Objectives: to produce improved low-altitude crustal magnetic field maps of selected lunar regions and apply them to two unresolved problems: (1) modeling of magnetic anomalies in impact basins to…

This is a $165,000 grant awarded by the National Aeronautics and Space Administration (NASA) to the University of Arizona (doing business as the Arizona Board of Regents) to conduct research addressing two key issues in lunar science. The research objectives are to: 1) Produce improved maps of the lunar crustal magnetic field within 13 lunar impact basins and use this data to determine more reliable paleomagnetic pole positions to evaluate evidence of true polar wander, and 2) Conduct a correlative study around the Orientale impact basin to distinguish whether magnetic anomalies are associated with subsurface dike intrusions or surface ejecta deposits. This grant is not associated with a set-aside program. The research will leverage data from NASA's Lunar Prospector and JAXA's Kaguya lunar orbiter missions to address high-priority topics identified in the Lunar Data Analysis Program. The results will be used to advance the scientific understanding of the Moon's magnetic field history and geological evolution.

Objectives: to produce improved low-altitude crustal magnetic field maps of selected lunar regions and apply them to two unresolved problems: (1) modeling of magnetic anomalies in impact basins to determine more reliable paleomagnetic pole positions and a better evaluation of evidence for true polar wander [1-4]; and (2) a correlative study of crustal fields with surface geology and grail gravity data around the orientale impact basin for the purpose of distinguishing between the dike intrusion and ejecta deposit hypotheses for the origin of magnetic anomalies in the lunar highlands [5-8]. Methods/techniques: in year 1, work will focus on (a) producing improved regional maps of the lunar crustal magnetic field within 13 lunar impact basins where magnetic sources can be reliably assumed to have formed by slow cooling in the former core dynamo magnetic field; and (b) begin applying parker's method [9,4] to determine paleomagnetic pole positions as a function of basin relative age. The field maps will be improved in two main ways [10]. First, only the best available measurements over a given region from either the lunar prospector or kaguya missions will be considered (lowest altitude and least external field contamination). Second, a refined equivalent source dipole technique will be used together with the observed radial field component to produce maps of the three vector field components and the field magnitude at a constant altitude. In year 2, the modeling/inversion of anomalies within impact basins will be completed and results will be submitted for publication. In year 3, a magnetic anomaly map will be produced for the region around the orientale basin, where high-resolution gravity gradiometry data for identifying likely dike intrusions are available and where the distribution of light plains representing outer ejecta deposits of this basin-forming impact has recently been mapped. Correlative studies will then evaluate whether magnetic anomalies are associated with linear gravity anomalies (indicative of subsurface dike intrusions) and/or with surface geologic units interpreted as ejecta from orientale. Results will be submitted for publication. Significance/relevance:the work addresses two unresolved problems in lunar science: the occurrence and history of true polar wander and the origin of strong magnetic anomalies in the lunar highlands. Quite different histories of true polar wander have been inferred by several groups [2,3] based on ``modeling the lunar gravitational field, global topography, and the global lunar figure'', which is listed as a high priority area of research in the most recent description of ldap (appendix c.8 of the roses 2016 NRA). The planned work will use magnetometer data from the ldap-supported lunar prospector and kaguya missions to determine more reliable and datable paleomagnetic pole positions with which to test these and other proposed true polar wander scenarios. Linear gravity anomalies implying igneous dike intrusions have been identified in grail gravity data. Linear magnetic anomalies have also been suggested to imply igneous dike intrusion sources [5,6]. The planned correlative study near orientale will establish whether linear magnetic and gravity anomalies are related in this region or whether anomalies instead correlate primarily with orientale ejecta deposits.