Tharsis

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Tharsis
Hellas
Argyre
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Outline - True Polar Wander (TPW) Causes and
consequences - TPW on ancient Mars? - TPW on
Late Hesperian/Amazonian Mars - Kite et al.,
Earth Planet. Sci. Lett., 2009 - Effects of TPW
on climate - Preliminary results from
Berkeley/Ames collaboration
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Outline - True Polar Wander (TPW) Causes and
consequences - TPW on ancient Mars? - TPW on
Late Hesperian/Amazonian Mars - Kite et al.,
Earth Planet. Sci. Lett., 2009 - Effects of TPW
on climate - Preliminary results from
Berkeley/Ames collaboration
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What is TPW?
True polar wander is the movement of a planets
entire lithosphere with respect to the spin axis
in response to changes in mass distribution.
Over geologic time, energy dissipation within a
planets viscous interior will align the spin
axis with the maximum moment of inertia.
To drive TPW, the load must be (at least partly)
uncompensated. Either supported by the elastic
strength of the lithosphere, or dynamically
supported by mantle motions.
Figures courtesy Isamu Matsuyama
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Positive loads go to the equator e.g., Tharsis
bulge Negative loads go to the
poles e.g., Enceladus plume terrain
Characteristics of TPW
TPW response timescale ltlt 3 Gyr Will assume the
'fluid limit' for Mars part of the talk.
NASA/JPL
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TPW in Earth history - Earth has a thin
lithosphere, so surface loads are almost
completely compensated. - Paleomag curves from
multiple plates discriminate between TPW and
continental drift
Snowball Earth precursor tied to inertial-intercha
nge TPW event ??
Steinberger Torsvik, Nature, 2008
TPW has played second-fiddle to plate tectonics
over the last 250 million years, possibly
because of the stabilizing influence of
long-lived superplumes
800 Mya Bitter Springs excursion Maloof et al.,
GSA Bull., 2006
On Earth, TPW has played a secondary role to
plate tectonics. But, on planets without plate
tectonics,TPW comes to the fore.
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Outline - True Polar Wander (TPW) Causes and
consequences - TPW on ancient Mars? - TPW on
Amazonian Mars - Kite et al., Earth Planet.
Sci. Lett., 2009 - Effects of TPW on
climate - Preliminary results from
Berkeley/Ames collaboration
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TPW on ancient Mars
Idea that equatorial, antipodal layered deposits
define a paleo-spin axis has a long history
Schultz Lutz, Icarus, 1988Niles Michalski,
Nature Geoscience, 2009
Boynton et al., JGR-Planets, 2007
Two major problems 1) High obliquity can
explain these observations! We know high
obliquity occurred no need to appeal to
TPW. 2) Tharsis limits TPW subsequent to Tharsis
formation.
Mischna Richardson, Granada Workshop, 2006
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Tharsis dominates the Martian geoid
The bulk of Tharsis was emplaced before the
valley networks. Any subsequent TPW that moves
Tharsis away from the equator will be strongly
resisted. However, TPW along the great circle
90 from Tharsis is still possible, because it
will leave Tharsis on the equator.
Phillips et al., Science, 2001
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Can TPW explain the mismatch between Mars ocean
shorelines and equipotentials?
Perron et al., Nature, 2007
??Shorelines proposed using Viking imagery
Parker et al., JGR Planets, 1993 ??MOLA
topography showed shorelines do not define an
equipotential, as expected for a coast ? Perron
et al. (Nature, 2007) showed that TPW can restore
the shorelines to an equipotential - and that
the best-fit TPW scenario lies close to the only
permissible TPW great circle! (90 º from
Tharsis) ??However, high resolution images cast
doubt on Parker's interpretation Malin
Edgett, GRL, 1999 Carr Head, JGR, 2003 The
ocean question is still unresolved, but the
burden of proof still lies with the ocean
hypothesis.
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Outline - True Polar Wander (TPW) Causes and
consequences - TPW on ancient Mars? - TPW on
Late Hesperian/Amazonian Mars - Kite et al.,
Earth Planet. Sci. Lett., 2009 - Effects of TPW
on climate - Preliminary results from
Berkeley/Ames collaboration
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TPW on Late Hesperian/Amazonian Mars
Kite et al., EPSL, 2009
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Elements of our hypothesis test- 1) Determine
young (Late Hesperian/Amazonian) volcanic
load. 2) Calculate net TPW response.
3) Compare with observations of
paleopolar deposits.
N.B. Not our paper!
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1) Determine young volcanic load.
We know area how do we get thickness?
Gravity/topography McGovern et al., 2004
Belleguic et al., 2005 Crater size-frequency
distribution Lunar maria Heisinger Head,
2002 Partially flooded craters Pre-Apollo
observations of maria Spectral signatures of
ejecta / central peaks Lunar Prospector UVVIS
OMEGA CRISM Geological backstripping e.g. Dohm
et al., 2001 Large uncertainties!
Kite et al., EPSL,2009
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Volcanic load recent developments
Radar sounding with SHARAD Campbell et al.,
JGR, 2008
Putzig et al., LPSC, 2008
Improved crater size-frequency estimates
Werner et al., Icarus, 2009
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2) Calculate TPW response.
Includes fossil rotational bulge elastic
lithosphere 2 cases Tharsis forms close to and
far from the equator.
For each response calculation Given lithospheric
thickness, young load mass x,y and fossil
bulge orientation, find the post-Tharsis, Pre-youn
g load pole position that minimizes the
deviation from present-day spin-axis orientation.
Geologically Insignificant
Geologically Significant

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What records polar wander? Paleopolar deposits
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The record of paleopolar deposits
S Polar region
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The record of paleopolar deposits
N Polar region
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Calculated TPW response to loading by individual
volcanic provinces
S Polar region
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Comparison with TPW tracks
A small load B medium load C large load
a small TPW b 100 km Te c 200 km Te d
300 km Te
Tharsis
S Polar region
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A small load B medium load C large load
Comparison with TPW tracks
a small TPW b 100 km Te c 200 km Te d
300 km Te
Tharsis
N Polar region
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Summary of results ??The hypothesis of
late-stage TPW survives our test. ?? Required
magnitude of volcanism is reasonable near the
low end of previous thermal evolution modelling
work, well below Viking-era young volcanism rate
estimates Very close to new ( independent)
estimates from Mars Express data ? Geologically
significant late-stage TPW requires that
Tharsis forms far from the equator.

• ??Geologic data and TPW theory are consistent
  with 8 deg of net TPW between Dorsa Argentea
  Formation time and the present day.
• ??This event shifts the boundary conditions for
  pre-TPW climate. We predict that this shift
  should have left recognizable geomorphic (and
  possibly tectonic) signatures.

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Outline - True Polar Wander (TPW) Causes and
consequences - TPW on ancient Mars? - TPW on
Late Hesperian/Amazonian Mars - Kite et al.,
Earth Planet. Sci. Lett., 2009 - Effects of TPW
on climate - Preliminary results from
Berkeley/Ames collaboration
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Outstanding Post-Hesperian Climate Puzzles
1) Asymmetry in 600m-baseline roughness
signature Crosscuts more recent latitudinal
belts which track latitude (MOLA spot IQR,
gullies, dissected mantle) Much older than
polygonal mantle (Kreslavsky Head, GRL,
2002) Overall latitudinal trend demands a
smoothing process sensitive to insolation (via
some combination of temperature and ice
stability) Consistent with Dorsa Argentea
Formation paleopole 2)
Ventifacts at Mars Pathfinder LZ are
misaligned with present-day winds Cannot be
explained by obliquity (Haberle et al., Icarus,
2003)
Kreslavsky Head, JGR, 2000
Kreslavsky Head, JGR, 2000
Greeley et al., JGR, 2000
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Ongoing research NASA Ames GCM run with
paleopolar topography by collaborators Fenton,
Kahre Hollingsworth
Results are backrotated to present-day
coordinates for comparison with geologic
observations (ventifacts, yardangs, and climate
belts).
1) Prediction If the smoothing process
responsible for the latitude-dependent roughness
signature was thermally activated, then
backrotated pre-TPW temperature contours should
be parallel to the edge of the smooth
terrain. Two classes of thermally activated
processes involving ground ice i) Ice creep,
sensitive to mean annual temperature -
Perron et al., GRL, 2003 Parsons Nimmo, JGR,
2009 ii) Gelifluction, periglacial solifluction
sensitive to maximum temperature Alternative
Smoothing process controlled by presence/absence
of ice, not ice flow
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Conclusions
??The hypothesis of late-stage TPW survives our
test, provided that Tharsis formed far from the
Equator. If Tharsis formed close to the
Equator, surface loads are probably insufficient
to drive geologically significant
TPW. ??Because TPW shifts the boundary
conditions for pre-TPW climate reconstruction,
it is worth simulating climate under pre-TPW
conditions. To see if TPW can account for
certain climate anomalies. These simulations
are underway. Further details Kite, E.S.,
Matsuyama, I., Manga, M., Perron, J.T.,
Mitrovica, J.X. (2009), True Polar Wander driven
by late-stage volcanism and the distribution of
paleopolar deposits on Mars, Earth Planet. Sci.
Lett. 280, 254-267
http//archive.seti.org/pdfs/csc-May20.pdf
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Backup slides
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Model of Zhong et al., Nature Geoscience,
2009 (net lithospheric rotation)
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Langlais Quesnel, Comprtes Rendus, 2008
It has also been suggested that magnetic
maps define paleopoles, but better maps make
this unlikely.
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Meteorological factors
Colaprete et al., Nature, 2005
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Did Tharsis form close to or far from the
equator?
Daradich et al., Icarus, 2008