The accretion during condensation mechanism, if it occurs during the early over-luminous stage of the Sun, can explain the differences in composition of the terrestrial planets and the Moon. An important factor is the variation of pressure and temperature with distance from the Sun, and in the case of the Moon and captured satellites of other planets, with distance from the median plane. Current estimates of the temperature and pressure in the solar nebula suggest that condensation will not be complete in the vicinity of the terrestrial planets, and that depending on location, iron, magnesium silicates and the volatiles will be at least partially held in the gaseous phase and subject to separation from the dust by solar wind and magnetic effects associated with the transfer of angular momentum just before the Sun joins the Main Sequence.
Many of the properties of the Moon, including the ‘enrichment’ in Ca, Al, Ti, U, Th, Ba, Sr and the REE and the ‘depletion’ in Fe, Rb, K, Na and other volatiles can be understood if the Moon represents a high temperature condensate from the solar nebula. Thermodynamic calculations show that Ca, Al and Ti rich compounds condense first in a cooling nebula. The high temperature mineralogy is gehlenite, spinel, perovskite, Ca-Al-rich pyroxenes and anorthite. The model is consistent with extensive early melting, shallow melting at 3 AE and with presently high deep internal temperatures. It is predicted that the outer 250 km is rich in plagioclase and FeO. The low iron content of the interior in this model raises the interior temperatures estimated from electrical conductivity by some 800°C. The lunar crust is 80% gabbroic anorthosite, 20% basalt and is about 250-270 km thick. The lunar mantle is probably composed of spinel, merwinite and diopside with a density of 3.4 g cm−3.
KeywordsSolar Wind Perovskite Diopside Main Sequence Terrestrial Planet
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