  The ice-rich dwarf planet Ceres is the largest object in the main asteroid
belt and is thought to have a brine or mud layer at a depth of tens of
kilometers. Furthermore, recent surface deposits of brine-sourced material
imply shallow feeder structures such as sills or dikes. Inductive sounding of
Ceres can be performed using the solar wind as a source, as was done for the
Moon during Apollo. However, the magnetotelluric method -- measuring both
electric and magnetic fields at the surface -- is not sensitive to plasma
effects that were experienced for Apollo, which used an orbit-to-surface
magnetic transfer function. The highly conductive brine targets are readily
separable from the resistive ice and rock interior, such that the depth to deep
and shallow brines can be assessed simultaneously. The instrumentation will be
tested on the Moon in 2023 and is ready for implementation on a Ceres landed
mission.
