Analysis of Enceladus’s Time-variable Space Environment to Magnetically Sound its Interior

Abstract

We provide a comprehensive study of Enceladus’s time-variable magnetic field environment and identify in measurements of the Cassini spacecraft signatures that appear to be consistent with induced fields from the moon’s interior. Therefore, we first analyze the background field Enceladus is exposed to within 21 flybys and 50 crossings of the moon’s orbit by the Cassini spacecraft. Considering magnetic field variability due to Enceladus’s eccentric orbit, Saturn’s planetary period oscillations, and local time effects within the magnetospheric current sheet, we construct predictive, time-variable background fields near Enceladus with a correlation coefficient of 0.75 and larger compared to the measured background fields. Subsequently, we build a geophysically based electrical conductivity model of Enceladus’s ocean from the equation of state for saline water and mixing laws for a porous core permeated by water. Using this conductivity model and the derived time-variable fields, we calculate expected induced fields. For close flybys, we identify within mostly plume-dominated magnetic field perturbations of 10–30 nT much smaller perturbations of 1–3 nT, which could be consistent with induction. The flybys over Enceladus’s north pole are best suited for induction studies, and the associated measurements suggest that a conductivity of the ocean with 1–3 S m–1 is not sufficient to produce an adequate induction response, but they support a highly conductive, porous core of 20–30 S m–1 and/or a more conductive ocean. Our study also provides strategies for future magnetic sounding of Enceladus.