Europa Scenarios: Physical Models Ice-cracks on surface consistent with either “warm-ice” or...

Europa Scenarios: Physical Models

• Ice-cracks on surface consistent with either “warm-ice” or water beneath the surface

• Near infrared mapping consistent with hydrated salts (Mc Cord et al. 2001)

• Magnetometer data from recent Galileo mission confirm presence of water (Kivelson et al., 2000)

The main facts

Europa Scenarios: Physical Models

• Is there a water ocean beneath the iced surface? How deep is the ocean? How thick is the ice shell?

• What is the thermal structure of the moon? Is tidal heating important as in Io? Where?

• Is there life???

The main questions

• Acquisition of seismic data has been proposed => need of physical models based on different composition and thermal structure (scenarios)

• We assume a three-layer composition: water-ice, silicatic mantle, metallic core consistent with the values of mass and moment of inertia

• The thermal structures are based on estimations of internal heating (radiogenic plus tidal heating) … we will see later on

water-iceComposition

• EOS used is the new standard international EOS for pure water: IAPWS-95 (Wagner and Pruss, 2002)

• For ice (and sea-ice), we implemented the recent EOS from Feistel and Wagner (2005)

• We compare the pure water EOS with the updated sea-ice EOS (at low P): main effect is the reduction of the melting temperature.

If other elements (e.g., ammonia) are present in the water-ice system, the melting T will be reduced favoring presence of a liquid water-rich ocean

Silicatic mantleComposition

• We use the recent thermodynamic consistent EOS for 5 oxides system (CFMAS) from (Stixrude and Lithgow-Bertelloni, 2005) implemented in PERPLEX (Connolly J.)

• We test 2 composition: pyrolite and L-LL type chondritic mantle

Pyrolite (a) Chondritic (L-LL type) (b)

CaO 3.50 2.13 FeO 5.72 13.98 MgO 48.53 39.68

Al2O3 3.59 1.43 SiO2 38.66 42.78

olivine 55.93 37.43 garnet 13.38 3.58 Opx 15.34 49.74 Cpx 15.35 9.25

Density (kg/m3) 3352.8 3456.2 VP (km/s) 8.09 7.61 VS (km/s) 4.57 4.32 3 Gpa (lower Europa mantle) , 1000 OC


Pyrolite L-LL type chondrite

• Density is lower for pyrolite than for low-iron chondrite• Uncertainties in density are between 0.1 and 0.2% at P-T of the Europa

mantle (Cammarano et al., 2003)


Pyrolite L-LL type chondrite

• And VP,S are higher for pyrolite.

• Note the stability fields of plagiocase and spinel at low pressure

Metallic CoreComposition

• We test either a pure iron (high-density, high melting T) or a mix of iron+sulfur (20%) core.

• Note that a solid iron core cannot contain more than 0.1% of light elements at core P (not exceeding 5.5 GPa) => if T is low enough, a solid iron may be favored

• Elastic data have been selected for stable iron phases () and liquid iron

• Data on eutectic melting T, its change with pressure and density change as function of S content at correct P-T range have been compiled (Sanloup et al., 2003, Boheler 1996)

I thank Sebastien Merkel for the help given to select these dataPressure (Gpa)

Thermal structures

solidus<Qr>

<Qr> + <Qi>

Hot

Cold

• Conductive curves for uniform internal heating in the mantle

Thermal structures

• Cold, conductive mantle coupled with solid iron core

• Hot convective mantle, with either no bottom boundary layer (no heat from the core, but only internal, maybe localized in the upper part) or with (T=400 K). Coupled with melted Fe+S core

• Isothermal structure in the core

Thermal structures

• Shallow thermal structures have been tuned for testing different thickness of the ice shell.

Inversion for ocean and CM boundary depth

• Cold scenario, pyrolitic mantle, pure iron core


• Ocean depth increase of circa 10km if mantle is chondritic (higher density) instead than pyrolitic


• Ocean depth is similar in hot or cold scenarios

The physical models

HotCold

(Pyrolitic mantle)

The physical models

The physical models

• Temperature dependence of Q confers very different dissipations between the cold and hot scenarios

Cammarano et al., 2003

An useful homologous temperature scaling is:

Conclusions

• Due to feeback between radiogenic and tidal heating, either hot or cold scenarios may be developed on Europa.

• We found a set of physical models for different scenarios (hot vs cold) that are consistent with mass and moment of inertia

• The ocean depth is constrained between 100 and 140, consistent with the result of Anderson (2000).

• Strong dissipation and dispersion• Case for occurrence of partial melt => volcanism ?• Possible source of EQ at lithospheric depths• Large(r) 3-D variation of seismic velocities related to large T to the

mantle convection• Possible case for a melted core of iron plus light elements

HOT scenario

Discussion

• Geodynamic models for the hot case should allow to assess the thickness of boundary layer and the degree of heterogeneity of the 3D structure. Perhaps it would also give indications about eventual strong mantle flow that will confer anisotropy…

• Test of different thermal models? What kind• Assess better the thermal structure of the ice shell, by computing how

effect of tidal heating change with thickness of ice (recent literature exists) and so compute the heat flow

• Possible Earthquakes source at a different depths for the two scenarios. In cold case brittle failure may happen below, not so in the hot mantle. EQ similar to moonquakes are possible.

• Use magnetic field constraints

The seismic response of the physical models???

TO BE CONTINUED….

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

• Effect of distance• Lp at 45sec

• Effect of shell thickness (5 km, 10km, 20km, 40km)

Europa Scenarios: Physical Models Ice-cracks on surface consistent with either “warm-ice” or...

Documents

Transcript of Europa Scenarios: Physical Models Ice-cracks on surface consistent with either “warm-ice” or...