Probing the Ice Shell Structure of Ocean Worlds with Gravity-Topography Admittance

Abstract:

The existence of subsurface oceans within the icy moons of Jupiter and Saturn is of great interest due to their potential habitability. Two such ocean worlds are Europa and Enceladus. Europa’s global subsurface ocean has been inferred from observations of magnetic induction by Galileo [Khurana et al.,1998], and Enceladus’s ocean from water vapor plume observations [Porco et al., 2006] and libration data from Cassini [Thomas et al., 2016]. Key to understanding the oceans is the structure of the overlying ice shell. In particular, ice shell thickness, heat transport mechanisms, and the effect of material properties of ice on tidal dissipation are important in assessing the long-term survivability of the ocean.

Gravity-topography admittance, defined as a wavelength-dependent ratio of gravity to topography, bears clues to ice shell structure such as viscosity profile and topography support mechanism. For example, Airy-compensated topography supported by buoyancy leads to low admittance compared to uncompensated topography. Other topography support mechanisms, such as elastic and viscous stresses as well as phase transitions at the base, yield characteristic admittances. Currently, only long-wavelength gravity and shape data are available for Europa and Enceladus. Our goal is to prepare for the future higher resolution data (e.g., from the Europa Clipper mission) by exploring the sensitivity of admittance to shell structure of Europa and Enceladus.

In this paper, we solve for Stokes flow with self-gravitation [Hager and Clayton, 1989] to obtain asymptotic admittance spectra. We explore the sensitivity of admittance to shell thickness, heat transport mechanism, and basal boundary condition [Cadek et al., 2019]. We find that as shell viscosity gradient steepens (e.g., due to a steeper conductive temperature gradient, or inclusion of a convective layer), viscous stresses dominate over buoyancy in supporting topography for progressively longer wavelengths, causing admittance to approach uncompensated values. This transition shifts toward longer wavelengths for thicker shells, as it becomes harder to propagate buoyancy produced stresses through the shell. Understanding the interplay between topography support mechanisms by measuring admittance would lead to new constraints on the ice shell structure.