Magnetic deposit on the Moon

Abstract

At the Fall Meeting, L. L. Hood reported on continuing studies at the University of California at Los Angeles (UCLA) of Apollo 16 subsatellite magnetic field measurements, which have recently indicated that a conspicuous light‐colored deposit superemposed on the western Ocean of Storms may be strongly magnetized. The 30 × 60 km deposit, known as Reiner Gamma, is visible from the earth with a small telescope near full moon and is the first visually prominent lunar feature to be clearly associated with a large magnetic anomaly detected with the UCLA magnetometers on board the Apollo 15 and 16 subsatellites.Like many other magnetic anomalies detected with the Apollo subsatellites, the Reiner Gamma anomaly is observed repeatedly over the same selenographic position on successive orbits, thus identifying itself as of lunar origin. However, its maximum amplitude of 21 gammas (for comparison, the strength of the earth's magnetic field at the equator is about 33,000 gammas) at 20‐km altitude is by far the largest in apparent magnitude (uncorrected for altitude) found in all 2600 orbital traverses of the Apollo 15 and 16 subsatellites. According to astrogeologists of the U.S. Geological Survey, the Reiner Gamma deposit can be no thicker than about 10m because no shadows are cast even under very low angles of the sun's illumination. Using this thickness estimate, the anomaly amplitude, and the subsatellite altitude, a minimum mean magnetization level 500 times greater than that of the most magnetic returned lunar sample and comparable to that of the most magnetic naturally occuring terrestrial rocks has been estimated. A relatively large free iron content (perhaps 5–10% by weight) is thus suggested if Reiner Gamma is indeed the source of the observed anomaly.