Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p43a..07w&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P43A-07
Mathematics
Logic
[1517] Geomagnetism And Paleomagnetism / Magnetic Anomalies: Modeling And Interpretation, [1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The vast majority of lunar crustal magnetic anomalies are not clearly associated with geological structures. A possible exception is that some of the largest anomalies appear to be located approximately antipodal to four relatively young large impact basins: Imbrium, Serenitatis, Crisium, and Orientale. A favored formation mechanism involves the amplification of ambient magnetic fields by an impact-generated plasma cloud, the deposition of iron-rich ejecta antipodal to the basin, and shock remanent magnetization resulting from either ejecta deposition or the antipodal focusing of seismic waves (e.g., Hood and Artemieva, 2008). The most prominent of these magnetic anomalies (antipodal to Imbrium and Serenitatis) are located in the northern portion of the South Pole-Aitken basin, and another prominent group (antipodal to Crisium) lies just exterior to this basin. Here, we show that these magnetic anomalies can be explained alternatively as the remnants of an iron-rich South Pole-Aitken forming projectile that were subsequently magnetized by a lunar dynamo-generated magnetic field. Geophysical and geochemical data show that the South Pole-Aitken basin is decidedly non-circular, with an elongation axis in roughly the north-south direction (e.g., Garrick-Bethell and Zuber, 2009). The most prominent magnetic anomalies in this region are located entirely in the northern half of this elliptical structure, and the strongest anomalies are close to the basin rim. This observation can be reconciled by the down-range deposition of iron-rich South Pole-Aitken ejecta resulting from an oblique impact. Based on the magnetic characteristics of the Apollo impact-melt breccias, which can contain more than 1 wt. % metallic iron, the South Pole-Aitken magnetic anomalies can be accounted for by the magnetization of several kilometers of metal-rich deposits in the presence of an Earth-like magnetic field. The long cooling timescales of such thick deposits indicate that the magnetizing field itself was long-lived, and long-lived fields are consistent with a core dynamo, but not with impact-generated fields that last for at most about 1 day (Hood and Artemieva, 2008). We find that several isolated anomalies have magnetic paleopoles coincident with the present rotation axis, consistent with the presence of an ancient dipolar magnetic field. The amount of iron delivered to the Moon by a 200-300 km diameter differentiated projectile corresponds to a globally averaged ejecta deposit with 1 wt. % iron that is about 10 km thick. We are conducting impact simulations in order to study the fate of iron-rich projectile materials in an oblique South Pole-Aitken forming impact event. Preliminary results suggest that although some iron eventually sinks in an impact melt pool in the middle of the basin, some is concentrated both in the downrange direction near the edge of the transient cavity and in the downrange ejecta blanket. Iron-rich impact deposits from the South Pole-Aitken impact might not only be able to account for the magnetic anomalies adjacent to this basin, but also those on the nearside as well.
Stewart Sarah T.
Weiss Benjamin P.
Wieczorek Mark A.
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