Stronger Evidence for a Subsurface Ocean in Callisto from a Multifrequency Investigation of its Induced Magnetic Field

The magnetometer investigation of the Galileo mission was able to produce the most compelling evidence that subsurface oceans exist within our solar system using the phenomenon of magnetic induction. 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 is due to the presence of a conductive ionosphere, which may also produce an induction 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 big-data 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. Further insight is gained because, on top of the flybys C03 and C09 used in all previous studies, we also use magnetic field measurements from flybys C10, C21, C22, and C23 to better estimate Jupiter's average driving field and also include measurements from flyby C10 to better constrain Callisto's induction response. Including the latter suggests that a subsurface ocean, likely deep, better explains the data than an ionospheric response alone.