How Thick is Europas Ice Shell Crust

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How Thick is Europas Ice Shell Crust?

• David Galvan
• ESS 298
• The Outer Solar System

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Outline

• Our interest in Europas ice shell crust
• Evidence for Ice/Water crust
• Methods of estimating thickness
• Gravity measurements
• Induced magnetization
• Impact Craters
• Surface Topography and Flexure model
• Convective Tidal Dissipation
• Summary of Estimates

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Europa

• Second major satellite from Jupiter.
• Smallest of the Galileans. (R1560 km, a little
  smaller than Earths Moon)
• Spectroscopic studies indicate primarily H20
  crust. (Malin and Pieri, 1986)
• Elliptical orbit yields tidal heating (e0.01)
• Surface is 30 My old (based on cratering
  record)
• Cassen Reynolds (1979) first suggested liquid
  water ocean could be sustained by tidal heating
• Kivelson et al (2000) showed that Europa has an
  induced magnetic field consistent with Jupiters
  field inducing a current in a conductive salty
  ocean within 100 km of the surface.

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Astrobiological Potential

• Life requires
• Energy source
• (tidal and radiogenic heating could fuel
  volcanism at base of H20 layer.)
• Liquid water
• (very likely)
• Organic chemistry
• (a strong possibility, due to observation of
  deposited salts on surface, organic compounds
  delivered by Jupiter-family comets, and possible
  convective action allowing transport of
  compounds/nutrients from surface to sub-surface.
• Based on reccomendation of NRC in 2000, which
  cited U.N. Document No. 6347 January 1967
• Galileo Spacecraft was intentionally crashed into
  Jupiter for the expressed purpose of eliminating
  the possibility of a future collision with and
  forward contamination of Europa.

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Ideas for a Biosphere
Image from Greenberg, American Scientist, Vol 90,
No. 1, Pg. 48
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Gravity Measurements

• Anderson et al (1997, 1998) used Doppler Shift of
  Galileos radio communication carrier to measure
  coefficients for a spherical harmonic
  representation of Europas gravitational
  potential to second order.
• Obtained an axial moment of inertia measurement
  of (C/MR2) 0.346. (Compare with 0.4 for
  uniform sphere, 0.378 for Io)
• Suggests a dense core and much less dense
  surface.
• Cant distinguish between solid and liquid H20
• For a 2-layer model (unlikely)
• A rock-metal (Fe-enriched) core and about 0.85
  Re and an ice/water crust of 150 - 250km in
  thickness. Considered unlikely for such a small
  body, since radiogenic heating in the silicate
  core would lead to differentiation, and formation
  of metal core.
• For a 3-layer model (most likely)
• A Fe or Fe-S metal core of 0.4 Re, a silicate
  mantle, and an ice/water crust of 80 170 km in
  thickness

Where ? longitude from Jupiter-Europa line, and
flatitude.
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Induced Magnetization

• Based only on observations of surface properties
  and gravity potential, there is no obvious way to
  tell if liquid water exists today, or if it froze
  thousands of years ago.
• Kivelson et al (2000) discovered an induced
  magnetic field at Europa, generated by the
  changing direction of Jupiters B-field at Europa
  as the satellite orbits the planet.

One model that explains this is a conducting
spherical shell (probably liquid salt water) at a
depth of at least 8 km below the ice crust.
Magnetometer measurements show that Europas
dipole moment changed due to a change in the
relative orientation of Jupiters magnetic field,
as Europa was in a different location in its
orbit.
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Induced Magnetization (contd.)

• Zimmer et al (2000) further constrained the
  spherical conducting shell model through in-depth
  analysis of the induced magnetic field, and
  variation of conductivity and depth.
• Assumes ocean thickness between 100 km and 200 km
  (from Anderson)
• Showed that the magnetic signature required an
  ocean within 175 km of the surface of Europa,
  with a minimum required conductivity of 72 mS/m
  and magnetic amplitude gt 0.7.

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Craters 1

• Central peaks in craters consist of deeply buried
  material uplifted immediately after impact.
• This means that the central peak craters on
  Europa should provide a lower limit of ice shell
  thickness, since if the impactor penetrates
  through the ice layer, a central peak will not
  form.
• Turtle Pierazzo (2001) conducted numerical
  simulations of vapor and melt production during
  crater formation in layers of ice overlying
  liquid water and warm, convecting ice.
• Used small and large (12 21km transient
  crater) objects, meant to represent
  Jupiter-family comet objects with 26.5 km/s
  vertical velocities.
• Also used a conducting ice layer with Tsurf 110
  K and Tbase 270 K

Solidno central peak Open with solid center
central peak Nested ring multiring basins
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Craters 1, (contd.)

• Found that
• At 9km thickness neither impactor vaporizes/melts
  through the ice crust. So 9km is not a lower
  bound.
• At 5 km thickness, large impactor melts through
  the crust, but small impactor does not. So 5 km
  not a lower bound.
• At 3 km thickness, large and small impactors
  mellt through ice crust to warm ice.
• Under a central peak 5km across and 500 m high,
  like at Pwyll Crater, viscosity of ice would be
  1013 Pa s, yielding relaxation time of lt 1yr.
• But, since Pwyll crater does exist, it must not
  have relaxed away, and hence the impactor that
  created Pwyll did not breach the ice crust.
• They claim that for 3km of ice over a liquid
  water layer, both large and small impactors would
  melt through the crust, precluding central peak
  formation as well.

Large (21km) Transient crater
Similar (21km) Transient crater
3km ice over warm ice
5 km ice over liquid water
9 km ice over liquid water
Hence, ice crust must be gt 3 km!
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Craters, 2
Central Peak (18 km)
Central Pit (30 km)
Central Dome (121 km)
Anomalous Dome (138 km)

• Morphology of impact craters depends on surface
  gravity and lithospheric properties.
• Since the Galileans and the Moon have fairly
  similar values of g, any differences in crater
  morphology between the satellites must be due to
  lithospheric rheology or composition differences.
• Schenk (2002) notices systematic differences
  between Europa craters and craters on Ganymede
  and Callisto.
• Depth as a function of Diameter (d/D) undergoes
  two breaks in trend, called transitions.
• 2 transitions occur at different diameters for
  Europa than for Ganymede and Callisto.

Ganymede/ Callisto
Europa
Anomalous Central Peak (27 km)
Multiring Basins (41 km)
Central Peak (8 km)
Central Pit (14 km)
Scalebars are 30 km for G/C and 10 km for Europa
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• Transition 1 From simple bowl to complex
  (central structure) craters.
• Similar on all 3 satellites.

C

• Transition 2 Anomalous changes in complex crater
  dimensions. Due to temperature dependent
  rheologic change with depth.
• Europa structures dont support as much
  topography, presumably due to weaker ice at a
  shallower depth than Ganymede or Callisto.

G

• Transition 3 Sharp reduction in crater depths
  and development of multiring basins. Consistent
  with impact into brittle crust resting on a fluid
  layer.
• Occurs for Europa at D 30 km, which implies a
  crust of 19 25 km. (according to laboratory
  transient crater studies)

E

• This constrains the ice shell to be at least 19 -
  25 km thick.

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Tidal Dissipation / Heat Flow

• Hussmann Spohn (2001) used a steady state model
  of tidal dissipation.
• Used viscoelastic rheology for Europas ice, and
  current values for orbital elements.
• Used the three-layer model proposed by Anderson
  et al (1998). With total water layer of 145 km.
• Model has tidal dissipation as a heat source in
  the viscoelastic ice, and radiogenic heat source
  in the silicate mantle.
• In the stagnant lid of ice crust, conduction
  allows surface heat flux.
• They vary the melting-point viscosity of ice
  while calculating heat production and heat flow
  through the ice crust as a function of thickness.

Thicknesses not to scale
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Tidal Dissipation / Heat Flow
They attempt to balance the heat budget of
Europas H20 layer by plotting tidal dissipation
(heat production rate) and heat flux through the
ice layer (convecting and conducting cases) for
different melting-point viscosities as a function
of ice thickness.
Ice Crust thickness range 30 km, and surface
heat flow 20mW/m2
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Elastically Supported Topography

• Nimmo et al (2003) used the wavelength of
  topography near Cilix crater to estimate elastic
  thickness Te.
• Then used a relation to infer actual crustal
  thickness Tc, based on temperature of surface Ts
  and base of crust Tb, and temperature of the base
  of the elastic layer Tr.

Cilix crater with topographic profiles. Derived
from Galileo stereographic images
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Elastically Supported Topography
Combined topographic profile for ice crust with
rigidity D loaded against by a trapesoidal mass,
with a best fit model of Te 6 km
Lowest value of the combined root mean square
misfit again shows best fit at Te 6 km
Conductive ice crust Tb melting temp, tc is
crust thickness. Convective ice crust Tb temp
of convecting ice, tc is conducting lid thickness.
Leads to crust thickness of 15 - 35 km!
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Summary of Estimates

• Gravity constraint total ice/liquid layer
• 80 - 170 km
• Magnetometer constraint
• Electrically conducting liquid water ocean must
  exist at a depth of within 200 km, otherwise
  poorly constrained.
• Craters
• Minimum ice shell thickness of 19-25 km
• Tidal Dissipation
• Heat conducting ice crust of 30 km
• Topography / Elastic Thickness
• Crustal thickness of 15 - 35 km.
• TOTAL
• Probably 25 km of ice crust, followed by liquid
  water ocean down to a depth of 150 km
• Get your swim trunks!

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Further constraints

• Could be brought by
• Another mission with
• Ground (Ice) Penetrating radar
• A Europa orbiter for more precise radio science
  and gravity measurements
• Seismometers?

JIMO would launch no earlier than 2015
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References

• Anderson, J. D., E. L. Lau, W. L. Sjogren, G.
  Schubert, and W. B. Moore. Europas
  differentiated internal structure Inferences
  from two Galileo encounters. Science 276,
  12361239. (1997)
• Anderson, J. D., E. L. Lau, W. L. Sjogren, G.
  Schubert, and W. B. Moore. Europas
  differentiated internal structure Inferences
  from four Galileo encounters. Science 281,
  20192022. (1998)
• Zimmer, C., K. Khurana, M. G. Kivelson.
  Subsurface Oceans on Europa and Callisto
  Constraints from Galileo Magnetometer
  Observations. Icarus 147, 329-347. (2000)
• Nimmo, F., B. Giese, and R. T. Pappalardo,
  Estimates of Europas ice shell thickness from
  elastically-supported topography, Geophys. Res.
  Lett., 30(5),1233 (2003)
• Schenk, P. M., Thickness constraints on the icy
  shells of the Galilean satellites from a
  comparison of crater shapes, Nature, 417, 41421
  (2002).
• Greenberg, R. Tides and the biosphere of Europa.
  Am. Sci. 90, 4855 (2002).
• Hussmann, H., T. Spohn, and K. Wieczerkowski,
  Thermal equilibrium states of Europas ice shell
  Implications for internal ocean thickness and
  surface heat flow, Icarus, 156, 143151 (2002)
• Hoppa, G. V., B. R. Tufts, R. Greenberg, and P.
  E. Geissler, Formation of cycloidal features on
  Europa, Science, 285, 18991902 (1999a)
• Pappalardo, R. T., et al., Geological evidence
  for solid-state convection in Europas ice shell,
  Nature, 391, 365368 (1998)
• Turtle, E. P., and E. Pierazzo, Thickness of a
  Europan ice shell from impact crater simulations,
  Science, 294, 1326 1328 (2001)

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Other Estimates

• Pappalardo et al (1998) interpret surface
  features as diapirs (warm, buoyant ice masses)
  yielding crust thickness of 3-10 km