Applications of Planetary Electromagnetic Sounding: Microhertz to Gigahertz

EM sounding formally encompasses both propagative (high-frequency) and inductive (low-frequency) methods. Orbital, surface-penetrating radars have been shown to be effective to depths of kilometers on the Moon and in icy parts of Mars, but have been ineffective over the most of Mars’ silicate surface. Fixed, surface-based radars will likely improve penetration to no more than several hundred meters. The optimum future application of surface-penetrating radar on Mars is to provide subsurface geological context for rovers. Deep sounding can be accomplished using natural-source induction, and has been used to probe the Moon and Galilean satellites. Applications include Mars groundwater, Venus geodynamics, and the interior of the Moon from crust to core. Magnetotellurics has the advantage of performing a complete sounding from a single station. 1. Surface-Penetrating Radar SPR operates propagatively where the loss tangent (tan) << 1, typically above kHz-MHz [e.g., 1]. The upper limit near 1 GHz is where scattering prevents significant subsurface penetration. Flown orbital SPRs comprise A17 Lunar Sounder, MARSIS, and SHARAD. Effectively dry lunar rocks have tan (loss tangent) ~10 at several MHz (attenuation  ~ 6 dB/km) [2], so absorption losses are negligible and sounder imaging to a few km depth [3] was dominated by scattering and the instrument’s limited dynamic range. SHARAD reflections from the subsurface of Mars are absentto-rare in all except ice-rich and young volcanic units [4], from which losses 0.065-0.27 dB/m for the radar-opaque units could be derived. Such attenuation is more Earth-like than Moon-like, and points to the ubiquitous presence of a few monolayers of adsorbed water. A future Mars orbiter would have to improve dynamic range by up to 50 dB to image to ~100 m depth at 20 MHz. The Netlander [5], now Netstation [6], mission would use fixed SPR, with the stated investigation depth “in excess of 1-2 km.” This performance relies on optimistic estimates of system dynamic range, large dielectric contrast of targets, and extreme signal integration (Table 1). In particular, signal averaging is limited in practice due to coherent, nonrandom effects such as electronic offsets and drifts [e.g., 7]. I too have been guilty of sanguine signal integration in predicting instrument performance [8]. Using more realistic parameters, I find maximum investigation depths for 2-20 MHz SPRs of hundreds of meters. The local 3D imaging capability of this instrument is innovative, but SPR cannot address goals of large-scale properties as can seismology, heat flow, and inductive EM, as was done for the Moon by Apollo. Rover-based Mars SPR has been under study for some time [e.g., 9] and will fly on ExoMars [10]. Subsurface context for surface investigations is the greatest benefit, but some information on material properties also follows, especially with broadband systems. If materials with a strong temperature dependence such as hematite are present [11], subsurface temperatures can also be estimated. 2. Natural-Source Induction Induction operates where tan >> 1. Ambient EM energy useful for sounding is widespread and derives from the solar wind, magnetospheres, ionospheres, and lightning [e.g., 12]. Skin depths increase with decreasing frequency. The lowest frequencies in terrestrial induction comprise periods up to 100 days. In general, two quantities must be measured at each frequency to determine the impedance. During Apollo, the magnetic field was measured on the surface and in distant lunar orbit. Two or more surface magnetometers separated by a skin depth can establish the impedance, as can comeasurement of the electric and magnetic fields at a single location—the magnetotelluric method. EPSC Abstracts Vol. 6, EPSC-DPS2011-1671, 2011 EPSC-DPS Joint Meeting 2011