Electrical conductivity and temperature of the lunar interior from magnetic transient‐response measurements

The response of the moon to magnetic‐field step transients in the solar wind has been investigated for over 100 events, by using simultaneous data from the Apollo‐12 lunar surface magnetometer and the lunar‐orbiting Explorer‐35 magnetometer. These transient events were all selected at times when the moon was in the free‐streaming solar wind and the Apollo‐12 magnetometer was on the lunar dark side. The lunar‐nighttime Apollo‐12 magnetometer data consistently show a distinct difference between radial and tangential surface magnetic‐field components for all step transients; this property strongly implies that the surface magnetometer is measuring a global rather than a local effect. The simplest model, although not unique, which qualitatively explains all the general aspects of the dark‐side transient‐response data is a spherically symmetric three‐layer model having a thin outer crust of very low electrical conductivity. The intermediate layer, of radial thickness R 1 – R 2 , where 0.95 R M ≤ R 1 < R M and R 2 = 0.6±0.1 R M , has an electrical conductivity σ 1 ∼10 −4 mhos/meter; the inner core has radius R 2 ∼0.6 R M and conductivity σ 2 ∼10 −2 mhos/meter. Temperatures are calculated from these conductivities for several possible lunar material compositions. For the example of an olivine moon, the temperatures of the layers are as follows: crust, <440°K; intermediate layer, ∼810°K; core, ∼1240°K.