Mars: The Middle Years

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Mars The Middle Years
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Summary of the Noachian

• Mars forms
• Accretion and core formation in about 13 Ma
• Crust forms from magma ocean
• ALH84001 crystallizes at 4.5 Ga
• Crust develops asymmetry
• Perhaps due to degree-1 mantle convection
• Or very large impact(s)
• Core-Dynamo switches off
• Magnetic remnants frozen in to crustal rocks
• Major impact basins form
• Both hemispheres are heavily cratered
• Remnant magnetism erased over large basins

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• Tharsis rise is constructed
• Vigorous volcanism outgasses significant
  atmosphere
• Polar wander
• Valley networks form
• Orientation controlled by pole-to-pole slope and
  Tharsis bulge
• Erosion rates orders of magnitude higher than
  Hesperian or Amazonian epochs
• Strong greenhouse needed to offset faint young
  sun
• Lack of carbonates from greenhouse atmosphere
  still unsolved

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In this lecture

• Hesperian epoch
• What?
• When?
• Hesperian volcanism
• Plains and Paterae
• Hesperian tectonics
• Wrinkle ridges
• Tharsis extension
• Changing planet
• Heat flow
• Atmospheric removal
• Evidence for environmental change
• Fluvial deposits at the Noachian/Hesperian
  boundary
• Evaporites
• Fans/Deltas
• Global water behavior
• Oceans
• Cryosphere
• Outflow channels

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• When is the Hesperian?
• Starts 3.5-3.7Ga
• Ends 2.0-3.1 Ga

Hartmann, 2005

• Whats important in the Late Noachian/Hesperian/Ea
  rly Amazonian?
• Volcanic resurfacing at a maximum
• Hesperian volcanic plains
• Fluvial resurfacing at a maximum
• Outflow channels
• Impacts much lower than Noachian
• Eolian activity low

Tanaka et al., 1992
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Hesperian Volcanism

• Extensive plains volcanism
• with subsequent wrinkle ridges
• Northern plains resurfaced
• 1km of fresh basalt
• Noachian impact crater population covered up
• Large amount of SO2 released
• Combines with water to form acid rain
• Shift from phyllosilicate to sulfate deposits

Head et al., 2002
Montési and Zuber, 2003
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• Paterae Volcanism
• Low relief
• Very low slopes lt1 degree
• Highland paterae - late Noachian
• Extensively dissected
• Easily erodible material consolidated ash

Tyrrhena Patera
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Hesperian Tectonics

• Growth of Tharsis causes circumferential
  extension
• Annulus of extensional features
• 1000s km in extent
• Opening of Valles Marineris
• Radial to Tharsis
• Extension stresses begin rift
• Unclear why most extension is in one place
• Another mantle plume under Valles Marineris?
• Canyon subsequently widened by landslides
• Later filled with layered deposits
• Probably from paleo-lakes

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A Changing Planet

• Early greenhouse atmosphere quickly removed
• Warm-wet transition to cold-dry
• Planetary heat flow declines
• Lithospheric thickening

Haberle, 1998
Montési and Zuber, 2003
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A Changing Planet

• Early greenhouse atmosphere quickly removed
• Warm-wet transition to cold-dry

Haberle, 1998
Lots of fluvial erosion
Not much fluvial erosion
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Evidence for change

• Hesperian lava plains are undissected
• Noachian terrains are highly dissected by valley
  networks
• Gusev crater basalts are not weathered at all
  (spirit rover)
• But the Columbia Hills rocks (older) are e.g.
  hematite, goethite, nanophase oxyhydroxides

Bibring et al., 2006

• OMEGA mineralogical results
• Clay formation ceases
• Transition to acidic environment to from sulfates
• Also requires evaporation
• Young terrains show no aqueous alteration

• Presence of olivine on the surface

• Erosion rates are drastically reduced

Christensen et al., 2003
Golombek and Bridges, 2000
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Ocean

• Original shorelines mapped by Tim Parker from
  Viking data in late 1980s
• One of the contacts he identified is close to an
  equipotental surface
• Makes sense since all surface water will drain
  into the low-lying northern hemisphere
• Still a lot of disagreement on whether to believe
  this
• No obvious shoreline features in high resolution
  imagery
• but
• Ocean may have been ice-covered
• Subsequent volcanic constructs have altered the
  shoreline elevation

Head et al., 1999
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• Polar wander can warp the shape of the planet
• Redistribute centrifugal forces
• Explains much of the long-wavelength shoreline
  topography

Perron et al., 2007
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• Shoreline erosional features are subtle
• Recent search for depositional features turned up
  nothing (Ghatan and Zimbelman, 2006)
• Effects of ice cover?

Clifford and Parker, 2001
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Cryosphere

• Mega-regolith ensures crust is porous to great
  depth

Clifford and Parker, 2001

• Mars starts freezing water in pore space
• Declining heat flux
• Environmental change
• Early ocean freezes over

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• What happens next?
• No ice covered ocean in the northern lowlands
  today

Clifford and Parker, 2001
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Outflow Channels

• Huge flood carved channels
• Contains streamlined Islands
• Similar to channeled scablands in Washington
• Likely that a large underground reservoir emptied
  catastrophically
• Source region collapses to chaos terrain
• Flood empties into northern lowlands

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• Terrestrial analogue
• End of the last ice-age
• Glacial lake Missoula- Ice-dam breaks

Channeled scablands, Washington
Outflow channel, Mars
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• Subterranean water table is higher than ocean
  surface
• Outflow occurs if cryosphere fails
• Cryosphere continues to thicken over time
• Failure become less and less likely
• Possibly no liquid water left beneath the
  cryosphere today
• Deep aquifer model for gully formation relies on
  expanding cryosphere pressurizing groundwater
• Important shift for Mars

Clifford and Parker, 2001
Pervasive fluvial activity transitions to
isolated outburst events
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Polar activity

• Evidence of first icy polar deposits
• Dorsa Argenta formation
• Deflated ice-sheet
• Eskers too big?
• Crater overflow channels
• Sub-glacial volcanoes
• Flat-topped morphology
• Initiates basal melting

Head and Pratt, 2001
Ghatan and Head, 2002
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• Features look like eskers a little large though
• Drainage toward Argyre basin and from there
  through Holden crater to the northern plains

Head and Pratt, 2001
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• Interconnected drainage basins point to water
  transport from lake to lake

Parker et al., 2000
Irwin et al., 2004
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Water at the Noachian-Hesperian Boundary

• Valley networks indicate surface water in the
  Noachian
• Inevitable to have this water pool into craters
  and low areas

• Ancient sedimentary rock layers found in craters
• Indicate active past
• Possible paleo-lake environments
• OMEGA indicates light toned layered deposits are
  sulfate rich in composition
• Implies evaporites

Bibring et al., 2006
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• E.g. Gale crater mound
• Clay minerals in Noachian
• Wet alkaline conditions
• Sulfates in Hesperian
• Drier acid conditions

Bibring et al., 2006
Milliken et al., 2010
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Evidence of sustained flow

• Distributary fans
• Indicate lengthy fluvial processes
• May have discharged into a lake (delta) or dry
  crater (alluvial fan)

Jerolmack et al., 2004
Lewis and Aharonson, 2006
Moore et al., 2003
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• Flow rates
• Moore et al. 700 m3s-1
• Delta forms slow (103-106 years)
• empirical meander wavelength/flow-rate
  relationship
• Jerolmack et al. 410 m3s-1
• Fan forms fast (10-102 years)
• numerical model of alluvial fan construction
• Lewis and Aharonson measured layer dips and
  argued against a progradational delta
• i.e. water level was not static

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Summary

• A time of transition away from pervasive fluvial
  activity to cold/dry conditions

• Change in alteration chemistry
• Phyllosilicates?Sulfates?Anhydrous ferrous
  chemistry
• Erosion rates drastically reduced
• Reduction in atmospheric greenhouse ? less
  available water
• Liquid water turning solid
• First evidence of polar ice caps
• Thickening cryosphere
• Water appears in flood outbursts rather than
  being pervasively present

Massive volcanic resurfacing and tectonic activity

• Plains volcanism resurfaces large areas
• Atmospheric infusion of SO2 may change chemical
  alteration of the surface
• Paterae volcanoes
• Pervasive wrinkle ridge formation
• Circum-Tharsis extension
• Opening of Valles Marineris
• Late Hesperian/Early Amazonian building of the
  big Tharsis shield volcanoes