The deep lunar electrical conductivity profile: Structural and thermal inferences

Abstract

Simultaneous Apollo 12 surface and Explorer 35 orbital magnetometer records have been reexamined for intervals suitable for measuring the lunar inductive response in the solar wind and terrestrial magnetosheath. Improvements in the selected time series are obtained by (1) editing of intervals containing extraneous surface field fluctuations caused by plasiha interaction with the local crustal magnetic field, and (2) minimization of magnetometer data errors in the Apollo 12 record. Power spectral estimates of the response tangent to the lunar surface, defined in terms of transfer and gain functions, are obtained for the frequency range 10−5 to 10−3 Hz using methods introduced earlier by Wiskerchen and Sonett (1977). Maximum consistency of estimates from different time intervals is found when the analysis is limited to the tangential direction of maximum incident power as measured at Explorer 35 for each interval, i.e., the direction in which the ratio of signal to background noise is greatest. A simplified version of spherically symmetric plasma (SSP) confinement theory is used to interpret the transfer function data via forward model calculations under the assumption that the electrical conductivity increases continuously with depth. The inferred bounds on the internal electrical conductivity profile are narrowest in the depth range 450–1350 km and are more restrictive at these depths than those obtained previously from plasma‐confined response data. An upper limit of ≤360 km for the radius of a metallic core with assumed conductivity ≥102 mhos/m is estimated from the one standard deviation error limits of the measurements. Using the bulk conductivity versus temperature model proposed by Huebner et al. (1979), rough limits on the selenotherm are calculated which appear to be in agreement with independent geophysical constraints at depths less than about 800 km.

Induced Magnetic Fields in Solar System Bodies

“…The conductivity was found to rise from 10 −4 -10 −3 S/m at a few hundred km depth to roughly 10 −2 -1S/m at about 1100 km depth. The study by Hood et al (1982) also allows for a metallic lunar core with a radius of about 360 km and a conductivity σ > 10 2 S/m. Knowing the conductivity profile furthermore enables one to constrain the compositional and thermal structure of the Moon (e.g., Khan et al 2006).…”

“…The bottom panel of Fig. 1 shows the electrical conductivity structure of the Moon as a function of radial distance from its center derived by Hood et al (1982). Upper and lower bounds for the conductivity are given.…”

“…The frequency dependent skin depth (6) Hood et al (1982) sounding allows a first view into the object under discussion. In case of the Moon the transfer function indicates an increasing electrical conductivity with depth.…”

Farside explorer: unique science from a mission to the farside of the moon

“…A variety of approaches are possible to measure the frequency dependent electromagnetic impedance [20]. The Apollo-era studies used the distantly orbiting Explorer 35 to determine the source magnetic field, and compared this to surface measurements that represent the sum of the source and induced fields [24]. This transfer function technique will be repeated for Farside Explorer using magnetometers on the two landers and relay satellite.…”

Ground calibration of the high-sensitivity SELENE lunar magnetometer LMAG

“…The SELENE/Lunar Magnetometer (LMAG) team aims at clarifying above mentioned issues by accumulating magnetic signatures originated from the moon such as those magnetic anomalies on the surface and also by identifying time-dependent magnetic responses which have information on the electrical conductivity of the interior of the moon (e.g. Dyal et al, 1975Dyal et al, , 1976Russell et al, 1981;Hobbs, 1973;Hood et al, 1982Hood et al, , 1999.…”