features
• Circular crater
• Ejecta rock and dust in surrounding region
• Brecchiated (shocked) rock in basin
• Increased gravity anomaly (mascon) due to thin
  crust and isostatic compensation
• Crustal magnetization may be different from
  surrounding region.

Manicougan Crater, Canada, 72 km across
Meteor Crater, AZ, 1km across
Chicxulub Crater, Yucatan, 200 km across
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Early Suggestions Giant Impact

• Wilhelms and Squyres (1984)
• Suggest a single giant impact event in early
  Martian history could account for dichotomy
• The impact would have created the Borealis
  basin, D7700 km.
• Identified 5 features (massifs A-D) that could be
  seen as remnant pieces of an ancient crater rim.
  Mainly mountains with steep slopes.
• Impact occurs, crustal material redeposits on
  basin periphery (southern highlands). Loss of
  mass is at least partly compensated by isostatic
  uplift of more dense mantle material, leaving
  depression.
• Partly filled later in history by lavas rising
  through weakened lithosphere.

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Giant Impact (contd.)

• Basin has diameter of 7700km, 130 deg of Lat.
  Centered at 50o N, 190o W
• Used Diameter-energy scaling relation based on
  experimental data and impacts of spacecraft on
  Moon
• DKEah(g)
• Where D is diameter of crater, E is kinetic
  energy of impactor, K and a are constants, and
  h(g) gives dependence on surface gravity.
  (Developed to describe craters on Moon by Housen
  et al., 1979)
• For impactor with density of 3 g/cc and velocity
  of 24km/s (orbital velocity of Mars), an object
  of diameter 600 km could have created Borealis
  Basin. If velocity is 12 km/s, youd need a body
  of 950 km diameter. Reasonable sizes for bodies
  near Mars orbit at end of accretion. (Hartmann
  Davis, 1977 Wetherill, 1985)

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Borealis Basin
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Proposed Borealis impact basin overlaid on MOLA
map.

• Accounts for
• Topographic depression
• Correlation of massifs not related to other
  basins
• Abrupt scarps between highlands and lowlands
• Extrusion of lava into northern lowlands
• Problems
• One impact alone cant explain entire dichotomy

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Qualification Multiple Impacts

• Frey and Schultz (1988)
• Tested the Borealis basin hypothesis by trying to
  fit observed impact basins to a 1/D2
  distribution (empirically derived for Mercury and
  Moon).
• Doesnt work for Borealis basin (Diameter
  7000km), since there would be too many missing
  impact craters of enormous size in order to fit
  the distribution.
• IE would expect 47 more craters with D 1000km
  than we actually observe.
• Works better if Chryse basin (Diameter 4300km)
  is the largest of impacts, and multiple
  overlapping impacts caused the dichotomy.
• IE would only expect 7 more craters 1000km than
  we observe. Fewer missing craters.

A single giant impact is unlikely to have caused
the entire dichotomy, but multiple overlapping
large impacts might. Multiple large impacts also
capable of depositing more heat and causing more
crustal thinning than single.
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Multiple Impacts
The Missing basins in the Chryse distribution
could fit into the southern highlands, while the
missing basins in the Borealis distribution could
not. -Frey and Schultz suggest a more thorough
examination of southern highlands looking for
large craters.
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Evidence of hidden impact basins

• McGill (1989)
• The distribution of knobs and partially buried
  structures in the Utopia Planitia area of the
  northern plains indicates a very large (D
  4600km) ring surrounding what is inferred to be
  an impact basin.
• Knobs are small, isolated hills standing above
  material in the plains of the northern highlands.
  Clusters of these knobs have been inferred to
  represent remnants of ancient impact crater rings
  that protrude up through the younger plains
  material.

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Fractured flat terrain
Sparse Knobs
Dense Knobs
Elysium Mons
Basin centered view of the inferred Utopia impact
basin. Red indicates dense knobby terrain Orange
indicates sparse knobby terrain and brown
indicates 'polygonal or fractured flat terrain
from the Hesperian outflow. The yellow dot is
Elysium Mons. The 3300km and 4715km rings are
indicated. (Fortes, 1999 after McGill, 1989.)
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Viking Gravity measurements indicate a coincident
mascon.
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Relative Age of Utopia materials
(McGill, 1989)
(Zuber, 2001)
-Tanaka (1986) considers the Isidis basin to be
Lower Noachian basement. -Isidis basin
interrupts outer Utopia ring -Thus, Utopia impact
must have occurred in early Noachian.
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Multiple Impacts Identified
Borealis Impact Basin (Wilhelm Squyres, 1984)
Elysium Basin (Schultz, 1984)
Utopia Impact Basin (McGill, 1989)
Tharsis Basin (Schultz Glicken, 1979)
Isidis Impact Basin (Tanaka, 1986)
North Polar Basin
Chryse Impact Basin (Schultz et al, 1982)
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An Evolving Model

• The problem with the single Giant Impact
  hypothesis (Borealis) is that it cannot account
  for the irregular shape of the northern lowlands
  by itself.
• The problem with the Multiple Large Impact
  hypothesis (Utopia Chryse Isidis, etc.) is
  that there should be towering piles of ejecta
  between the large basins (as there is around
  Hellas).
• It at least seems reasonable that multiple
  impacts are involved, since weve found multiple
  basins.
• Further discovery of northern plain impact basins
  would be helpful, but seemed difficult before
  MOLA.

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Crustal Thickness

• Zuber (2001)
• MGS topography and gravity measurements show
  that, in general, the Martian crust is thinner in
  the northern lowlands than in southern highlands.

-It is worth noting, however, that the crustal
thickness dichotomy does not exactly match the
topographical dichotomy (ex Arabias
Terra). -Zuber states that, if the northern
depression formed as a result of impacts, it must
have occurred very early in martian history (IE
before Utopia) and subsequent processes must have
modified the topographic crustal thickness
signatures.
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Quasi-Circular Depressions

• Frey, et al. (2002)
• MOLA reveals a very large population of
  Quasi-Circular Depressions (QCDs) many shallow
  (100s of meters vs. 1.6 km for a 50 km
  diameter visible impact crater) basins in
  Northern Lowlands which have no visible
  expression in Viking or MOC images.
• Frey, et al. interprets these newly discovered
  objects as impact craters which have been long
  buried by the northern plains material.

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• Out of 644 QCDs larger than 50 km, only about 90
  are visible impact craters.
• But they are well distributed, supporting the
  idea that they are impact basins.
• Based on Garvin et al. 2000, overlying cover must
  not exceed 5-6 km or basins would not reveal any
  relief. Most are probably buried under 1.5 km of
  younger crust.

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QCDs imply very old Lowlands

• Frey et al compares crater counts of the QCDs in
  the lowlands to QCDs and visible craters in the
  highlands, again using the empirical 1/D2 law to
  fit.
• Finds that visible lowland impacts lie on lower
  line, consistent with Hesperian resurfacing
• But there are more lowland QCDs than highland
  visible impacts!
• Implies that the buried lowlands are about the
  same age as the highlands. The Northern Lowlands
  were formed during the early Noachian, and have
  been around for most of the history of Mars!
• Constrains formation of dichotomy to mechanisms
  that operate quickly and early. Large scale
  impacts near end of accretion (early Noachian)
  fit the bill.
• QCDs superimposed on Isidis/Utopia. Thus Utopia
  and Isidis pre-date QCDs, and are some of the
  oldest features on Mars.
• If internal mechanisms like mantle convection
  caused lowland formation, they had to have
  operated very early (ie before Utopia)

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Conclusions

• The case for the northern lowlands being caused
  by impacts has been criticized, but not ruled
  out.
• Recent evidence supporting the idea that the
  lowlands have been low since the early Noachian
  lends favor to possibility of large impacts as a
  cause. (Frey et al 2002)
• Remains a viable option for creation of the
  hemispheric dichotomy.
• As in many cases in science, the real solution is
  probably a combination of multiple mechanisms.

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References

• Wilhelms, D. E. Squyres, S. W. The Martian
  hemispheric dichotomy may be due to a giant
  impact. Nature 309, 138140 (1984).
• Tanaka, K. L., The stratigraphy of Mars, Proc.
  Lunar Planet. Sci. Conf. 17, J. Geophys. Res.
  Suppl., 91, E139E158, 1986.
• Frey, H. V., and R. A. Schultz, Large impact
  basins and the mega-impact origin for the crustal
  dichotomy on Mars, Geophys. Res. Lett., 15,
  229232, 1988.
• McGill, G. E., Buried topography of Utopia, Mars
  Persistence of a giant impact depression, J.
  Geophys. Res., 94, 2753 2759, 1989.
• Smith, D. E., et al., The global topography of
  Mars and implications for surface evolution,
  Science, 284, 14951502, 1999.
• Fortes, A.D., Origin of the Martian Hemispheric
  Dichotomy. Department of Geological Sciences,
  University College, London. 1999.
• Garvin, J. B., S. E. H. Sakimoto, J. J. Frawley,
  and C. Schnetzler, North polar region craterforms
  on Mars Geometric characteristics from the Mars
  Orbiter Laser Altimeter, Icarus, 144, 329 352,
  2000.
• Zhong, S., and M. T. Zuber, Degree-1 mantle
  convection and the crustal dichotomy on Mars,
  Earth and Planetary Science Letters, 189, 75 84,
  2001.
• Zuber, M.T., et al., The crust and mantle of
  Mars. Nature 412, 220 227 (2001)
• Frey, H., et al., Ancient lowlands on Mars.,
  Geophys. Res. Lett, Vol. 29, No. 10, 2002.

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