Ganymede and Callisto

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Ganymede and Callisto
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In this lecture

• Ganymede and Callisto
• Formation
• Tidal interactions
• Internal structure
• Differentiation
• Oceans and heat-flow
• Surface compositions
• Craters surfaces ages
• Geology of Callisto
• Impact structures
• Mass wasting
• Geology of Ganymede
• Dark terrain
• Impact structures
• Furrows
• Bright terrain
• Smooth bright terrain
• Grooved bright terrain

Callisto-like
Europa-like
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Galilean Satellites
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• Io
• Volcanism
• Tectonics
• No impact craters
• Europa
• Tectonics
• Subsurface ocean
• Few impact craters
• Ganymede
• Everything
• Tectonics, impact basins, Differentiated
  interior, magnetic field, sub-surface ocean,
  polar volatiles etc etc
• Largest solar system moon only Earth, Mars and
  Venus are larger
• Callisto
• Impacts craters
• Not fully differentiated

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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

• Ganymede
• e is very small
• Tidal heating much weaker than Io and Europa
• Callisto
• Tidal heating extremely weak
• Not in resonance with other satellites
• e is small and distance is large (26 RJ)

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• Tidal heating
• Ganymede and Callisto have minimal tidal heating
  compared to Io and Europa

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Internal structure

• Bulk density suggests 40 ice by mass
• Silicates probably similar to primitive
  carbonaceous

• Ganymede is extremely differentiated
• Iron core
• Silicate mantle
• Thick ice shell
• Intrinsic magnetic field
• Core still convecting
• Callisto appears very weakly differentiated
• Density increases slightly towards center
• No intrinsic magnetic field
• Induced magnetic fields on both bodies
• Oceans suggested
• Shallower than ice/rock interface
• Suggests ice III below ocean

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Homogenous I 0.4 MR2
Self-compaction I 0.38 MR2
Callisto I 0.36 MR2

• Callistos curious interior
• Not fully differentiated
• Near-surface low density layer probably icy
  surface layer
• Convecting layer though necessary to dissipate
  heat
• Ice-rich layer near surface
• How did Callisto avoid runaway differentiation?

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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

• Solids in Jovian disk mirror the solar nebula
• Primative c-type meteroites
• Bulk composition of Galilean satellites close to
  nearby asteroids

• 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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• Late heavy bombardment
• Ganymede receives many more impacts and more
  energetic impacts
• Increased melting causes differentiation

Barr and Canup, 2010
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Oceans

• Radiogenic heat can produce sub-surface oceans
• Models are complicated by
• Convection is very efficient at removing heat
• Liquid water can exist at 173K with dissolved NH3
• Induced magnetic fields on Ganymede and Callisto
• Conducting shell 10 km thick, 100 km deep
• Reexamination of Ganymedes dipole field
• Small non-dipole component

• Liquid ocean sandwiched
• Ice I above
• Probably ice III below Ganymede
• Probably ice III silicates below Callisto
• Callisto paradox
• Not enough internal heat for complete
  differentiation
• No appreciable tidal heat
• Yet, ocean is still liquid??
• Possibly spiked with ammonia?

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• Ganymede processes gravity anomalies
• So far unique among Galilean satellites
• Not correlated with surface features

Anderson et al, 1999
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Surface Composition

• Dominated by water ice
• With dark spectrally neutral material
• Callistos water ice surface is more masked
• Dark material unknown composition
• Low albedo, red colour
• Similar to primitive carbonaceous meteorites
• Hydrated silicates
• Complex organic compounds (tholins)
• Lab results suggest small amount have large
  effects

• Bright material pokes through dark covering
• Dark material thought to be a sublimation lag
• Bright regions cold-trap sublimating water
• Positive feedback keeps bright/dark material
  separate
• Spectral evidence indicate no intimate mixing

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• Irradiation of cyanogens with water
• Jupiters magnetic field lines sweep past every
  10 hours
• Irradiation of particles more pronounced on
  trailing hemispheres
• Similar to Saturnian system
• CO2 adsorbed onto regolith grains
• 4.25 microns
• CN and C-H bonds
• 4.57 and 3.4 microns
• Sulphur provided by Io volcanics
• S-H (3.88 microns) and SO2 (4.05 microns)
• Unique to the Jovian system
• Atmospheres generated by sputtering
• H2 and O on Ganymede
• CO2 on Callisto
• Ganymede polar caps
• Thermal redistribution of volatiles
• Radiation brightening of ice matches field
  lines
• Intrinsic magnetic field funnels particles to
  pole (like an Aurora)

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Surface Ages
Dombard and McKinnon, 2006

• Callisto
• 4 Gyr old cratered terrain everywhere
• Interiors of large basins have fewer craters
• Fewer small craters than expected
• Efficient erosion of 3km craters
• Ganymede
• Dark terrain basically Callisto
• Bright terrain younger 2Gyr old
• Ages very uncertain
• All large craters have viscously relaxed
• Maxwell time
• Stress causes elastic deformation and creep
• Time after which creep strain equals elastic
  strain
• tM eel / (?ecreep/t) ?/µ
• µ is the shear modulus (rigidity), ? is the
  viscosity
• On Earth
• tM for rock gt109 years

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Crater morphology

• Small craters have simples bowl shapes
• Central peaks appear at D3km
• No lunar-like central peak rings
• At Dgt35km you get central pits instead
• At Dgt60km these pits have domes in their centers
• Possibly cryovolcanic analogous to lava domes
  in terrestrial calderas
• Most large craters are viscously relaxed
• Penepalimpsest
• Subtle topographic rings remaining
• Palimpsest
• Only a depressed smooth patch remains
• Sometimes an outward facing scrap also
• Palimpsests found in oldest terrain
• Heat flow higher in the past
• Faster relaxation rates
• Heat-flow transition
• About the same time as Ganymedes
• bright terrain formation

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Asymmetric Crater Distribution

• More impacts expected on the leading hemisphere
• True for Callisto
• Only weakly true for Ganymede
• Chains of craters
• Break up of comets due to Jupiters gravity
• Due to tidal effects
• E.g. Shoemaker Levy 9
• Comet makes close pass to Jupiter and breaks up
• Hits a moon as its leaving the Jovian system
• Produces a chain of craters
• Crater chain catena
• Catenae should all be on the Jupiter facing side
• True for Callisto
• 1/3 of Ganymedes catenae are on the other side
• Distributions indicate Ganymedes shell has
  rotated
• Similar situation to Europa non-synchronous
  rotation

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Surface Geology Callisto

• Surface is uniform and primitive
• Icy version of the lunar highlands
• No sign of volcanics or tectonics
• Dominated by impacts
• And viscous relaxation of impact structures

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Impact Basins Callisto

• Large impact basins differ from Lunar examples
• Evenly spaced concentric fractures
• Extension of upper brittle layer
• Flow of lower ductile layers towards crater
• E.g. Valhalla 1000km across, 20 rings
• Large palimpsest at center of rings
• Higher albedo from cleaner subsurface ice

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• Mass wasting on Callisto
• Material behaves like a Bingham fluid (non-zero
  yield stress)

Chuang and Greeley, 2000
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Surface Geology Ganymede

• Half way between Europa and Callisto
• Dark Terrain 1/3
• Callisto-like cratered
• Bright Terrain 2/3
• Grooved terrain - Europa-like Tectonics
• Smooth terrain - Cryovolcanics

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Ganymedes Dark Terrain

• About 1/3 of the body
• Very similar to Callisto
• Impact dominated
• Furrows
• 2 ridges, 10-20 km apart
• Intervening area is low
• Ridges are concentric arcs
• Some furrows have palimpsests at the center
• Old Valhalla-type landforms

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Ganymedes Bright Terrain

• Covers 2/3 of the surface
• Bright terrain made up of grooved and smooth
  terrain
• Tectonism and volcanism
• Extension common
• Compression rare

• Grooved terrain
• Dense network of overlapping sets of parallel
  ridges and troughs
• Wavelength 5-10 km
• Voyager ridges composed of many smaller ridges at
  higher resolution
• Extensional formation
• Strains of up to 1!
• Confirmed by crater shape distortion
• Tilt-block normal faulting or graben formation
• Produces many parallel faults
• Bright sub-surface ice exposed on slip-faces
• Grooved terrain is just a sliced up version of
  the dark terrain

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• Similar in appearance to Europas extensional
  zones
• but Ganymede doesnt have much net extension

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• Smooth terrain
• Patches or lanes
• Patches are bounded on all sides by grooved
  terrain
• Lanes of smooth terrain (10s of km wide) cut
  through dark terrain
• Flooding of grooved terrain by low-viscosity
  cryovolcanic fluid
• No volcanic constructs
• A few caldera-like collapse pits though
• Lineations
• Faint dark parallel markings
• Possibly narrow valleys ( extension again)

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• Perhaps.
• Ganymede and Callisto start off the same
• High-e period causes tidal heating on Ganymede
• Triggers differentiation (if it hasnt happened
  already) leads to more energy release etc
• Core still cooling off and so still generates a
  magnetic field
• Callisto too far from Jupiter for tidal heating
• Age of bright terrain is 2 Gyr
• Hard to understand a once-off event in the middle
  of solar system history

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Summary

• Ganymede and Callisto
• Formation
• Tidal interactions
• Internal structure
• Differentiation
• Oceans and heat-flow
• Surface compositions
• Craters surfaces ages
• Geology of Callisto
• Impact structures
• Mass wasting
• Geology of Ganymede
• Dark terrain
• Impact structures
• Furrows
• Bright terrain
• Smooth bright terrain
• Grooved bright terrain

Callisto-like
Europa-like