Magnetotelluric Sounding of the Interior of Venus

Introduction. Electromagnetic (EM) sounding has been widely used to reveal earth structure from depths of meters to hundreds of kilometers [e.g. 1-5] and has also probed the deep interiors of the Moon [e.g. 6-9] and the Galilean satellites [10]. The mag-netotelluric method [MT: 11,12] enables natural-source soundings from a single platform, without any external measurements or prior knowledge as was required for previous planetary EM applications. MT can in principle be performed anywhere within about 1 skin depth of the target, allowing measurements from the ground, the air, or in space (as long as the last is within any shielding ionosphere or current sheath). For Venus, MT measurements from the surface or an aerial platform can reveal lithospheric and crustal structure, providing key constraints on thermal and geodynamic evolution. Terrestrial and Planetary EM Sounding. Time-varying natural or artificial EM source 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 [2,11,12]. The EM response is often expressed as the apparent resistivity ρ a , the resis-tivity of a halfspace giving the same signal as the structure under test. Alternative EM response parameters such as the impedance, admittance or the transfer function can also be expressed as ρ a [9,11-13]. To date, planetary EM sounding has used one or more magnetometers by (1) a priori knowledge of the amplitude and phase of the source field, such as the time variation introduced byt he motion of the Galilean satellites through Jupiter's main field [10], or (2) use of a second, distantly orbiting magnetometer to measure the source field, as was done using the Apollo 12 surface magnetometer and Explorer 35 spacecraft [6-9]. A single magnetometer is also sufficient at very low frequencies where the source wavelength can be specified (e.g., a diurnal variation) or where the target can be approximated as a perfect conductor. Magnetotelluric Method. A classical EM-exploration approach offers a complete shallow-to-deep sounding from a single station, without specific knowledge of the source field. The magnetotelluric method (MT) uses orthogonal horizontal components of the …