Magnetic Properties of Lunar Samples: an Exhaustive Survey of the Apollo Collection

Detailed paleomagnetic studies of lunar samples shed light on the existence and timing of the ancient lunar dynamo, with insights to the inner structure and thermal evolution of the Moon, as well as constraints for the lunar dynamo models [e.g., 1-6]. However these studies are usually performed on small cm-scale samples, typically below 100 mg. Such a small size, combined with anisotropy and other spurious effects have been shown to be the source of additional complexity [7].

We measured the natural remanent magnetization and magnetic susceptibility of 105 large Apollo samples (mass range 40 g to 2.9 kg, median mass 350 g). For this, following the approach utilized for the initial paleomagnetic evaluation of Apollo 11 samples [8], we developed a dedicated magnetometer using a fluxgate sensor and a rotating stage, which allowed measuring the bulk samples in their original Teflon and aluminum packaging under nitrogen atmosphere.

Despite a number of caveats (no demagnetization steps, existence of viscous magnetization and other soft secondary magnetization), the ratio of natural remanent magnetization to susceptibility gives a rough estimate of the paleointensity. The evolution of the paleointensity with the estimated age of the samples will provide a broad picture of the evolution of the lunar dynamo. Susceptibility, as a proxy to the bulk metal content in lunar rocks [9], is also a valuable source of information per se but is currently available only for a small fraction of the Apollo collection. Our survey will allow identification of rocks with unusual magnetic properties, and therefore potentially unusual petrogenesis.

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