"For decades it has been known that portions of the lunar crust are strongly magnetized [1-4]; yet the origin of magnetization is not understood. Difficulties discerning a source for these anomalies begin with most of them having no consistent association with geologic structures. Impact basins and ejecta, and antipodes are geologic structures that sometimes associate with magnetic anomalies [5], but most are weak relative to the global dynamic range. Further, many of these same structures do not show magnetic anomalies. It is also difficult to reconcile the strengths of many of these anomalies with lunar samples, as most lunar materials are weakly magnetic relative to terrestrial materials. Magnetic measurements of mare basalt and pristine highlands rock show weaker magnetism relative to mid-ocean ridge basalts (~3 orders of magnitude) [6]. Another complication is the mineralogy of lunar magnetic anomalies is not rigorously constrained. As a result, it is difficult to discern if the magnetization is derived from crystallization or from impact shock [7, 8].
Wieczorek et al. [9] suggest that magnetic anomalies on the southern farside of the Moon are attributable to remnant metallic iron from the impactor that created South Pole-Aitken basin (SPA) [9]. They argue that the distribution of modeled projectile materials roughly coincide with the distribution of magnetic anomalies near the northern rim of SPA. Wieczorek et al. [9] note that chondritic projectiles are approximately two times more magnetic than average lunar crustal materials. If the SPA-forming projectile were similar to an undifferentiated chondrite, then the thickness of the ejecta needed to account for the magnetism of materials north of SPA would only need to be a few hundred meters thick. This thickness could be less if the projectile were differentiated into core, mantle, and crustal components. We evaluate this hypothesis combining Lunar Prospector Gamma Ray Spectrometer (GRS) and Clementine reflectance (CSR) FeO products. Our results are ultimately inconclusive. Delta FeO (i.e., GRS-CRS) is higher north of SPA as observed by GRS and might suggest detection of remnant metallic iron. However, excess GRS FeO is evenly distributed throughout the farside highlands. Any remnant high-Fe materials would need to cover the farside highlands in ejecta, and then avoid being covered with a subsequent 10 cm of regolith from impact processes. Furthermore, δFeO and magnetic anomalies are not spatially correlated and do not show corresponding dynamic intensity ranges. Ultimately, due to the old age of SPA and subsequent impact mixing, it may be that any magnetic materials are now too deep to be detected by either instrument.
Acknowledgments: Supported by NASA LASER grant NNH09AL71I.
References: [1] Dyal et al. (1974) RGSP, 12, 568; [2] Hood et al. (2001) JGR, 106, 27,825; [3] Mitchell et al. (2008) Icarus, 194, 401; [4] Purucker and Nicholas (2010) JGR, 115; [5] Richmond (2005) JGR, 110, E05011; [6] Wang et al. (2005) Geosphere, 1, 138; [7] Fuller and Cisowski, Lunar paleomagnetism, in Geomagnetism, 1987; [8] Gattacceca (2010) EPSL, 299; [9] Wieczorek et al. (2012) Science, 335, 1212."