Magnetic study of large Apollo samples: Possible evidence for an ancient centered dipolar field on the Moon

We present new rock magnetic and paleomagnetic results obtained from seventeen Apollo samples. Measurements of the magnetic properties of the samples confirm that the magnetic properties of lunar rocks are dominated by about 0.1 wt.% of multidomain FeNi grains. The ferromagnetic mineral assemblage is to the first order the same in all lunar lithologies, with higher concentration in basalts than in anorthosites (some of which are diamagnetic). Impact processing leads to an increase of the ferromagnetic content. Out of the seventeen samples, eleven show at least one stable component of natural remanent magnetization. In some cases, this magnetization may be a magnetic contamination. However, five basalt samples have a component of magnetization that may have recorded a lunar paleofield acquired on the Moon. For these samples (plus two similar samples from a previous study), using the rock magnetic fabric as a paleohorizontal proxy, paleoinclinations can be estimated from the paleomagnetic data. These paleoinclinations are best explained with a dipolar field and a magnetic paleopole located at 75 N. Therefore our results suggest the existence on the Moon, at least between 3.8 and 3.3 Ga, of a dipolar field whose axis is close to the present-day rotation axis of the Moon. The estimated surface field intensities of several tens of μT support previous result that the Moon had a dynamo field in the past, and the proposed paleofield geometry suggests that this dynamo was centered on the lunar present-day rotation axis.