Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p31c..01t&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P31C-01
Other
3017 Hydrothermal Systems (0450, 1034, 3616, 4832, 8135, 8424), 3225 Numerical Approximations And Analysis (4260), 5418 Heat Flow, 5430 Interiors (8147), 6221 Europa
Scientific paper
Data from the Galileo mission indicate that Europa possesses a liquid ocean beneath its icy shell. However, the thickness of the outer ice shell and the depth of the ocean remain uncertain. The past state of the outer H2O layer is even less constrained. We present results of a computational model of the thermal evolution of Europa's interior, that suggest an ocean could have existed beneath an ice shell throughout most of Europa's history, maintained in part by pore water convection in the silicate mantle. Our numerical simulator predicts a thermal history by solving the time-dependent governing equations of mass, momentum and energy conservation in spherical coordinates. The various processes included in the simulations are hydrothermal convection (through silicate mantle pores), thermal diffusion, salt transport, phase changes, radiogenic heating, parameterized convection in the ocean and ice layers, and tidal dissipative heating (tdh) in an ice shell. The vigor of hydrothermal convection is characterized by the Rayleigh number, which depends on the product, kH, where k is mantle permeability and H is the thickness of the convecting layer. Studies of terrestrial permeabilities are used to constrain the permeability of Europa's silicate mantle. Significant permeability has been found on Earth at depths up to 25 km. Accounting for differing gravity, this scales to about 200 km on Europa. There is considerable uncertainty as to when tidal dissipative heating may have commenced in Europa; we adopt Yoder's analysis in which the Laplace resonance among Io, Europa, and Ganymede formed about 500 Myr ago. Despite the 4 Gyr period in our model without tdh, we find that an ocean layer can be maintained by pore water convection in the mantle. This ocean layer, not as thick as when tdh is active, varies in depth with latitude, and thins slowly over time. Concurrently, parameterized convection in the ice shell occurs, and is non-uniform in space and time. Water heated in the silicate mantle is released into the ocean at sites of hydrothermal venting as warm, buoyant plumes that tend to rise through the ocean to the base of the ice shell. The presence of salt in mantle pores, but not initially in the ocean, will limit plume rise for a period of time, but eventually the bulk of the ocean becomes homogenized due to turbulent mixing. Further, a brine layer slowly forms at the base of the ice shell, resulting in an unstable concentration profile. The brine accumulation results from exclusion of water as cold brine cools further, and from the high viscosity of sub-zero brines. The heat delivered by the rising plumes can contribute to local melting and thinning of the ice shell. Without tdh, significant topography at the base of the ice shell develops due to coupling between fluid flow and thermodynamics of ice melt, but most of this topography is lost when tdh is activated. Some variation remains with ice shell thicknesses ranging from 22 km at the poles to 26 km at 11° latitude to 15 km at the equator. The thickest ice occurs near the latitude of Conamara, suggesting that convection in the ice shell could contribute to chaotic terrain.
Palguta Jennifer
Schubert Gerald
Travis Bryan
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