Physics – Geophysics
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...56z&link_type=abstract
Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N
Physics
Geophysics
Geochemistry, Geophysics, Lunar Evolution, Maria, Stratification, Taylor Instability, Thickness, Viscosity, Selenology, Pre-Imbrian Period, Clementine Spacecraft, Peridotite, Ilmenite
Scientific paper
The asymmetric distribution of maria is fundamental to understanding the early evolution of the Moon. The apparent correlation between mare and surface elevation has prompted the idea that mare basalt flooded topographic low areas. However, Clementine topographic data indicate that while mare basalts clearly fill low areas, large areas of low elevation do not contain mare basalt. The nearside-farside (l=1) crustal thickness asymmetry that has been considered responsible for the center-of-figure/center-of-mass offset of the Moon may be a consequence of the redistribution of crustal material excavated from the SP-Aitken Basin, rather than an endogenic process that might also subsequently control the mare distribution. Therefore, it is necessary to examine how the spatial distribution of mare basalt may be a natural consequence of the Moon's internal dynamics. A previous study suggested that the asymmetry may be related to the formation of a metallic core at the very early stage of lunar evolution. However, this study did not address the timing and distinct geochemical signatures of mare basalts that indicate an origin from the melting of differentiated materials. Anorthositic crust indicates that chemical differentiation is important in early lunar evolution. Although details of this differentiation are not fully understood, a layer of ilmenite cumulates (IC) that has a greater density than peridotite mantle should develop beneath the crust. This IC material may also contain a high concentration of incompatible elements, including heat-producing U and Th. Previous studies have suggested that the IC materials should sink into the deep interior due to Rayleigh-Taylor (R-T) instability, perhaps forming an IC core. We have developed models of internal evolution with temperature-dependent rheology that show that IC material initially in a layer at the base of the anorthositic crust may partially or completely sink into the deep interior, depending on the viscosity of the crust and mantle. IC materials that sink due to R-T instability of a about 100-km thick layer produce small wavelength structure that cannot be related to the l=1 distribution of maria. Rayleigh-Taylor instability of an IC-rich core or layer that overlies a metallic core may, however, explain this distribution. If, because of high viscosity or chemical stratification, radioactive heat within the IC layer cannot be efficiently transferred out by thermal convection in the overlying mantle, the IC layer will heat up at a rate that depends upon the concentration of U and Th and may melt to form a liquid with a density less than that of the overlying peridotites. Depending on the intrinsic density of its melt, further heating and thermal expansion of the IC materials may be required to make it buoyant relative to the overlying mantle. The ensuing R-T instability could then give rise to mare volcanism. Our analyses have shown that the dominant wavelength of R-T instability for a buoyant IC layer between a metallic core and peridotite mantle depends on the size of the metallic core, the thickness of the IC layer, and the viscosity structure of the mantle. When the IC layer has the same viscosity as the mantle, the fastest growth rate occurs at l = 1 only for a small metallic core(<250 km in radius) and a certain range of IC-layer thickness. However, if the IC layer is less viscous than the overlying mantle, growth rate for the instability is fastest at l=1 for a much wider range of model parameters with the metallic core as 350 km in radius (consistent with geophysical observations and an IC rich layer of any reasonable thickness.
Parmentier Marc E.
Zhong Sijia
Zuber Maria T.
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