Physics – Geophysics
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...10g&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
Earth-Moon System, Lunar Geology, Moon, Earth Planetary Structure, Planetary Evolution, Lunar Evolution, Magma, Solar System, Geophysics, Solar System Evolution, Lunar Rocks
Scientific paper
Consider an alternative to a giant impact origin of the Earth-Moon system. Could coaccretion and orbital evolution have led to substantial melting of the Moon early in its history, independent of the thermal evolution of the Earth? On the Moon, the early history of differentiation processes from 4.5 to 3.9 Ga are well preserved in the lunar highland samples and in the record provided in lunar mare basalts that erupted during the equivalent of the Earth's preserved Archean record. In contrast, the record of processes preserved in the Earth's early crust is incomplete. The Earth record has been largely eradicated by recycling of crustal material into the interior via plate tectonics. However. the rare remnants of ancient crust that are preserved (4.0 and 3.8 Ga) paint a picture of early Earth that differs little from modern Earth. When the preserved crust and mantle records of both planets are compared, different early evolutionary histories emerge. On Earth, granitoids are important components of the early Archean crust, and their existence implies cool, hydrous melting. The igneous pressure-temperature conditions preserved in the Earth's ancient tectosphere indicate mantle temperatures similar to those present in the modern mantle. Early ultramafic mantle melts, such as the 3.49 Ga Barbeton komatiites, are hydrous, subduction zone magmas that record mantle temperatures similar to those inferred for modern midocean ridges. The earliest record from the crust indicates differentiation conditions similar to those that occur today. Therefore, Earth preserves no early record of high-temperature, global differentiation events. The moon, a much smaller body, and one depleted in heat-producing elements (U, Th, and K) preserves a record of a profound differentiation event. A significant fraction fo the mass of the planet was melted during the 0.5 b.y. of lunar history and a magma ocean enveloped 30-50% of the planet. Isotopic constrains limit the time interval of magma ocean formation to 4.4 to 4.1 Ga. By 3.7 Ga the Moon had cooled significantly and formed a rigid which lithosphere sufficient to support elastically the lunar maria. Mare magma generation ended at approximately 3.2 Ga. The thermal histories of these two bodies seem to follow independent paths. Could the larger body that is relatively enriched in heat-producing elements record a cooler early history? What process made the moon "hot enough" to melt significantly early in its history? Do the thermal histories of the Earth and MOon need to be coupled? A model that is consistent with these contrasting thermal histories involves coaccretion of the Earth and mOon. A modest magma ocean forms in the outer portion of the early Earth and the core segregates in a partly molten mantle. The abundant earth producing elements and large volatile inventory trapped and preserved during accretion lead to early whole-Earth convection. Volatile-induced melting generates a Hadean water ocean, and processes much like those on modern Earth are established by the time the Moon's magma ocean has solidified. Could orbital instabilities in the Earth-moon system induce frictional heating in the early Moon and cause extensive melting and lunar magma ocean formation? This scenario could account for the formation of a large magma ocean on the Moon, despite the Moon's geochemically depleted characteristics. Such a scenario is not unique in the solar system. The relation between Io and Jupiter is more complex than that of the Earth-Moon system, but it provides an analogous example of satellite-planet interaction that resulted in extensive melting of Io, as well as eruption temperatures that are similar to those recorded to lunar ultramafic glasses.
Bowring Samuel A.
Grove Timothy L.
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