Stronger Evidence of a Subsurface Ocean Within Callisto From a Multifrequency Investigation of Its Induced Magnetic Field

Abstract

The magnetometer investigation of the Galileo mission used the phenomenon of magnetic induction to produce the most compelling evidence that subsurface oceans exist within our solar system. Although there is high certainty that the induced field measured at Europa is attributed to a global-scale subsurface ocean, there is still uncertainty around the possibility that the induced field measured at Callisto is evidence of an ocean. This uncertainty is due to the presence of a conductive ionosphere, which will also produce an induction signal in response to Jupiter's strong time-varying magnetic field. Therefore, it is not yet known whether the observed induced field is attributable to the ionosphere, an ocean, or a combination of both. In this work, we use previously published simulations of Callisto's plasma interaction in combination with both an inverse and an ensemble forward modeling method to highlight the plausible range of interior properties of Callisto. We further constrain the ocean thickness and conductivity, ice shell thickness, and ionospheric conductivity that are required to explain the Galileo magnetometer observations. This is the first study to jointly consider all flybys to constrain the driving field and three flybys (C03, C09, and C10) to assess the induction response. Our results suggest that Callisto's response more likely arises from the combination of a thick conductive ocean and an ionosphere rather than from an ionosphere alone.