What Ocean Salinity Is Consistent With The Large Thickness Variation Of The Enceladus Shell?

(a) The density anomaly of water near the freezing point as a function of temperature for various salinities and two pressures. From cold to warm colors denotes increasing salinity (see color-coded legend). Pressure underneath thick ice at the equator is used to compute the solid curves and pressure underneath the south polar ice shell is used for the dashed curves. The density at −5 ◦C for each case is chosen as the reference point. (b) Ice shell thickness of Enceladus (solid curve, left y-axis). The freezing rate (positive) and melting rate (negative) is plotted required to maintain a steady state implied by an ice-flow model (dashed curve, right y-axis). Schematics of circulation and associated transport of heat (red wiggly arrows) and freshwater (blue wiggly arrows) for (c) the alpha ocean in which temperature dominates density variations with sinking at the poles and (d) the beta ocean in which salinity dominates with sinking at the equator. The dark brown arrows denote sinking of dense water, light yellow arrows denote rising of buoyant water. The circulations are forced by the freezing/melting required to counterbalance the down-gradient ice flow (thick black arrows marked at the top) and by variations in the freezing point of water due to pressure.

Of profound astrobiological interest is that not only does Enceladus have a water ocean, but it also appears to be salty, important for its likely habitability.

Here, we investigate how salinity affects ocean dynamics and equilibrium ice shell geometry and use knowledge of ice shell geometry and tidal heating rates to help constrain ocean salinity. We show that the vertical overturning circulation of the ocean, driven from above by melting and freezing and the temperature dependence of the freezing point of water on pressure, has opposing signs at very low and very high salinities.

In both cases, heat and freshwater converges toward the equator, where the ice is thick, acting to homogenize thickness variations. In order to maintain observed ice thickness variations, ocean heat convergence should not overwhelm heat loss rates through the equatorial ice sheet. This can only happen when the ocean's salinity has intermediate values, order 20~psu.

In this case polar-sinking driven by meridional temperature variations is largely canceled by equatorial-sinking circulation driven by salinity variations and a consistent ocean circulation, ice shell geometry and tidal heating rate can be achieved.

Wanying Kang, Tushar Mittal, Suyash Bire, Jean-Michel Campin, John Marshall

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (physics.ao-ph)
Cite as: arXiv:2104.07008 [astro-ph.EP] (or arXiv:2104.07008v1 [astro-ph.EP] for this version)
Submission history
From: Wanying Kang
[v1] Wed, 14 Apr 2021 17:39:11 UTC (2,469 KB)

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