Europa: Estimates of Ice Layer Thickness

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Europa Estimates of Ice Layer Thickness

• Paul J. Thomas, Marc R. Goulet, Alex J. Smith,
  Anna Kindt, Derrick G. Whitelaw, Jon Mitchell,
• Christopher F. Chyba?
• Departments of Physics and Astronomy and
  Mathematics, UW-Eau Claire
• ?SETI Institute, Mountain View, CA

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Life in the Solar System?
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Missions to Europa

• Voyager 1 and 2, 1979
• 50 of surface at 0.5-20 km/pxl
• Galileo, 1995-2000
• Selected areas at 20m/pxl
• Europa Orbiter, 2006
• Imaging of entire surface to 20 m/pxl
• 50 MHz radar sounder
• Europa Lander, gt2010?
• Surface analysis

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The First Human Mission - 2045
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Galilean Satellites
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Europa Bulk Properties

• R1565?8 km
• ?3.01 g/cm3
• T0100 K
• 1-2 ?m absorption ? H2O ice surface (Kuiper,
  1959)
• H20 layer lt100 km (density constraint)
• 124 orbital resonance with Io and Ganymede

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(No Transcript)
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Tidal Heating Model

• Rate of tidal energy dissipation in a
  synchronously rotating satellite (Peale and
  Cassen, 1978)
• Time varying tidal bulge 30 m (ocean) or 1 m
  (no ocean).

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Tidal Heating Model

• For both core and decoupled shell, tidal heating
  rate is
• Q dissipation function
• (1/Q tidal phase lag in radians)
• Q ? 100-25 for both core and shell
• tidal heating 5-20 ? radiogenic heating

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Geology

• Lineaments
• Flood features
• Chaos Terrain
• Craters
• Flexi

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Lineaments
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Ridges and Floods
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Conamara Chaos
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Pywll Crater
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Pwyll Crater
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Flexi

• Periodic flexing during one revolution (3.551 d)
  produces cycloid cracks (Hoppa et al., 1999)
• Propagation speed 2.4 km/h
• Requires thin elastic ice layer decoupled from
  interior

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Inside Europa

• Thick Ice or Internal Ocean
• Models are unstable towards both thin and thick
  ice models
• Geological models are inconclusive
• Magnetic field evidence implies existence of ocean

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Crater Relaxation Constraints

• Viscous relaxation of impact craters is
  controlled by ice viscosity and the lower
  boundary of the ice.
• A thin (25 km) rigid based ice layer will
  retain impact craters for gt108 y for all
  plausible rates of tidal heating.

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Age of Europas Surface

• 90 of impactors are short-period Jupiter family
  comets
• Impact speeds and fluxes are increased by
  proximity to Jupiter
• at Europa, impact speeds are 25 higher and
  impact fluxes 4? values at Callisto
• Impact-based age of surface 10 Myr (Zahnle et
  al., 1999).

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Melt Through Events

• Chaos regions appear to preserve record of
  catastrophic melt through of ice crust
• Melt through could occur as a result of
• localized tidal heating
• impact by large comet
• Ice crust is self-healing (Lvap7Lh)
• How long will it take for crust to refreeze?

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SPH Impact Simulations

• 3-D particle hydrocode with EOS data for ice,
  water
• Impactors will frequently penetrate 10-20 km ice
  layers

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SPH Impact Simulations
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The Stefan Problem

• Freezing of solid layer above liquid.
• No heat generated except for release of latent
  heat Lh.
• Solve for ice thickness vs. time.

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Model Requirements

• Must be time-dependent
• Incorporate heat generation in ice shell q(x) and
  basal heat flow H
• Allow for variations in k(T), c(T), ?(P, T), ?(P,
  T)
• Incorporate phase transition at moving boundary
  for melting and freezing

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Numerical Freezing Model
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Phase Change Algorithm

• Melting
• Test lowest layer for
• If yes, apply excess heat to phase transition.
• When cumulative heat latent heat for layer
  volume, remove layer.

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Phase Change Algorithm

• Freezing
• Test lowest layer for
• If yes, create new layer with temperature held at
  Tm and apply deficit heat to phase transition.
• When cumulative heat latent heat for layer
  volume, add new layer.

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Numerical vs. Analytical Result
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Freezing of Ice Layer
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Viscoelastic Rheology

• Maxwell viscoelastic rheology
• Maxwell time
• temperature dependent viscosity

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Viscoelastic Shell Heating

• Volumetric heating rate is highest where strain
  rates are greatest (Ojakangas and Stevenson, 1989)

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Temperature Gradients
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Sounding for Water on Europa
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Challenges of Europa IPR

• Radar sounder maximum depth 10 km.
• Cannot assume layered geometry.
• Convection onset for ice layers 25 km may make
  them very thick.