Interior Conductivity Structure of the Moon

Interior Conductivity Structure of the Moon

Determining the interior structure of the Moon provides a key constraint on the history and evolution of the Moon. Probing the interior via induced magnetic fields is one of the few ways of determining this structure from orbiting vehicles. This technique depends on the effects of the changing magnetic environment of the Moon on the electrically conducting interior. The Moon passes through several different magnetic environments as it orbits the Earth. The largest fraction of the lunar month is spent in the solar wind, a moderately dense, cool plasma (~7 protons and electrons per cubic cm with a temperature of about 10 eV or 110,000 K). Embedded in this plasma is a magnetic field of about 6 nT whose direction is quite variable. Many of these changes in direction are quite abrupt. Because the velocity of the solar wind is highly supersonic, the resulting interaction with the Moon is very local-time-dependent with solar wind plasma intersecting the Moon’s surface on the sunlit side, and a near vacuum cavity arising on the dark side. For a few days each month, the Moon passes into the Earth’s geomagnetic tail, which consists of two near-vacuum magnetic lobes, sandwiching a dense, hot plasma called the plasma sheet. On a traversal of the tail region, the Moon could find itself immersed in a strong unidirectional field for 4 or 5 days, pass from one lobe to the other, or see the irregular plasma environment of the plasma sheet. Acting as a buffer between the solar wind and the magnetotail is the magnetosheath, a region of much greater turbulence than in the solar wind or the tail. ARTEMIS measures the changing magnetic field in all three of these regions, and in different circumstances each of these regionscould be used to sound the interior conductivity of the Moon.

Sounding in the Magnetotail

On the Apollo project, the Apollo subsatellites were used to measure the conductivity structure of the interior of the Moon using tail-lobe entries of the Moon and the orbiting satellite, and observing the resultant induced magnetic moment of the Moon. This measurement was repeated on Lunar Prospector, and a consistent two-layer model was determined with a highly electrically conducting core of about 500 km radius, surrounded by a low-conductivity mantle. The ARTEMIS mission allows us to go beyond the two-layer model because the two spacecraft in lunar orbit allow the input to the Moon and the response of the Moon to be separately measured over a range of frequencies. The distant spacecraft will measure the input spectrum with no lunar response detectable, and the near spacecraft close to 100 km from the Moon will measure the response plus the input. The difference signal then provides the lunar response. By inverting the response, a conductivity profile can be determined. There are many terrestrial and solar-wind drivers that could produce sounding transients, including travelling compression regions and interplanetary shocks. Thus, ARTEMIS will take us beyond the Apollo and LP results.

Sounding in the Solar Wind

During the Apollo project, Explorer 35 and the ALSEP magnetometers were used to explore the conductivity profile of the Moon, using sudden changes in the solar wind magnetic field called magnetic discontinuities. This technique assumed that the surface magnetometer measured the undisturbed solar wind input to the Moon, plus the lunar response and the Explorer 35 magnetometer measured accurately the input. A test of this technique provides excellent results, but a failure of the Explorer 35 magnetometer ended this line of research. ARTEMIS can return to this measurement technique by exploiting both spacecraft. In this case, the low altitude spacecraft should beinthe lunar wake where it is free from plasma effects. The high-altitude spacecraft provides the input to the Moon, and the measurement in the wake provides the sum of the input and the lunar response. This measurement is complementary to the tail sounding described above, but has the advantage that many more magnetic discontinuities occur in the solar wind than shocks or traveling compression regions in the tail. Again, the response can be inverted to provide a conductivity profile. This technique may not work at 100 km altitude on the dayside because of plasma currents beneath the spacecraft. The turbulent magnetosheath might also provide a region of good signals for sounding, but this region has not yet been tested for conductivity sounding.