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Title:
An impact origin of the Earth-Moon system
Authors:
Canup, R. M.; Asphaug, E.
Affiliation:
AA(Southwest Research Institute, 1050 Walnut Street Suite 426, Boulder, CO 80302 ; ), AB(University of California Santa Cruz, Department of Earth Sciences, Santa Cruz, CA ; )
Publication:
American Geophysical Union, Fall Meeting 2001, abstract #U51A-02
Publication Date:
12/2001
Origin:
AGU
AGU Keywords:
5455 Origin and evolution, 6250 Moon (1221)
Abstract Copyright:
(c) 2001: American Geophysical Union
Bibliographic Code:
2001AGUFM.U51A..02C

Abstract

In the leading hypothesis for lunar origin, the Moon forms from debris ejected as a result of the collision of a roughly Mars-sized impactor with early Earth (Hartmann & Davis 1975; Cameron & Ward 1976). The likelihood of giant impact events has been substantiated by over a decade of planetary accretion simulations (e.g., Wetherill 1985, 1992; Agnor et al. 1999; Chambers 2001). The most recent simulations predict a median accretion time of 50 million years for an Earth analogue to reach 90% of its final mass (Chambers 2001), in good agreement with lunar and terrestrial formation times derived from Hf-W systematics (e.g., review by Halliday et al. 2000). Simulations of potential lunar forming impacts using a method known as smooth particle hydrodynamics, or SPH, can now achieve resolutions sufficient to study the production of bound debris necessary to yield the Moon. A wide variety of works have found that off-center, low-velocity collisions yield material in bound orbit from which a satellite may then accumulate. However, identifying impacts capable of producing the Earth-Moon system has proven difficult (Cameron 1997, 2000, 2001; Cameron & Canup 1998, Canup et al. 2001). Previous works (Cameron 1997, 2000, 2001) identified only two types of impacts capable of producing the Moon. The first involved an impact by an object with about 3 times the mass of Mars, and about twice the angular momentum of the Earth-Moon system; the second involved an the impact of an object with about twice the mass of Mars with an Earth that was only about half formed. Both scenarios are more restrictive and problematic than that originally envisioned, since they require that the Earth-Moon system's mass or angular momentum be significantly modified after the Moon-forming event by either multiple large impacts, or selective subsequent accretion of material onto only the Earth and not the Moon. Recent scaling trends identified in the SPH simulation results (Canup et al. 2001) implied that a smaller, Mars-mass impactor would be better able to simultaneously account for the Earth-Moon system mass and angular momentum (Canup & Asphaug 2001). This smaller scale impact had not been considered viable since early low-resolution SPH simulations found that it placed too much iron into orbit to yield an appropriately iron-poor Moon (Benz et al. 1986). However, recent work using high-resolution simulations (Canup & Asphaug 2001) found that impacts by an object with 10 to 12% of the Earth's mass produce orbiting debris that is less than 3% iron by mass, and that contains sufficient mass and angular momentum to yield the Moon outside the Earth's Roche limit. This type of impact leaves the Earth-Moon system with approximately its final mass and angular momentum, and implies that the Moon formed near the very end of Earth's accretional history.
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