Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1698z&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1698
Other
[5724] Planetary Sciences: Fluid Planets / Interiors, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
A dense ilmenite-rich cumulate layer is suggested to exist at the base of the lunar mantle, caused by overturn after the lunar magma ocean fractional solidification. This ilmenite-rich cumulate layer is enriched in radioactive elements. It may ascend due to the progressively increasing of thermal buoyancy caused by the continuous radioactive heating. This rise-up of the ilmenite-rich cumulates is proposed to explain the lunar asymmetry and the transient lunar dynamo. On the other hand, the ilmenite-rich cumulates may sink to form a stable outer core surrounding a previously formed metallic core. The inversion of lunar free oscillation data indicates a density structure that may be more compatible with a metallic core surrounded by stable ilmenite-rich cumulates. Here, we investigate the stability of the ilmenite-rich cumulates at the base of the lunar mantle by a numerical convection model with a three-dimensional spherical geometry. The convection model investigates two modes of heating. The mixed heating mode has the internal heating from the ilmenite-rich cumulates and the basal heating from the core while the pure internal heating mode has no heating contribution from the core. These two possible heating modes are based on two different ideas for the lunar metallic core. The first one proposed that the lunar metallic core has been keeping cooling and hence heating the lunar mantle from below while the second idea suggested that the lunar metallic core is too small and can only heat the mantle in very short period of time. Our results show that the behavior of the ilmenite-rich cumulate layer depends on its strength and the heating modes of the lunar mantle. With a relatively large viscosity, the ilmenite-rich cumulate layer acts like a thermal blanket and forms a single upwelling as previous study demonstrated. However, with a small viscosity, the ilmenite-rich layer is much more stable because it is efficient for heat transport and will not accumulate much buoyancy. The ilmenite-rich cumulate layer stays on the lunar core-mantle boundary and is slowly entrained to the overlying mantle. The entrainment rate of the ilmenite-rich cumulates to the mantle is largely influenced by the heating modes. The mixed heating generates many upwellings from the lunar core-mantle boundary, and leads to a much faster entrainment of the ilmenite-rich cumulates. At the meanwhile, with a pure internal heating mode, the ilmenite-rich cumulates can exist at the base of the lunar mantle for a long time (> 1Ga), which provides a possibility for differentiating of the ilmenite-rich cumulates to an outer core.
Liang Yanbin
Parmentier Michel
Zhang Nan
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