Feasibility of Exploring Europas Subsurface Ocean

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Feasibility of Exploring Europas Subsurface Ocean
Susan McDonald Sophomore- Planetary Science Ge
151a - June 2003
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Outline

• Background/Theories of Subsurface Ocean on Europa
• Recent Magnetometer Data- Proof of Conducting
  Layer Near Surface in Present Epoch
• Craters- Thickness Constraints on Ice Shell
• Cratering morphology indicates rheological
  differences
• Depth/diameter ratio analysis
• Conclusions/Future Missions

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Galileo Europa Mission (GEM)

• Double Ridges indicate lighter, pure water-ice
  at top of slope and darker, soft material welling
  up in valley floor, where crust has pulled apart
• Famous chaos regions show break-up and
  movement of icy crust on soft, mobile material
• Surface topography could be caused by conducting
  ice, or ancient, frozen ocean

so how do we know if there is an ocean?
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Magnetometer Observations

• Jupiters magnetic dipole axis tilted with
  respect to rotation axis- Galiliean moons
  experience magnetic field varying with rotation
  frequency of Jupiter
• Time varying primary field can induce secondary
  field given a sphere of sufficient electrical
  conductivity
• Magnetic signature consistent with gt 70 of
  induced dipole expected of perfectly conducting
  sphere the size of Europa

Kivelson et al., 1997
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Magnetometer Results

• Model required currents of gt 60 mS/m within
  200-300 km of surface.
• Subsurface oceans with salinity of earths
  oceans and a few km thick produce observed
  induction response
• Rule out Other Possible Conductors
• solid ice, ionosphere, or cloud of pick-up ions
  (too resistive)
• conducting core (amplitude too small to produce
  measured field perturbation)

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so where is this ocean- can we send probes
there?
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Craters- Measurements
(central peak, D18km) (central pit,
D30km) (central dome, D121 km)
(dome, D138km)
Callisto/ Ganymede
low high
(2km)
SCALE BAR 30 km
(central peak, D8km) (modified pit,
D14km) (central peak, D27km)
(multiring basins, D 41km)
Europa
low high
(0.5km)
SCALE BAR 10 km
Schenk, 2002
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Craters- Depth/Diameter Ratios
Callisto

• Three transitions in crater shape for each icy
  satellite
• Transition I (simple to complex crater
  morphology)
• Same on all 3 satellites
• Transition II (shallower craters)
• Ganymede/Callisto- occurs at D26km
• Europa- occurs at D8km
• Transition III (anomalous morphologies)
• Ganymede/Callisto- dome craters
• Europa- shallow, multiring craters (Tyre)
• Extreme drop in crater depth unique to icy
  satellites
• Not due to viscous relaxation (youth of craters
  shown in presence of bright rays)
• Most likely due to warm ice at shallow depths

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Ganymede
Europa
Schenk, 2002
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Craters- Analysis

• Transitions II and III are 24 times shallower on
  Europa than on Ganymede or Callisto gt outer ice
  shell is similarly thinner on Europa
• McKinnon and Schenk (1995) scaling relationship
  for icy satellite craters estimates transient
  (pre-modified) crater dimensions (within 10)
  from the observed crater diameters
• Final crater shape influenced when weak layer is
  roughly 1 to 1.3 times as deep as the transient
  crater width (Schenk, 2002)
• Europa- Transition II craters (D 8 km) scale to
  transient craters about 6 km wide, for a weak
  layer depth of 78 km
• Europa- Transition III craters (D 30 km) scale
  to transient craters about 19 km wide, and a
  depth to the second transition of 1925 km
• Lower bound on ice thickness gt 19-25 km

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Conclusions

• Magnetometer data
• Definite proof of conducting shell- salty ocean
  is most likely electrolyte
• Constrains ocean to existing at least in present
  epoch
• Cratering Analysis
• Constrains Depth of Ocean to lower bound of
  19-25 km under surface
• Feasibility of ocean probe- not very good
  (thicker than earlier predictions of 3-4 km)
• Future missions
• SOUNDERS (Surface Observatories for UNDErground
  Remote-Sensing) will let us probe seismic and
  magnetic data from the surface itself