Magnetotelluric Sounding of Terrestrial Planet and Satellite Interiors

Introduction. Electromagnetic (EM) sounding of the Moon and the Galilean satellites has yielded important insights on the interior structures of these bodies [e.g., 1-7]. These investigations measured magnetic fields only and required auxiliary information on the nature of the source fields. The magnetotelluric (MT) method, by measuring both magnetic and electric fields, can perform soundings from a single platform, without specific information on source fields. MT soundings can be performed from the ground, the air, or close orbit. Investigation depths vary depending on the spectra of source fields, but could span hundreds of meters to hundreds of kilometers. Greater investigation depths are enabled by the lower electrical conductivities of the outer shells of most targets due to the lack of liquid water. Component technology is mature and comparatively low-mass and low-cost. MT would be a useful addition to many missions investigating the solid bodies of the Solar System. Terrestrial and Planetary EM Sounding. Time-varying natural or artificial EM fields induce eddy currents in planetary interiors, whose secondary EM fields are detected at or above the surface. These secondary fields shield the deeper interior according to the skin-depth effect, so that EM fields fall to 1/e amplitude over depth δ (km) = 0.5√ρ/f, where ρ is the resistivity and f is the frequency. EM sounding exploits the skin-depth effect by using measurements over a range of frequency to reconstruct resistivity over a range of depth [8,9]. Natural EM signals (mag-netospheric pulsations, ionospheric currents, lightning) are used instead of transmitters at the low frequencies necessary to penetrate kilometers to hundreds of kms into the Earth. The fundamental quantity that must be derived in any sounding is the frequency-dependent EM impedance Z, and it is the variety of approaches to Z that lead to more individual techniques in EM than in any other geophysical method [e.g., 9]. The impedance is related to the apparent resistivity ρ a the most commonly used parameter beause of its dimensional ana-log to true resistivityas ρ a = Z 2 /µω, where µ is the permittivity and ω is the angular frequency. Alternative EM response parameters such as the admittance or the transfer function can also be related to Z [10,11]. Two known quantities are necessary to determine the impedance, e.g., Ohm's Law Z = V/I. One of those quantities is nearly always the magnetic field near the target, i.e., the sum of source + induced magnetic