How Thick is Europa’s Ice Shell Crust? David Galvan ESS 298 The Outer Solar System

Outline  Our interest in Europa’s 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

Europa  Second major satellite from Jupiter.  Smallest of the Galileans. (R=1560 km, a little smaller than Earth’s Moon)  Spectroscopic studies indicate primarily H20 crust. (Malin and Pieri, 1986)  Elliptical orbit yields tidal heating (e=0.01)  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 Jupiter’s field inducing a current in a conductive salty ocean within ~100 km of the surface.

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 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.

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

Gravity Measurements  Anderson et al (1997, 1998) used Doppler Shift of Galileo’s radio communication carrier to measure coefficients for a spherical harmonic representation of Europa’s gravitational potential to second order.  Obtained an axial moment of inertia measurement of (C/MR^2) = (Compare with 0.4 for uniform sphere, for Io)  Suggests a dense core and much less dense surface.  Can’t 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 km 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 λ = longitude from Jupiter-Europa line, and φ=latitude.

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 Jupiter’s B-field at Europa as the satellite 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.)  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 > 0.7.

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 Solid=no central peak Open with solid center = central peak Nested ring = multiring basins

Craters 1, (cont’d.)  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 10^13 Pa s, yielding relaxation time of < 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. 3km ice over warm ice 5 km ice over liquid water 9 km ice over liquid water Large (21km) Transient crater Similar (21km) Transient crater Hence, ice crust must be > 3 km!

Craters, 2  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. 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) Anomalous Central Peak (27 km) Multiring Basins (41 km) Scalebars are 30 km for G/C and 10 km for Europa

 Transition 1: From simple bowl to complex (central structure) craters.  Similar on all 3 satellites. C G E  This constrains the ice shell to be at least km thick.  Transition 2: Anomalous changes in complex crater dimensions. Due to temperature dependent rheologic change with depth.  Europa structures don’t support as much topography, presumably due to weaker ice at a shallower depth than Ganymede or Callisto.  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)

Tidal Dissipation / Heat Flow  Hussmann & Spohn (2001) used a steady state model of tidal dissipation.  Used viscoelastic rheology for Europa’s 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

Tidal 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 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

Elastically Supported Topography Leads to crust thickness of 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 Estimates  Gravity constraint: total ice/liquid layer – 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 km  Tidal Dissipation –Heat conducting ice crust of ~ 30 km  Topography / Elastic Thickness –Crustal thickness of km.  TOTAL: –Probably ~ 25 km of ice crust, followed by liquid water ocean down to a depth of ~150 km –Get your swim trunks!

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

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