Initial measurements of the lunar induced magnetic dipole moment using lunar prospector magnetometer data

Twenty-one orbits of Lunar Prospector magnetometer data obtained during an extended passage of the Moon through a lobe of the geomagnetic tail in April 1998 are applied to estimate the residual lunar induced magnetic dipole moment. Editing and averaging of individual orbit segments yields a negative induced moment with amplitude -2.4 +/- 1.6 x 10(22) Gauss-cm(3) per Gauss of applied field. Assuming that the induced field is caused entirely by electrical currents near the surface of a highly electrically conducting metallic core, the preferred core radius is 340+/-90 km. For an iron-rich composition, such a core would represent 1 to 3% of the lunar mass.