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We have applied a direct mapping method (see Hood, Icarus, 2011 for details) to produce more complete maps of lunar magnetic anomalies in the north and south polar regions, extending from 60N to 90N and from 60S to 90S. Only Lunar Prospector magnetometer measurements obtained at altitudes less than 35 km during periods when external field fluctuations were minimal were considered. The maps are normalized to a common altitude of 25 km assuming an approximate fall-off with altitude proportional to the -2.5 power (Richmond and Hood, JGR, 2008).
The south polar region contains the relatively young Schrodinger impact basin, centered near 134E, 75S as well as part of the ancient South Pole Aitken basin. Schrodinger is approximately 320 km in diameter and has been determined via crater counts and superposition relationships to be either slightly younger or slightly older than Imbrium. It is therefore one of the three youngest basins on the Moon. Detailed mapping confirms the existence of weak but distinct magnetic anomalies within the inner ring of Schrodinger. Since lunar basin-forming impacts heated the interior subsurface to temperatures exceeding the Curie temperature of metallic iron (the main lunar remanence carrier), it follows that any pre-existing shock magnetization or shock magnetization imparted at the time of impact would have been thermally erased subsequent to the impact. The existence of anomalies within the inner ring is therefore difficult to explain unless the subsurface acquired thermoremanent (or chemical remanent) magnetization in the presence of a steady magnetic field as it cooled slowly over many thousands of years. This result therefore supports recent evidence from sample studies that the lunar dynamo persisted at least into the Imbrian epoch.
The north polar region is dominated geologically by ancient (preNectarian and Nectarian) terrain. The largest cluster of strong anomalies in this region occurs near the antipode of the Schrodinger basin in terrain that has not been greatly modified by subsequent impacts or mare flooding. Further investigation is therefore warranted to evaluate whether this anomaly cluster may be genetically associated with the Schrodinger impact. If so, then this would represent the fifth young large basin with strong anomalies concentrated near its antipode. Similar anomaly clusters have previously been mapped near the antipodes of Orientale, Imbrium, Serenitatis, and Crisium. A physical model for the formation of such antipodal anomaly clusters has been developed, involving amplification of a pre-existing ambient magnetic field by the partially ionized impact vapor-melt cloud as it expanded thermally around the Moon (Hood and Artemieva, Icarus, 2008). The strongest transient field amplification occurs in the vicinity of the antipode. Although the ambient field could, in principle, have been either a lunar dynamo field or an early solar wind field, current constraints from both sample studies and orbital studies indicate that it was probably a dynamo field. One approach toward evaluating whether the mapped anomaly cluster in the north polar region is genetically associated with Schrodinger is to investigate whether unusual terrain is present, similar in morphology to that which is found antipodal to Imbrium, Orientale, and Serenitatis (as well as antipodal to the Caloris basin on Mercury). Such terrain may have been produced either by the convergence of seismic compressional waves or by enhanced impacts of secondaries near the antipode. The latter, in combination with an amplified antipodal field, could have imparted strong shock magnetization in these regions (Gattacceca et al., EPSL, 2010). Preliminary evidence for the possible occurrence of such terrain, obtained from examinations of LROC imagery, will be presented and discussed at the conference.
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