Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p12d0524r&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P12D-0524
Physics
1221 Lunar Geodesy And Gravity (6250)
Scientific paper
A self-gravitating elastic spherical model is used to derive the present state of lateral variations of density and stress within the lunar interior. The model is allowed to deform under the load of an initial surface topography and internal density distribution such that the resulting deformed body gives rise to the observed surface topography and gravity anomalies specified by the spherical harmonics of degree and order up to n = 70. The model also results in a minimum total shear strain energy within the Moon. The density perturbations are largely concentrated near the surface and diminish quickly with depth, becoming negligible below a depth of 300 km. The major stress differences in the model are also in the upper 300 km, reaching highs of approximately 8-9 MPa beneath the large impact basins. The effect of different compositions such as a pure Fe and Fe-S core of 300-400 km radius is shown to have less than 10% effect on the magnitudes of density and stress in the upper parts of the Moon and reducing with depth. The use of a solid or liquid core is even less influential on the distribution of density and stress within the model. This is attributed to the comparatively small core in relation to the large mantle within the Moon. In comparing the Nakamura (J. Geophys. Res., 88, pp. 677-686, 1983) and Khan (Geophys. Res. Lett., 28, 1791-1794, 2001) seismic models, the Khan model has reduced density perturbations within the crust, resulting in smaller stresses in the upper mantle. The introduction of a soft layer in the upper mantle only moderately alters the density perturbations in the overlying strata, but has a major effect on the stress distribution with stresses increasing to the order of 10-14 MPa beneath the largest basins for the Nakamura model, and even larger for the Khan model. The increased stresses in the Khan model are directly due to the soft layer in the upper mantle removing the deeep-seated support of surface features by a more rigid deep interior in the Khan model. This soft layer model may be representative of the period soon after the cessation of major volcanism on the Moon, some 3 billion years ago. The stresses proposed in the soft layer models are on the order of those associated with the shallow moonquake events, and may support the idea that the Moon cooled very rapidly and due to its rigidity and high viscosity has been able to rigidly support remnant stresses within its upper interior for several billion years.
Arkani-Hamed Jafar
Reindler Lucas
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