Magnetism and the History of the Moon

Antipodal effects of lunar basin-forming impacts: Initial 3D simulations and comparisons with observations

“…Apollo surface magnetic field measurements showed that fields are relatively weak at sites dominated by mare basalt but are much stronger at sites such as Apollo 16, which is dominated geologically by basin ejecta materials in the form of the Cayley and Descartes formations (Dyal et al, 1974;Strangway et al, 1973b). Statistical and correlative studies of Apollo subsatellite and Lunar Prospector magnetometer and electron reflectometer data confirmed that magnetic anomalies on the geologically less complex near side correlate best with units interpreted as basin ejecta materials Halekas et al, 2001;Richmond et al, 2003).…”

“…3, recent improved mapping of low-altitude Lunar Prospector magnetometer data shows that, in addition to being concentrated antipodal to young large basins, magnetic anomalies on the lunar far side tend to be distributed along the northwestern periphery of the South Pole-Aitken basin (Purucker et al, 2006;Richmond and Hood, 2007). In the context of the antipodal seismic shock model, this result can be understood as due to: (a) increased ejecta impact shock amplitudes in the thick, unconsolidated SPA ejecta mantle near the Imbrium and Serenitatis antipodes; and (b) enhanced concentrations of microscopic Fe remanence carriers in these ejecta materials as compared to other crustal materials (Strangway et al, 1973a(Strangway et al, , 1973b. It had previously been emphasized by Schultz and Gault (1975) that seismic amplifications would occur preferentially around old impact basins where ejecta deposits are relatively thick.…”

“…Since this is an average over the source volume, even larger intensities would probably be required in local parts of the source. For comparison, the strongest stable natural remanent magnetization observed for any lunar sample or portion thereof is of order 1 A m −1 (e.g., Strangway et al, 1973aStrangway et al, , 1973b. Also, as mentioned in the Introduction, strong shocking of lunar soils yielded maximum magnetization intensities of ∼3 A m −1 .…”

Initial mapping of Mercury's crustal magnetic field: Relationship to the Caloris impact basin

“…The measured magnetic field intensity and orientation along the Apollo 16 surface traverse varied widely implying localized, near‐surface sources. Early sample analyses showed that highland breccias contain relatively more metallic iron remanence carriers and stronger magnetizations than pristine igneous rocks (mare basalts and anorthosites) [ Strangway et al , ; Fuller and Cisowski , ]. More recently, Rochette et al [] have found that the enhanced metallic iron content of lunar breccias is due at least in part to the addition of meteoritic metal during impacts.…”

Magnetic anomalies concentrated near and within Mercury's impact basins: Early mapping and interpretation

“…On the Moon, ejecta materials are known to be enriched in microscopic metallic iron remanence carriers, due partly to reduction of preexisting iron silicates by impact-generated heat and shock and partly to the addition of meteoritic iron [Strangway et al, 1973a;…”