Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p53b1238p&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P53B-1238
Other
6218 Jovian Satellites, 6221 Europa
Scientific paper
Observations by the Galileo spacecraft have shown that Europa likely possesses a liquid water ocean beneath a solid icy outer shell although the depth to the top of the ocean and its thickness remain unknown. The ice shell thickness has important implications for the formation of chaotic terrain and the accessibility of the ocean to future exploration. Hydrothermal plumes have been suggested as a possible mechanism for thinning the ice shell and facilitating the formation of chaotic terrain. The plumes could transport heat from Europa's silicate mantle to the ice shell and possibly melt through the ice. We present new numerical simulations of hydrothermal circulation in the silicate mantle and ocean of Europa that allow us to quantify its impact on overall heat transport and the global ice shell thickness. The processes in our models are hydrothermal convection in the mantle and ocean, thermal diffusion in the core and mantle, parameterized thermal diffusion to account for enhanced heat transfer in the ocean and ice layer, radiogenic heating in the rocky mantle, latent heat of melting/freezing, and tidal heating in the ice shell. Our models start with a differentiated Europa. We select a reasonable value of 750 km for the radius of Europa's metallic core. The silicate mantle extends from the top of the metallic core to a radius of 1465 km. The ocean is approximately 100 km thick and is surrounded by a thin ice shell. The metallic core and silicate mantle have an initial conductive temperature profile appropriate for Europa immediately after differentiation. Our model assumes that tidal heating starts much later than hydrothermal convection. Consequently, hydrothermal convection is not initially aided by tidal heating. The convective pattern is established within a few tens of million years; thereafter a fairly steady state is reached for a few billion years. During this time the ocean thickness is reduced to roughly 50-60 km. However, as a consequence of hydrothermal convection, heating at the base of the ice is spatially heterogeneous and variations in ice shell thickness that directly correlate with the hydrothermal plumes are maintained. Tidal dissipative heating starts around 4 Gyr when the Laplace resonance between Io, Europa, and Ganymede likely formed. The ice shell thins to about 20 km, corresponding to an ocean thickness of 80 km. This process is complete in 20-30 Myr. Additionally, the topographic variations initially present in the ice shell decrease and the ice becomes much more uniform once tidal heating starts. Over time, there is a gradual loss of energy, but the interior is very warm and has high heat content, so the overall dynamical activity decreases only very gradually. Our model indicates that it would be difficult to completely melt through the ice shell. However, even without tidal heating, hydrothermal convection is able to maintain an ocean through most of Europa's history. Additionally, hydrothermal convection promotes the transport of salt from the silicate mantle to the ice shell. The transport of salt through convective flow leads to the formation of a brine layer at the bottom of the ice shell due to exclusion of salt upon freezing. The average salt concentration at the base of the ice is significantly higher than in the bulk of the ocean. Convection within the ice shell will likely be affected by the presence of a brine layer at its base.
Palguta Jennifer
Schubert Gerald
Travis Bryan J.
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