Conditions for present-day magmatism in Europa's mantle

Introduction

The presence of an internal ocean in direct contact with silicates and possible seafloor magmatic activity drew attention to the potential habitability of Jupiter's moon, Europa. Here, we address the conditions for melt production in its silicate mantle until the present day, an essential ingredient for assessing the habitability potential of Europa.

Method

To reach our goal we employ Oedipus/Antigone (Choblet et al., 2007; Běhounková et al., 2010) – a numerical tool solving consistently thermal convection and tidal dissipation in 3d planetary shells. For the thermal convection part, the mass, momentum and energy conservation equations for viscous material are solved in the extended Boussinesq approximation. The solved equations are supplemented by an instantaneous melt extraction model once the solidus temperature is reached. The efficiency of heat transfer is mainly determined by viscosity, characterized by viscosity at the melting point 𝜂_melt and strong temperature dependency. We consider both radiogenic and tidal heating as energy sources. The former is computed from long-lived radioisotope abundances assuming concentrations based on LL (upper limit) and CM (lower limit) chondrites. Two families of models are tested: homogeneous distribution and a model with a simple 50:50 partitioning of radioactive elements between the crust and the mantle. The time-evolution follows the decay law and is progressively diminishing with time. The latter is controlled by internal structure and Europa’s orbital eccentricity. For the tidal dissipation part, the mass and momentum conservation of tidal deformations are solved over the tidal time scale assuming a Maxwell viscoelastic rheology with a temperature-dependent effective viscosity capturing an Andrade-like behaviour. The orbital eccentricity and the eccentricity evolution are parameterized. Two families of eccentricity models are taken into account assuming either constant or periodically varying eccentricity (cf. Hussmann and Spohn, 2004).

Results

Our simulations show that the evolution of Europa's silicate mantle and associated melt production are controlled by three primary parameters: i) the viscosity at the melting temperature 𝜂_melt, ii) the amount of radiogenic heating and its partitioning, and iii) Europa’s orbital eccentricity. 

For viscosity equal or larger than 10²⁰Pa.s, no onset of convection is observed, the melt production is concentrated into the layer above the core-mantle boundary, and melt production can be sustained until the present day. 

For lower viscosity (𝜂_melt=10¹⁸ or 10¹⁹ Pa.s), more effective heat transfer is obtained. All the performed simulations follow a similar pattern. The onset of convection is observed within 1 Gyr and 100 Myr for 10¹⁹ Pa.s and 10¹⁸ Pa.s, respectively. Due to the large volumetric heating, the mantle temperature is close to the dry silicate solidus except in the cold lid and the temperature differences are relatively small below the lid. The onset of convection can, therefore, occur after a relatively long time, especially for 𝜂_melt=10¹⁹ Pa.s, and the exact timing is sensitive to the radiogenic heating model. The onset of convection is characterized by abrupt changes in melt production and in temperature, and by increased flux from the core. The temperature during the onset is represented by long-wavelength features following the pattern of heterogeneous tidal dissipation. After the onset of convection, the melting production concentrates into the layer just under the stagnant lid, the thickness of this layer depends on the viscosity.

For models with constant eccentricity equal to the present-day value, no melt production is expected at present (except for non-convective models with high reference viscosity). Only simulations with enhanced eccentricity can lead to melt production during the last billion years. Increased tidal dissipation during periods of enhanced eccentricity leads to prolonged melt production at high latitudes, even at present.

We predict that melt can be currently produced in the polar regions if Europa experienced a recent period of enhanced eccentricity. This would also imply the eccentricity is presently decreasing. The volume of melt that is produced during such a magmatic pulse and the associated volatile release depends on the assumed initial rock composition, which determines the abundance in radioactive elements and volatile species. 

Conclusions

We have investigated the thermal evolution of Europa's mantle and conditions for sustaining melt production until the present. Present-day seafloor magmatism might be possible if Europa experienced a period of increased eccentricity in the recent past. Such recent magmatism should influence the topography of the seafloor and composition of the ocean (Zolotov and Kargel, 2009; Dombard and Sessa 2019). Characterization of the ocean composition, and possible detection of gravity anomalies, hydrothermally derived volatile species at high latitude, and changes in Europa’s orbital motion by Europa Clipper and JUICE may confirm possible ongoing large-scale seafloor activity. 

Acknowledgements

The research leading to these results received funding from the Czech Science Foundation through project No. 19-10809S, from the French ”Agence Nationale de Recherche” ANR (OASIS project, ANR-16-CE31-0023-01), from CNES (JUICE and Europa Clipper missions), from the Région Pays de la Loire - GeoPlanet consortium project (No. 2016-10982). The computations were carried out in IT4Innovations National Supercomputing Center (project no. LM2015070).

References

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