Icy Worlds

Different Ice Shell Geometries On Europa And Enceladus Due To Their Different Ssizes: Impacts Of Ocean Heat Transport

By Keith Cowing
June 30, 2022
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Different Ice Shell Geometries On Europa And Enceladus Due To Their Different Ssizes: Impacts Of Ocean Heat Transport
Panel (a) sketches the primary sources of heat and heat fluxes, which include: heating due to tidal dissipation in the ice Hice, the heat flux from the ocean to the ice Hocn and the conductive heat loss to space Hcond. Ocean heat transport is shown by the horizontal arrow. Panel (b) shows the default ice shell thickness profile considered here a black solid curve, which is thinner over the poles because ice dissipation amplifies going poleward (Beuthe 2019). The gray dashed curve shows the freezing (positive) and melting rate (negative) required to maintain a steady state based on an upside-down shallow ice flow model (see appendix for details). In this calculation, the default 2500 km radius is considered. Panel (c) shows the profiles of Hice, Hcond and Hlatent given the information in panel (b). Panel (d) sketches the key physical processes in an ocean covered by an ice shell with varying thickness (see main text for description). Panel (e) shows how thermal expansion coefficient under the ice shell varies with the satellite’s size (gravity), at 10 psu (blue) and 60 psu (brown) ocean salinities. Panel (f) shows the salinity forcing (equatorial minus polar salinity flux, dots) and the temperature forcing (the freezing point difference under the equatorial and polar ice shell, crosses) as a function of the moon’s radius.

On icy worlds, the ice shell and subsurface ocean form a coupled system — heat and salinity flux from the ice shell induced by the ice thickness gradient drives circulation in the ocean, and in turn, the heat transport by ocean circulation shapes the ice shell.

Therefore, understanding the dependence of the efficiency of ocean heat transport (OHT) on orbital parameters may allow us to predict the ice shell geometry before direct observation is possible, providing useful information for mission design. Inspired by previous works on baroclinic eddies, I first derive scaling laws for the OHT on icy moons, driven by ice topography, and then verify them against high resolution 3D numerical simulations. Using the scaling laws, I am then able to make predictions for the equilibrium ice thickness variation knowing that the ice shell should be close to heat balance.

Ice shell on small icy moons (e.g., Enceladus) may develop strong thickness variations between the equator and pole driven by the polar-amplified tidal dissipation in the ice, to the contrary, ice shell on large icy moons (e.g., Europa, Ganymede, Callisto etc.) tends to be flat due to the smoothing effects of the efficient OHT. These predictions are manifested by the different ice evolution pathways simulated for Enceladus and Europa, considering the ice freezing/melting induced by ice dissipation, conductive heat loss and OHT as well as the mass redistribution by ice flow.

Wanying Kang

Comments: arXiv admin note: substantial text overlap with arXiv:2203.16625
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2206.15325 [astro-ph.EP] (or arXiv:2206.15325v1 [astro-ph.EP] for this version)
Submission history
From: Wanying Kang
[v1] Thu, 30 Jun 2022 14:56:16 UTC (10,432 KB)

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.