An ancient lunar magnetic dipole field

Abstract

SINCE the landing of the first Luna spacecraft it has been known that the present magnetic moment of the Moon is negligible. Later, the Explorer 35 data¹ gave an upper limit of 4 γ (= 4 nT) to the dipole field at the surface. Now, Russell et al.² by analysing the Apollo 15 subsatellite magnetometer data obtained during passages of the Moon through the geomagnetic tail (where the magnetic field is reasonably steady) have calculated an upper limit of 0.05 γ. They have shown that both the induced and the permanent magnetic dipole moments can be determined by separately analysing data obtained during passages of the Moon through the north and south lobes of the tail, in which the field is directed towards and away from the Sun, respectively. Only the permanent dipole component in the plane of the subsatellite orbit is determinable and is less than 1.3 × 10¹⁸ gauss cm³: it is unlikely that the component perpendicular to it is appreciably greater. Russell et al.² estimated the error to be 1.33 × 10¹⁸ gauss cm³, and therefore concluded that the lunar magnetic moment is not significantly different from zero. Paradoxical as it may seem, it follows from this observation that the Moon possessed a magnetic field of internal origin in its early history.