Io and Europa

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Io and Europa
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In this lecture

• Io and Europa
• Formation
• Tidal interactions
• Heating
• Differential rotation
• Internal structure
• Oceans and heat-flow
• Surfaces ages
• Surface compositions
• Geology of Io
• Volcanism
• Paterae
• Shields
• Eruption styles
• Magma composition
• Tectonics
• Mountains
• Fissures
• Geology of Europa

Voyager I II 1979
Galileo 1995-2003
Bummer!
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The Jovian System

• Jupiter
• Fast rotating, strong magnetic fields
• Very high radiation environment
• and its friends
• Faint gossamer ring system
• Irregular satellites
• Himalia group - Prograde
• Carme, Ananke and Pasiphae groups - Retrograde
• Regular satellites
• Amalthea group inner moons
• Galilean satellites

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Formation

• Jupiter accretes as large rock/ice core
• Begins runaway gas capture when core 10 ME
• A miniature solar system
• Jupiter develops accretion disk
• Gravitation contraction generates heat
• Jovian disk has a temperature and density gradient

Phil Armitages internet page.

• Solids in Jovian disk mirror the solar nebula
• Composition of Galilean satellites close to
  asteroids

• Ice
• not stable at Ios location form Mars-like body
  instead
• barely stable at Europas location
• lots of ice for Ganymede and Callisto
• Formation timescales slower further away
• Gravitational PE release diluted over formation
  time
• Heat not generated fast enough to differentiate
  Callisto

Canup and Ward, 2002
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Tidal interaction

• Eccentric orbits tides heating
• Satellite rotation cannot be synchronous
• Bulge position moves around surface causes
  deformation and heating
• Satellite distance varies
• Size of bulge varies causes deformation and
  heating
• Repeated squeezing can cause a lot of energy
  dissipation

• Eccentricity get damped down by tidal dissipation
• So what up with Io?
• Still getting tidally pumped because e?0
• Io is in a 21 resonance with Europa
• Europa is in a 21 resonance with Ganymede
• Laplace resonance
• Energy transfer between the Moons keeps Ios e
  high
• Ios volcanism predicted just weeks before
  voyager encounter
• Slow leakage of energy out of the system
• Orbital energy is passed from Moon to Moon

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Internal structure

• Moments of inertia indicate differentiation
• Mean density indicates terrestrial-planet like
  compositions

• Io core size depends on sulfur content
• Europas shell inferred to be about 100km thick

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Geissler et al., 1998
Europa ocean

• Non-synchronous rotation
• Cracks dont agree with current stresses
• Shell appears to have drifted 60o
• Expected from tidal interactions
• Mantle and core likely synchronously
• locked outer shell moves independently
• i.e. shell and mantle separated by low-strength
  layer
• Chaos regions
• Apparent melt-through areas
• Magnetic data
• Jupiters field induces currents
• Currents create weak magnetic field
• Modeling of magnetometer data suggests a shell of
  weakly conducting material 100 km deep

• Ice III layer under the ocean?
• 0.12 GPa

Khurana et al., 1998
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Ios heat

• Ios heat flux estimated at 2500 mW m-2
• Earths average 60-70 mW m-2
• Mostly concentrated at hot-spots
• Plains are cool (surface Temp 90 K)
• Resurfacing rate of 1cm/yr
• Hot material can be buried
• Loki dominates heat flow
• 200km lava lake that periodically (540 days)
  overturns
• Solar systems most powerful volcano
• Mantle inferred to be mushy
• From magma composition and temperature
• Large amount of partial melt
• Perhaps isothermal if convecting
• Heat flow gt heat production
• Excess energy from somewhere?
• Io spiraling into Jupiter?
• Tidal dissipation was higher in recent past?
• Ios Q 120 today
• Lithosphere

Keszthelyi et al., 2004
Keszthelyi et al., 2004
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Surface Ages

• Dynamical simulations of cratering rates
• Jupiter family comets dominate
• Ages not scaled from dated lunar samples
• Expect a 10 km crater every few Myr
• Uncertainty is a factor of 3
• Io
• Essentially no craters
• Resurfacing rate estimate at 1cm yr-1
• Buries a 10km crater (2km deep) in 200,000 yrs
• Europa
• 24 craters gt 10km identified
• Mean surface age about 60Myr inferred
• Small craters
• Over abundant
• Clustered
• Cumulative size-frequency power law slope gt3
• Most likely small craters are
• dominated here by secondaries

Zahnle et al., 2003
Bierhaus et al., 2005
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Surface Composition - Europa

• Substantial differences in overall spectra
• Water ice dominates Galilean satellites except
  Io
• Dark patches contain impurities
• Hydrated salts
• Sulfates
• Possibly carbonates, sulfuric acid
• Radiolysis drives chemistry
• Jupiters intense radiation belts
• Sulfur combines into long chains to give reddish
  colors
• Sulfur probably comes from Io
• Youngest terrains are darkest
• Contain the most impurities
• Surface brightens again with age

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Surface Composition Io

• Surface composition dominated by sulfur and SO2
• Vivid colors come from allotropes of sulfur
• Color units
• 40 Yellow cyclo S8 (orthorhombic)
• 30 Red/Orange short chain sulfur S3 and S4
• Concentrated poleward of 30 degrees
• Breakdown of cyclo S8
• 27 white/grey Coarse to medium grained SO2
• 3 black ultramafic basalt
• Mg rich orthopyroxene
• Concentrated around lava lakes and flows
• Small greenish-yellow patches
• Contamination by iron or olivine
• Composition of plumes is SO2
• Plume material spreads out to form Io plasma
  torus

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Surface Geology Io

• Three surface types
• Broad flat layered plains
• Pyroclastic covering
• Volcanic terrain
• Paterae caldera-like depressions
• Flucti lava flow fields
• Tholi shield volcanoes (not common)
• Mountains
• Anti-correlated with volcanic centers

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Io Mountains

• Defined as being over 1km in height
• 135 found, 104 definitely tectonic
• Average height 6km, max height 17km
• Steep sided with asymmetric shape
• Uplifted crustal blocks from compression
• Tidally generated
• Lithosphere is at least as thick as mountains are
  high
• Distribution
• Cluster near equator but no global pattern
• Near longitudes of 65 and 265 W
• Appears 90 degrees out of phase with volcanics,
  but
• Local association between tectonics and volcanics
• 40 of tectonic mountains (out of 104)
  associated with volcanic Paterae
• 13 of paterae (out of 415) associated with
  mountains

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Io Volcanoes

• Extreme local surface changes
• Continuous reworking of surface markings
• But plains material is stable
• 90 of Io has not changed since Voyager
• Eruption styles
• Pillanian
• Explosive, intense, short-lived
• Extensive lava field
• Extensive dark pyroclastic deposits 2x105 km2
• Silicate composition (Mg rich)
• Promethean
• Long-lived, less-intense
• Dominated by flow fronts
• Prometheus plume has moved 80km in 20 years
• Intra-Patera (Lokian)
• Confined to calderas
• Overturn of thin cooling crust

Spencer et al., 2007
Williams and Howell, 2007
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• Lava Lakes plume rises through mushy mantle
• Melting of SO2 to from collapse caldera
• Paterea thought to contain cooled lava lakes
• Largest is Loki
• Only a handful of terrestrial volcanoes have
  permanent lava lakes
• Periodic overturn
• Loki period of 540 Days

Nyiragongo, Zaire
Williams and Howell, 2007
Rathburn et al., 2004
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Magma Composition

• Long pre-Galileo debate about sulfur vs silicate
  volcanism
• Initial measured temp.s were low
• Sulfur volcanism lt 700K
• Silicate volcanism 1300-1450K
• High temperatures detected by Galileo confirm
  basaltic volcanism
• Composition appears Mg-rich ultramafic
• Probably dominated by Olivine and Pyroxene
• Some flows observed at 1800K !
• Stereo topography shows magma had very low
  viscosity
• Secondary melting of Sulfur country rock
• Komatiites?
• Mg rich very high (1800K) melting Temperture
• Very fluid viscositywater
• Occurred on Earth in the Archean
• Implies that Io is basically erupting mantle
  material
• Problem?
• Why arent Ios magmas more evolved (higher
  Silica)
• Time to cycle all of Io through volcanoes 61
  Myr
• Why doesnt Io have a crust? mantle appears to
  extend to the surface?

Spencer et al., 2007
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Surface Geology Europa

• Europa has two terrain types
• Ridges plains Tectonic
• Mottled material Volcanic
• Surface is dominated by tectonics
• Tectonics
• Ridges double and cycloidal
• Troughs and cracks
• Extensional bands
• Volcanism
• Lenticulae
• Chaos
• Future evolution?
• Paradigm of Europas geology
• Ocean
• Overlain by warm ice
• Overlain by brittle ice
• Viscous relaxation of landforms

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• Double ridges most common landform
• V-shaped groove in center
• 0.5-2km wide
• 1000s km long
• Surface texture preserved on slopes
• Favourite model is frictional heating from
  strike-slip faulting
• Warm ice rises buoyantly to form the ridges
• Melting in fault plane causes drainage of liquid
  and subsidance

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• Cycloidal cracks
• Extensional features
• Chain of distinct arcs
• Tidal explanation
• Direction and magnitude of tidal force changes
  throughout Europas day
• Force is strong enough to crack ice when Europa
  is close to Jupiter confined to sub-Jovian
  point and antipode
• Change in tidal direction cause change in
  direction of propagating crack
• Cracks propagate at about 3 km hr-1
• Stresses are low (max 40 kPa) so ice must be weak
• Good fit for initiation strength of 25 kPa,
  propagation strength of 15 kPa
• Tensile stresses overwhelmed by compressive
  pressure at depths of 65m

Hoppa et al., 1999
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• Pull-apart Bands
• Enormous amounts of extension
• Pre-extension topography can be recreated
• Inward facing parallel fault scarps
• Some pull-aparts start as cycloidal cracks
• Supposedly shallow!

Prockter et al., 2002
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• Analogous to seafloor spreading on Earth

Prockter et al., 2002
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• Triple bands
• Alternating dark/bright/dark bands
• Fissure eruption through trough in ridge center?

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Volcanism Europa

• Lenticulae
• Spots of high or low elevation
• Low elevation spots preserve past topography
• Often discoloured (probably by salts)
• Related to convection in icy shell
• If plume breaks through then plume material is
  deposited
• If not, then material withdraws and collapse
  feature forms

Pappalardo et al., 1998
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• Chaos
• Regions of mottled terrain
• Still contain islands or previous crust
• Essentially a big lenticula
• Material has disappeared
• No net extension of contraction

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Europas unsolved problems

• Lots of extension
• Wheres the corresponding compression?
• Planet is expanding from freezing ocean?
• When ice thickens heat conduction slows down
• start convection in ice layer
• Stratigraphy shows transition from ridges to
  lenticulae and chaos
• but record is only 60Myr long surely this
  isnt a once off?
• Cyclic on timescales of 100Myr?

Ios unsolved problems

• Heat budget doesnt match expectations
• Io loosing more heat than tides should be
  producing
• Perhaps Io wasnt in this resonance for long?
• Perhaps we dont understand tidally heating as
  well as we think we do
• Composition of Magmas
• Why arent they more evolved
• Io can be cycled through its volcanoes in only 61
  Myr
• Perhaps tidal heating hasnt been in effect very
  long?

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Summary

• Io and Europa
• Formation
• Tidal interactions
• Heating
• Differential rotation
• Internal structure
• Surfaces ages
• Surface compositions
• Geology of Io
• Volcanism
• Paterae
• Shields
• Eruption styles
• Magma composition
• Tectonics
• Mountains
• Fissures
• Geology of Europa
• Tectonics