46th Lunar and Planetary Science Conference (2015) 1319.pdf A STUDY OF PALEOMAGNETIC POLE POSITIONS USING ISOLATED MAGNETIC ANOMALIES ON THE LUNAR NEAR SIDE. S.-M. Baek1, K.-H. Kim1, and H. Jin1, 1School of Space Research, Kyung Hee University (1732, Deogyeong-daero, Yongin-si, Korea) Introduction: Analysis of the Lunar Prospector magnetometer (LP-MAG) data has yielded information on the distribution and nature of lunar crustal magnetization . It has been reported that strong magnetic anomalies are clustered on the lunar far side corresponding to the antipodal regions of large impact basins: Crisium, Serenitatis, Imbrium, and Orientale. These basin-antipodal anomalies have been explained by transiently enhanced magnetic fields associated with the basin-forming impacts, and their structures are complex. Unlike these far side anomalies, LP-MAG data show that well-isolated strong magnetic anomalies have been widely distributed on the lunar near side. Such anomalies show magnetic field perturbations, which can be expected from a simple dipole source (Figure 1). Assuming that a well-isolated anomaly is associated with a simple (i.e., dipole) magnetic source body, we examine whether the source bodies of the observed widespread isolated anomalies on the near side were magnetized by a dipole core field. In this study, we identify eight isolated magnetic anomalies on the lunar near side listed in Table 1 and use a simple dipole model to determine the magnetization direction and depth of the source for each isolated anomaly. We find that the depth of anomaly’s source systematically changes with the inferred latitude of the paleomagnetic pole as shown in Figure 3a. Assuming that the magnetization direction of a single source represents the core field direction and that the depth of the source of the magnetic anomaly corresponds to the age of the magnetization, we suggest that there was the migration of the paleomagnetic pole from south to north for the formation times of the eight anomalies. We also find that the modeled amplitude of dipole moment is negatively correlated with the source’s depth as plotted in Figure 3b, implying a strong core dynamo in early age. Dipole parameters and Paleomagnetic pole positions: Using a simple dipole model for magnetic anomalies, we determine the direction of the dipole moment vector (M) and the position of the dipole source (rDIP) from the lunar surface, where rDIP = (θ, φ, d) is the selenographic system in which θ, φ, and d are the latitude, longitude, and depth from the mean lunar surface. In our study we use dipole parameters obtained from a minimum RMS deviation, which is calculated along a single orbit passing over the estimated center of each magnetic anomaly and as changing the depth of the source location increased downward from the lunar surface (d = 0) with 5 km intervals. Paleomagnetic pole positions are calculated with θp and φp obtained from the dipole model . Description of results: Using a dipole model for magnetic anomalies, we examine M, d, and paleomagnetic pole positions. Source location and angles of inclination (I) and declination (D) of M with respect to lunar surface are listed in Table 1. I is the angle between the local radial direction and M, and D is the azimuth measured from the local eastward direction counterclockwise. Figure 1 Examples of lunar magnetic anomalies. Magnetic field perturbations in the local coordinates of (a) Reiner Gamma on March 23th 1999, (b) Descartes on April 13th 1999, and (c) Crisium on March 14th 1999. Three components (BE, BN, BR) in the local coordinates indicate eastward, northward, and radially outward, respectively. Figure 2 shows comparison of observed and modelcalculated field component for Descartes. The minimum RMS deviation is 2.35 nT for d = 30 km. The 46th Lunar and Planetary Science Conference (2015) inferred directions of the dipole moment vector are I = 36° and D = 305°, indicating radially outward and inclined toward the south-east. Paleomagnetic pole positions calculated with the angles of I and D and magnetic ages for eight anomalies are listed in Table 2. We find that the depth of anomaly’s source systematically changes with the inferred latitude of the paleomagnetic pole as shown in Figure 3a. Assuming that the magnetization direction of a single source represents the core field direction and that the depth of the source of the magnetic anomaly corresponds to the age of the magnetization, we suggest that there was the migration of the paleomagnetic pole from south to north for the formation times of the eight anomalies. We also find that the modeled amplitude of dipole moment is negatively correlated with the source’s depth as plotted in Figure 3b, implying a strong core dynamo in early age. 1319.pdf 83 5 Airy 21 -60 St W 265 21 St E 88 -2 Des W 64 34 Des E 40 -13 Cri N 34 -21 Cri SW 111 70 Cri SE 90 12 Abel N = Nectarian system ; I = (related to Imbrium ejecta) [6, 7] I I N N N Imbrian Reference:  Richmond, N.C. and Hood, L.L. (2008). J. Geophys. Res., 113, E02010.;  Butler, R. F. (2004). Paleomagnetism: Magnetic Domains to Geologic Terranes.;  Takahashi, F. et al., (2014). Nature 7, 409-412.;  Hood, L.L. (2011). Icarus 211., 11091128.;  Hood, L.L. et al., (2001). J. Geophys. Res., 106, 27825-27839.;  Norman, M.D. (2010), Icarus 74, 763-783. Table 1 Dipole parameters Location Direction angles °E I °N D 278 -10 96 101 Hartwig 302 7 100 106 RG W 302 11 104 160 RG E 302 -12 152 25 Ri-Sir 4 -18 108 8 Airy 6 -41 11 113 St W 5 -36 47 235 St E 17 -11 123 2 Des W 16 -11 36 305 Des E 57 21 160 287 Cri N 58 12 159 299 Cri SW 61 13 88 286 Cri SE 88 -33 27 271 Abel RG = Reiner Gamma; Ri-Sir = Rima Sirsalis; Dipole Moment (Am2) Depth (km) 2.9E+13 1.0E+13 5.2E+12 5.0 E+12 5.8 E+12 1.2 E+13 1.2 E+13 1.9 E+13 1.8 E+13 8.5 E+12 5.1 E+12 2.2 E+13 7.1 E+12 50 10 10 20 15 45 35 20 30 30 30 50 20 St = Stöfler; Des = Descartes; Cri = Crisium Table 2 Paleomagnetic pole positions Palopole Magnetic Oisition Age °E °N 238 73 Hartwig 219 71 I RG W 218 21 I RG E 344 9 I Ri-Sir Figure 2 Comparison between observation and model over Descartes. (a), (b), (c), and (d) indicate total, radial, east, and north components, respectively. Figure 3 Change tendency of latitude of paleomagnetic pole positions and dipole moment, respectively. Here red and blue mean Imbrian and Nectarian, respectively.
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