Constraining Spatial Asymmetry in Europa's Oceans

Strong evidence for present-day liquid water oceans under Europa's icy crust comes from magnetic measurements by the Galileo spacecraft. Based on surface geomorphology, gravity measurements, and surface spectra, the best explanation for the time-varying magnetic moments of Europa is a deep subsurface ocean with a high dissolved salt content. However, all previously published research has assumed that the ice-ocean boundary is perfectly spherically symmetric as a simplifying assumption. To first order, this assumption is likely correct, as Europa is differentiated. The shape of the boundary between the highly conducting, saline ocean and the non-conducting ice shell has a significant effect on the magnetic moments induced by the time-varying field applied by Jupiter. Asymmetry in the ice-ocean boundary is expected due to gravitational, thermal, compositional, and other inconsistencies. In this work, we relax the assumption of spherical symmetry and constrain the spatial asymmetry that may be present in Europa's ice-ocean boundary.

Expansion in spherical harmonics of the ice-ocean boundary radius permits calculation of induced magnetic moments for a proposed boundary shape. The resulting induced magnetic fields for characteristic shapes are compared to smoothed Galileo MAG data from close flybys, as in past studies assuming spherical symmetry. Limits of asymmetry are determined by the largest expansion coefficients that may be considered consistent with Galileo data. Magnetic measurements from the upcoming Europa Clipper mission will provide greater detail and resolution, as well as very close flybys, which will emphasize higher order magnetic moments. The relative signal from spatial asymmetries in Europa's oceans will be dramatically enhanced in measurements from these close flybys, so accounting for these asymmetries in modeling is vital to interpretation of this data.