Evidence for a weakly stratified Europan ocean sustained by seafloor heat flux
Journal of Geophysical Research-Planets, 106 (E6):
12355--12365 (2001)Abstract
We interpret chaos-type features on the surface of Europa as melt-through
structures formed by rotationally confined, steady and/or episodic
oceanic plumes that rise to the base of the ice shell from magmatically
heated regions of the seafloor. Smaller lenticular features in the
vicinity of chaos-type regions might be formed by baroclinically
unstable vortices that spin off the main convective plume or by persistent
heating from localized hydrothermal venting sites. The ocean is assumed
to be weakly stratified because of turbulent convection generated
by heating from below and cooling from above. Seafloor heating, maintained
by tidal dissipation in the rocky interior, generates an estimated
global heat flux of 8.7 x 10(12) W and limits the mean ice thickness
to 2-5 km. For seafloor heat sources with radii r that are less than
the ocean's deformation radius, r(D) = ND//f/ (N is the Brunt-Vaisala:
frequency, D is the water depth, and f is the Coriolis parameter),
the diameters of chaos-type regions are expected to diminish from
O(100 km) within equatorial regions to O(10 km) at high latitudes
(assuming spatially uniform water depth and density structure). Where
r > r(D), the scale of the source region determines the scale of
the melt-through features. Provided there is sufficient time before
refreezing, ice rafts in large melt-through regions are imbedded
in episodes of preferentially anticyclonic circulation, corresponding
to clockwise (counterclockwise) motions in the northern (southern)
hemisphere. We calculate that 10(21) J were required to melt the
ice in the similar to 100 km diameter Conamara Chaos region and that
for a steady, localized heat flux F approximate to 10(11) W (similar
to1% of the global heat flux) it took similar to 1000 years for the
initial melt-through to occur. Assuming that ice raft displacements
in Conamara Chaos occurred during a major melt-through event, maximum
current speeds in the region were O(10 cm s(-1)), and refreezing
occurred within similar to 20 hours. A lack of well-defined ice drift
in other major melt-through regions suggests that these regions formed
through episodes of melting and refreezing that modified the existing
structure but left little time for the establishment of organized
advective motion.