The moon’s permanent magnetic field: a cratered-shell model |
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Authors: | H Weiss P J Coleman Jr |
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Institution: | (1) Dept. of Geophysics and Space Physics and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif., USA |
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Abstract: | As part of our study of the larger-scale remanent magnetic field of the Moon, we have examined the effects of cratering in
an otherwise spherically symmetrical shell magnetized by a concentric dipolar magnetic fieldH
o to an intensity of magnetizationc
H
o, wherec is a constant. In our initial model, we assume that the material excavated from the craters is distributed with random orientation
and thus does not contribute to the remanent dipole momentM
g
. We further assume that the mare fill does not contribute significantly toM
g
. We choose the magnetizing dipole momentM
o and the constantc such that the magnitude of the productcH
o ≃ 3 × 10−4Г at the outer surface of the shell in the equatorial plane of the dipole. This value of the intensity of remanent magnetization
was chosen to be within the range 10−7−10−3Г’; the intensities of thermo-remanent magnetization exhibited by Apollo samples. Finally, we use the locations and diameters
of the 10 largest craters on the Moon and the depth-to-diameter ratios of Pike’s formulation to model approximately the excavation
of the magnetized shell.
The remanent dipole momentM
g
was calculated for each of three orthogonal orientations of the magnetizing dipoleM
o. The three magnitudes ofM
g
fall in the range 4 × 1018−1 × 1019Г cm3 which is close to the upper limit of 1019Г cm3 estimated forM
g
from the field measurements obtained with the Apollo subsatellites. Further, the distribution of the craters is such as to
produce a significant transverse component ofM
g
with acute angles between the spin axis andM
g
in the range 51°–77°. |
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Keywords: | |
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