  One objective of a lander mission to Jupiter's icy moon Europa is to detect
liquid water within 30 km as well as characterizing the subsurface ocean. In
order to satisfy this objective, water within the ice shell must also be
identified. Inductive electromagnetic (EM) methods are optimal for water
detection on Europa because even a small fraction of dissolved salts will make
water orders of magnitude more electrically conductive than the ice shell.
Compared to induction studies by the Galileo spacecraft, measurements of
higher-frequency ambient EM fields are necessary to resolve the shallower
depths of intrashell water. Although these fields have been mostly
characterized by prior missions, their unknown source structures and plasma
properties do not allow EM sounding using a single surface magnetometer or the
orbit-to-surface magnetic transfer function, respectively. Instead, broadband
EM sounding can be accomplished from a single surface station using the
magnetotelluric (MT) method, which measures horizontal electric fields as well
as the three-component magnetic field. We have developed a prototype Europa
Magnetotelluric Sounder (EMS) to meet the measurement requirements in the
relevant thermal, vacuum, and radiation environment. EMS comprises central
electronics, a fluxgate magnetometer on a mast, and three ballistically
deployed electrodes to measure differences in surface electric potential. In
this paper, we describe EMS development and testing as well as providing
supporting information on the concept of operations and calculations on water
detectability. EMS can uniquely determine the occurrence of intrashell water on
Europa, providing important constraints on habitability.
