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Lunar asymmetry and palaeomagnetism

Stevenson, D. J. (1980) Lunar asymmetry and palaeomagnetism. Nature, 287 (5782). pp. 520-521. ISSN 0028-0836. doi:10.1038/287520a0.

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The compositional asymmetry between the nearside and farside of the Moon and the natural remanent magnetism (NRM) of lunar rocks are poorly understood. The compositional asymmetry is indicated by the 2-km offset towards the Earth of the centre of mass relative to the centre of figure and the concentration of both KREEP and mare basalts on the nearside. Wasson and Warren recently noted that these asymmetries may be better explained by an asymmetric crystallization of a primordial magma ocean than by the often proposed greater thickness of farside anorthositic crust. The NRM5 has been attributed to an ancient lunar dynamo. I propose here a model for the early lunar evolution in which the preferred gravitational energy state consisted of an asymmetric accumulation of a liquid iron alloy (Fe–Ni and a small amount of sulphur) which displaces upwards the cold, primordial, undifferentiated core. The resulting depth asymmetry of the outer partially molten zone leads eventually to the subcrustal accumulation of light, magnesium-rich pyroxenes and olivine, preferentially in one hemisphere, sufficient to explain the offset and also indirectly providing a possible explanation for the nearside concentration of KREEP and mare basalt. Meanwhile, slow downward migration of the iron releases gravitational energy sufficient for convection and dynamo generation in an iron layer for about 10^9 yr. The proposed present state of the Moon has a symmetrically placed iron core (radius ≃ 500 km), unlike a previous model for the lunar asymmetry.

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Stevenson, D. J.0000-0001-9432-7159
Additional Information:© 1980 Macmillan Journals Ltd. Received 16 June; accepted 1 August 1980. I thank D. Shirley and J. Wasson for useful discussions. This work is supported by NASA grant NGL 05-007-002.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
NASANGL 05-007-002
Issue or Number:5782
Record Number:CaltechAUTHORS:20140311-103748134
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44241
Deposited By: Ruth Sustaita
Deposited On:11 Mar 2014 21:07
Last Modified:10 Nov 2021 16:49

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