How Thick is Europa’s Ice How Thick is Europa’s Ice Shell Crust?Shell Crust?

David GalvanDavid Galvan

ESS 298ESS 298

The Outer Solar SystemThe Outer Solar System

OutlineOutline

Our interest in Europa’s ice shell crustOur interest in Europa’s ice shell crust Evidence for Ice/Water crustEvidence for Ice/Water crust Methods of estimating thicknessMethods of estimating thickness

– Gravity measurementsGravity measurements– Induced magnetizationInduced magnetization– Impact Craters Impact Craters – Surface Topography and Flexure modelSurface Topography and Flexure model– Convective Tidal DissipationConvective Tidal Dissipation

Summary of EstimatesSummary of Estimates

EuropaEuropa Second major satellite from Jupiter.Second major satellite from Jupiter. Smallest of the Galileans. (R=1560 km, a little Smallest of the Galileans. (R=1560 km, a little

smaller than Earth’s Moon)smaller than Earth’s Moon) Spectroscopic studies indicate primarily H20 crust. Spectroscopic studies indicate primarily H20 crust.

(Malin and Pieri, 1986)(Malin and Pieri, 1986) Elliptical orbit yields tidal heating (e=0.01)Elliptical orbit yields tidal heating (e=0.01) Surface is ~ 30 My old (based on cratering record)Surface is ~ 30 My old (based on cratering record) Cassen & Reynolds (1979) first suggested liquid Cassen & Reynolds (1979) first suggested liquid

water ocean could be sustained by tidal heatingwater ocean could be sustained by tidal heating Kivelson et al (2000) showed that Europa has an Kivelson et al (2000) showed that Europa has an

induced magnetic field consistent with Jupiter’s induced magnetic field consistent with Jupiter’s field inducing a current in a conductive salty ocean field inducing a current in a conductive salty ocean within ~100 km of the surface.within ~100 km of the surface.

Astrobiological PotentialAstrobiological Potential Life requires:Life requires:

– Energy sourceEnergy source (tidal and radiogenic heating could fuel (tidal and radiogenic heating could fuel

volcanism at base of H20 layer.)volcanism at base of H20 layer.)

– Liquid waterLiquid water (very likely)(very likely)

– Organic chemistryOrganic chemistry (a strong possibility, due to observation of (a strong possibility, due to observation of

deposited salts on surface, organic deposited salts on surface, organic compounds delivered by Jupiter-family compounds delivered by Jupiter-family comets, and possible convective action comets, and possible convective action allowing transport of compounds/nutrients allowing transport of compounds/nutrients from surface to sub-surface.from surface to sub-surface.

Based on reccomendation of NRC in Based on reccomendation of NRC in 2000, which cited 2000, which cited U.N. Document No. U.N. Document No.

6347 January 1967:6347 January 1967:

Galileo Spacecraft was intentionally Galileo Spacecraft was intentionally crashed into Jupiter for the expressed crashed into Jupiter for the expressed purpose of eliminating the possibility of a purpose of eliminating the possibility of a future collision with and forward future collision with and forward contamination of Europa.contamination of Europa.

Ideas for a BiosphereIdeas for a Biosphere

Image from Greenberg, American Scientist, Vol 90, No. 1, Pg. 48

Gravity MeasurementsGravity Measurements Anderson et al (1997, 1998) used Doppler Shift of Galileo’s Anderson et al (1997, 1998) used Doppler Shift of Galileo’s

radio communication carrier to measure coefficients for a radio communication carrier to measure coefficients for a spherical harmonic representation of Europa’s gravitational spherical harmonic representation of Europa’s gravitational potential to second order.potential to second order.

Obtained an axial moment of inertia measurement of Obtained an axial moment of inertia measurement of (C/MR^2) = 0.346(C/MR^2) = 0.346. (Compare with 0.4 for uniform sphere, . (Compare with 0.4 for uniform sphere, 0.378 for Io)0.378 for Io)

Suggests a dense core and much less dense surface.Suggests a dense core and much less dense surface.

Can’t distinguish between solid and liquid H20Can’t distinguish between solid and liquid H20

For a 2-layer model: (unlikely)For a 2-layer model: (unlikely) A rock-metal (Fe-enriched) core and about 0.85 Re and an A rock-metal (Fe-enriched) core and about 0.85 Re and an

ice/water crust of ice/water crust of 150 - 250km150 - 250km in thickness. Considered in thickness. Considered unlikely for such a small body, since radiogenic heating in the unlikely for such a small body, since radiogenic heating in the silicate core would lead to differentiation, and formation of silicate core would lead to differentiation, and formation of metal core.metal core.

For a 3-layer model: (most likely)For a 3-layer model: (most likely) A Fe or Fe-S metal core of A Fe or Fe-S metal core of 0.4 Re0.4 Re, a silicate mantle, and an , a silicate mantle, and an

ice/water crust of ice/water crust of 80 – 170 km80 – 170 km in thickness in thickness

2coscos3)1sin3(

2

11 2

2

222

2

2 r

RC

r

RJ

r

GMV Where λ = longitude

from Jupiter-Europa

line, and φ=latitude.

Induced MagnetizationInduced Magnetization Based only on observations of Based only on observations of

surface properties and gravity surface properties and gravity potential, there is no obvious potential, there is no obvious way to tell if liquid water exists way to tell if liquid water exists today, or if it froze thousands of today, or if it froze thousands of years ago.years ago.

Kivelson et al (2000) discovered Kivelson et al (2000) discovered an induced magnetic field at an induced magnetic field at Europa, generated by the Europa, generated by the changing direction of Jupiter’s changing direction of Jupiter’s B-field at Europa as the satellite B-field at Europa as the satellite orbits the planet.orbits the planet.

Magnetometer measurements show that Europa’s dipole moment changed due to a change in the relative orientation of Jupiter’s magnetic field, as Europa was in a different location in its orbit.

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.

Induced Magnetization (cont’d.)Induced Magnetization (cont’d.) Zimmer et al (2000) further Zimmer et al (2000) further

constrained the spherical conducting constrained the spherical conducting shell model through in-depth shell model through in-depth analysis of the induced magnetic analysis of the induced magnetic field, and variation of conductivity field, and variation of conductivity and depth.and depth.

Assumes ocean thickness between Assumes ocean thickness between 100 km and 200 km (from Anderson)100 km and 200 km (from Anderson)

Showed that the magnetic signature Showed that the magnetic signature required an ocean required an ocean within ~175 kmwithin ~175 km of the surface of Europa, with a of the surface of Europa, with a minimum required conductivity of ~ minimum required conductivity of ~ 72 mS/m and magnetic amplitude > 72 mS/m and magnetic amplitude > 0.7.0.7.

Craters 1Craters 1 Central peaks in craters consist of deeply buried Central peaks in craters consist of deeply buried

material uplifted immediately after impact.material uplifted immediately after impact. This means that the central peak craters on This means that the central peak craters on

Europa should provide a lower limit of ice shell Europa should provide a lower limit of ice shell thickness, since if the impactor penetrates thickness, since if the impactor penetrates through the ice layer, a central peak will not form.through the ice layer, a central peak will not form.

Turtle & Pierazzo (2001) conducted numerical Turtle & Pierazzo (2001) conducted numerical simulations of vapor and melt production during simulations of vapor and melt production during crater formation in layers of ice overlying liquid crater formation in layers of ice overlying liquid water and warm, convecting ice.water and warm, convecting ice.

Used “small” and “large” (12 & 21km transient Used “small” and “large” (12 & 21km transient crater) objects, meant to represent Jupiter-family crater) objects, meant to represent Jupiter-family comet objects with 26.5 km/s vertical velocities.comet objects with 26.5 km/s vertical velocities.

Also used a conducting ice layer with Tsurf = 110 Also used a conducting ice layer with Tsurf = 110 K and Tbase= 270 KK and Tbase= 270 K

Solid=no central peakOpen with solid center = central peakNested ring = multiring basins

Craters 1, (cont’d.)Craters 1, (cont’d.) Found that:Found that:

– At 9km thickness neither impactor At 9km thickness neither impactor vaporizes/melts through the ice vaporizes/melts through the ice crust. So 9km is not a lower bound.crust. So 9km is not a lower bound.

– At 5 km thickness, large impactor At 5 km thickness, large impactor melts through the crust, but small melts through the crust, but small impactor does not. So 5 km not a impactor does not. So 5 km not a lower bound.lower bound.

– At 3 km thickness, large and small At 3 km thickness, large and small impactors mellt through ice crust to impactors mellt through ice crust to warm ice. warm ice.

– Under a central peak 5km across Under a central peak 5km across and 500 m high, like at Pwyll Crater, and 500 m high, like at Pwyll Crater, viscosity of ice would be 10^13 Pa viscosity of ice would be 10^13 Pa s, yielding relaxation time of < 1yr. s, yielding relaxation time of < 1yr.

– But, since Pwyll crater does exist, it But, since Pwyll crater does exist, it must not have relaxed away, and must not have relaxed away, and hence the impactor that created hence the impactor that created Pwyll did not breach the ice crust.Pwyll did not breach the ice crust.

– They claim that for 3km of ice over They claim that for 3km of ice over a liquid water layer, both large and a liquid water layer, both large and small impactors would melt through small impactors would melt through the crust, precluding central peak the crust, precluding central peak formation as well.formation as well.

3km ice over warmice

5 km ice over liquidwater

9 km ice over liquidwater

Large (21km)Transient crater

Similar (21km)Transient crater

Hence, ice crust must be > 3 km!

Craters, 2Craters, 2 Morphology of impact craters depends on Morphology of impact craters depends on

surface gravity and lithospheric properties.surface gravity and lithospheric properties.

Since the Galileans and the Moon have Since the Galileans and the Moon have fairly similar values of g, any differences in fairly similar values of g, any differences in crater morphology between the satellites crater morphology between the satellites must be due to lithospheric rheology or must be due to lithospheric rheology or composition differences.composition differences.

Schenk (2002) notices systematic Schenk (2002) notices systematic differences between Europa craters and differences between Europa craters and craters on Ganymede and Callisto.craters on Ganymede and Callisto.

Depth as a function of Diameter (d/D) Depth as a function of Diameter (d/D) undergoes two breaks in trend, called undergoes two breaks in trend, called transitions.transitions.

2 transitions occur at different diameters 2 transitions occur at different diameters for Europa than for Ganymede and for Europa than for Ganymede and Callisto.Callisto.

Europa

Ganymede/Callisto

Central Peak(8 km)

Central Peak(18 km)

Central Pit(14 km)

Central Pit(30 km)

Central Dome(121 km)

Anomalous Dome(138 km)

AnomalousCentral Peak(27 km)

MultiringBasins(41 km)Scalebars are 30 km for G/C

and 10 km for Europa

Transition 1Transition 1: From simple bowl to : From simple bowl to complex (central structure) craters.complex (central structure) craters.

Similar on all 3 satellites.Similar on all 3 satellites.C

G

E

1

1

1

2

2

2

3

3

3

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

Transition 2Transition 2: Anomalous changes in : Anomalous changes in complex crater dimensions. Due to complex crater dimensions. Due to temperature dependent rheologic change temperature dependent rheologic change with depth.with depth.

Europa structures don’t support as much Europa structures don’t support as much topography, presumably due to weaker topography, presumably due to weaker ice at a shallower depth than Ganymede ice at a shallower depth than Ganymede or Callisto.or Callisto.

Transition 3Transition 3: Sharp reduction in crater : Sharp reduction in crater depths and development of multiring depths and development of multiring basins. Consistent with impact into brittle basins. Consistent with impact into brittle crust resting on a fluid layer.crust resting on a fluid layer.

Occurs for Europa at D = 30 km, which Occurs for Europa at D = 30 km, which implies a crust of 19 – 25 km. (according implies a crust of 19 – 25 km. (according to laboratory transient crater studies)to laboratory transient crater studies)

Tidal Dissipation / Heat FlowTidal Dissipation / Heat Flow Hussmann & Spohn (2001) used a steady state model of Hussmann & Spohn (2001) used a steady state model of

tidal dissipation.tidal dissipation. Used viscoelastic rheology for Europa’s ice, and current Used viscoelastic rheology for Europa’s ice, and current

values for orbital elements.values for orbital elements. Used the three-layer model proposed by Anderson et al Used the three-layer model proposed by Anderson et al

(1998). With total water layer of 145 km.(1998). With total water layer of 145 km.

Model has tidal dissipation as a heat source in the Model has tidal dissipation as a heat source in the viscoelastic ice, and radiogenic heat source in the silicate viscoelastic ice, and radiogenic heat source in the silicate mantle.mantle.

In the stagnant lid of ice crust, conduction allows surface In the stagnant lid of ice crust, conduction allows surface heat flux.heat flux.

They vary the melting-point viscosity of ice while They vary the melting-point viscosity of ice while calculating heat production and heat flow through the ice calculating heat production and heat flow through the ice crust as a function of thickness.crust as a function of thickness.

Thicknesses not to scale

Tidal Dissipation / Heat FlowTidal Dissipation / Heat Flow

They attempt to balance the heat budget of Europa’s 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/m^2

Elastically Supported TopographyElastically Supported Topography Nimmo et al (2003) used the Nimmo et al (2003) used the

wavelength of topography near Cilix wavelength of topography near Cilix crater to estimate elastic thickness Te. crater to estimate elastic thickness Te.

Then used a relation to infer actual Then used a relation to infer actual crustal thickness Tc, based on crustal thickness Tc, based on temperature of surface Ts and base of temperature of surface Ts and base of crust Tb, and temperature of the base crust Tb, and temperature of the base of the elastic layer Tr.of the elastic layer Tr.

Cilix crater with topographic profiles.

Derived from Galileo stereographic images

Elastically Supported TopographyElastically Supported Topography

Leads to crust thickness of 15 - 35 km!

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.

Summary of EstimatesSummary of Estimates Gravity constraint: total ice/liquid layerGravity constraint: total ice/liquid layer

– 80 - 170 km80 - 170 km Magnetometer constraint:Magnetometer constraint:

– Electrically conducting liquid water ocean must exist at a depth of within 200 km, Electrically conducting liquid water ocean must exist at a depth of within 200 km, otherwise poorly constrained.otherwise poorly constrained.

CratersCraters– Minimum ice shell thickness of 19-25 kmMinimum ice shell thickness of 19-25 km

Tidal DissipationTidal Dissipation– Heat conducting ice crust of ~ 30 kmHeat conducting ice crust of ~ 30 km

Topography / Elastic ThicknessTopography / Elastic Thickness– Crustal thickness of 15 - 35 km.Crustal thickness of 15 - 35 km.

TOTAL:TOTAL:– Probably ~ 25 km of ice crust, followed by liquid water ocean down to a Probably ~ 25 km of ice crust, followed by liquid water ocean down to a

depth of ~150 kmdepth of ~150 km– Get your swim trunks!Get your swim trunks!

Further constraintsFurther constraints

Could be brought by:Could be brought by:– Another mission with:Another mission with:– Ground (Ice) Penetrating Ground (Ice) Penetrating

radarradar– A Europa orbiter for more A Europa orbiter for more

precise radio science and precise radio science and gravity measurementsgravity measurements

– Seismometers?Seismometers?JIMO: would launch no earlier than 2015

ReferencesReferences Anderson, J. D., E. L. Lau, W. L. Sjogren, G. Schubert, and W. B. Moore. Europa’s differentiated internal structure:

Inferences from two Galileo encounters. Science 276, 1236–1239. (1997)

Anderson, J. D., E. L. Lau, W. L. Sjogren, G. Schubert, and W. B. Moore. Europa’s differentiated internal structure: Inferences from four Galileo encounters. Science 281, 2019–2022. (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 Europa’s 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, 41–421 (2002).

Greenberg, R. Tides and the biosphere of Europa. Am. Sci. 90, 48–55 (2002).

Hussmann, H., T. Spohn, and K. Wieczerkowski, Thermal equilibrium states of Europa’s ice shell: Implications for internal ocean thickness and surface heat flow, Icarus, 156, 143–151 (2002)

Hoppa, G. V., B. R. Tufts, R. Greenberg, and P. E. Geissler, Formation of cycloidal features on Europa, Science, 285, 1899–1902 (1999a)

Pappalardo, R. T., et al., Geological evidence for solid-state convection in Europa’s ice shell, Nature, 391, 365–368 (1998)

Turtle, E. P., and E. Pierazzo, Thickness of a Europan ice shell from impact crater simulations, Science, 294, 1326– 1328 (2001)

Other EstimatesOther Estimates

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